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Chapter 7Chapter 7Components of Components of Optical InstrumentsOptical Instruments
IInstruments for the ultraViolet nstruments for the ultraViolet (UV)(UV),,ViSibleViSible , and infrared , and infrared ((IRIR)) regions have enough features inregions have enough features in
common that they are often common that they are often called optical instruments even called optical instruments even
though the human eye is not though the human eye is not sensitive to ultraviolet or sensitive to ultraviolet or
infrared wavelengths.infrared wavelengths.
7A7A GENERAL GENERAL DESIGNS OFDESIGNS OF
OPTICAL OPTICAL INSTRUMENTSINSTRUMENTS
Optical spectroscopic methods Optical spectroscopic methods are are based upon based upon sixsix phenomena: phenomena:
11. Absorption. Absorption
22. Fluorescence. Fluorescence
33. Phosphorescence. Phosphorescence
44. Scattering. Scattering
55. Emission. Emission
66. Chemiluminescence. Chemiluminescence
Components of typical Components of typical spectroscopic instruments:spectroscopic instruments:
1.1. A stable source of radiant energy (sources A stable source of radiant energy (sources of radiation).of radiation).
2.2. A transparent container for holding the A transparent container for holding the sample (sample cell).sample (sample cell).
3.3. A device that isolates a restricted region A device that isolates a restricted region of the spectrum for measurement of the spectrum for measurement (wavelength selector, monochromator or (wavelength selector, monochromator or grating).grating).
4.4. A radiation detector, which converts A radiation detector, which converts radiant energy to a usable electrical radiant energy to a usable electrical signal.signal.
5.5. A signal processor and readout, which A signal processor and readout, which displays the transduced signal.displays the transduced signal.
FIGURE7-1FIGURE7-1 Components of various Components of various types of instruments for types of instruments for optical spectroscopy. In optical spectroscopy. In
(a),(a),the arrangement for the arrangement for
absorption absorption measurements is shown. measurements is shown.
Note that source Note that source radiation of theradiation of the
selected wavelength is selected wavelength is sent through the sample, sent through the sample,
and the transmitted and the transmitted radiation is measuredradiation is measuredby the detector-signal by the detector-signal
processing-readout unit. processing-readout unit. With some instruments, With some instruments,
the position of thethe position of thesample and wavelength sample and wavelength selector is reversed. In selector is reversed. In
(b), the configuration for (b), the configuration for fluorescence fluorescence
measurementsmeasurementsis shown. Here, two is shown. Here, two
wavelength selectors are wavelength selectors are needed to select the needed to select the
excitationexcitationand emission and emission
wavelengths. The wavelengths. The selected source radiation selected source radiation is incident on the sample is incident on the sample
and theand theradiation emitted is radiation emitted is measured, usually at measured, usually at right angles to avoid right angles to avoid scattering. In (c), the scattering. In (c), the
configurationconfigurationfor emission for emission
spectroscopy is shown. spectroscopy is shown. Here, a source of Here, a source of
thermal energy, such as thermal energy, such as a flame ora flame or
plasma, produces an plasma, produces an analy1evapor that emits analy1evapor that emits radiation isolated by the radiation isolated by the
wavelength selectorwavelength selectorand converted to an and converted to an
electrical signal by the electrical signal by the detector.detector.
FIGURE7-2FIGURE7-2 (a)construction materials and (b) wave (a)construction materials and (b) wave length selectors for spectroscopic instruments.length selectors for spectroscopic instruments.
7B7B Sources of Sources of RadiationRadiation
Sources of RadiationSources of Radiation
In order to be suitable for In order to be suitable for spectroscopic studies, a source spectroscopic studies, a source must generate a beam of radiation must generate a beam of radiation with sufficient power for easy with sufficient power for easy detection and measurement and its detection and measurement and its output power should be stable for output power should be stable for reasonable periods. Sources are of reasonable periods. Sources are of two typestwo types..
11. . Continuum sourcesContinuum sources
22. . Line SourcesLine Sources
7B-17B-1 Continuum Continuum SourcesSources
Continuum Sources:Continuum Sources:
Continuum sources emit radiation that Continuum sources emit radiation that changes in intensity only slowly as a changes in intensity only slowly as a function of wavelength. It is widely used function of wavelength. It is widely used in in absorption absorption andand fluorescence fluorescence spectroscopy. For the spectroscopy. For the ultravioletultraviolet region, region, the most common source is the the most common source is the deuterium deuterium lamplamp. High pressure gas filled arc lamps . High pressure gas filled arc lamps that contain argon, xenon, or mercury that contain argon, xenon, or mercury serve when a particular intense source is serve when a particular intense source is required. For the required. For the visible region visible region of the of the spectrum, the spectrum, the tungsten filament lamp tungsten filament lamp is is used universally. The common infrared used universally. The common infrared sources are inert solids heated to 1500 to sources are inert solids heated to 1500 to 2000 K.2000 K.
7B-2 7B-2 Line SourcesLine Sources
Line Sources:Line Sources:
Sources that emit a few discrete lines Sources that emit a few discrete lines find wide use in find wide use in atomic absorption atomic absorption spectroscopy, spectroscopy, atomicatomic and and molecular molecular fluorescence fluorescence spectroscopy, and spectroscopy, and RamanRaman spectroscopy. Mercury and sodium spectroscopy. Mercury and sodium vapor lamps provide a relatively few vapor lamps provide a relatively few sharp lines in the ultraviolet and visible sharp lines in the ultraviolet and visible regions and are used in several regions and are used in several spectroscopic instruments. spectroscopic instruments. Hollow Hollow cathode lamps cathode lamps and and electrodelesselectrodeless discharge lamps discharge lamps are the most are the most important line sources for atomic important line sources for atomic absorption and fluorescence methods.absorption and fluorescence methods.
7 B-3 7 B-3 Laser SourcesLaser Sources
Laser SourcesLaser Sources
The term ‘LASER’ is an acronym The term ‘LASER’ is an acronym for for LLight ight AAmplification by mplification by SStimulated timulated EEmission of mission of RRadiation. Laser are highly adiation. Laser are highly useful because of their very high useful because of their very high intensities, narrow bandwidths, intensities, narrow bandwidths, single wavelength, and coherent single wavelength, and coherent radiation. Laser are widely used radiation. Laser are widely used in high-resolution spectroscopyin high-resolution spectroscopy..
FIGURE 7-2 FIGURE 7-2 (a) sources and (b) detectors for (a) sources and (b) detectors for spectroscopic in struments.spectroscopic in struments.
Components of Components of LasersLasers
Component of Lasers:Component of Lasers:
The important components of The important components of laser source are lasing medium, laser source are lasing medium, pumping source, and mirrors. The pumping source, and mirrors. The heart of the device is the lasing heart of the device is the lasing medium. It may be a solid crystal medium. It may be a solid crystal such as ruby, a semiconductor such as ruby, a semiconductor such as gallium arsenide, a such as gallium arsenide, a solution of an organic dye or a solution of an organic dye or a gas such as argon or krypton.gas such as argon or krypton.
““LASER”LASER” LLight ight AAmplification by mplification by SStimulated timulated EEmission of mission of
RRadiationadiation Emits very intense, Emits very intense, monochromaticmonochromatic light at light at
high power (intensity)high power (intensity) All waves All waves in phasein phase (unique), and parallel (unique), and parallel All waves are polarized in one planeAll waves are polarized in one plane Used to be expensiveUsed to be expensive Not useful for Not useful for scanningscanning wavelengths wavelengths
LaserLaser SetupSetup
FIGURE 7-2 FIGURE 7-2 schematic schematic representation of a typical laser representation of a typical laser
source.source.
Lasing MechanismLasing Mechanism
Four processesFour processes in in Lasing Lasing
MechanismMechanism::
11. Pumping. Pumping
22. Spontaneous emission . Spontaneous emission
(fluorescence)(fluorescence)
33. Stimulated emission. Stimulated emission
44. Absorption. Absorption
1.1. PumpingPumping Molecules of the active Molecules of the active
medium are excited to higher medium are excited to higher
energy levelsenergy levels
Energy for excitation Energy for excitation
electrical, light, or chemical electrical, light, or chemical
reactionreaction
PumpingPumping
Spontaneous: Incoherent radiationDiffers in direction and phase
FIGURE 7·5 FIGURE 7·5 Four processes Four processes important in laser action: (a) important in laser action: (a)
pumping (excitation by pumping (excitation by electrical,electrical,
radIant, or chemical energy), radIant, or chemical energy), (b) spontaneous emission, (c) (b) spontaneous emission, (c)
stimulated emission, andstimulated emission, and(d) absorption.(d) absorption.
22.. Spontaneous EmissionSpontaneous Emission A molecule in an excited state can A molecule in an excited state can
lose excess energy by emitting a lose excess energy by emitting a
photon (this is fluorescence)photon (this is fluorescence)
E = hE = h = hc/ = hc/; E = E; E = Eyy – E – Exx
E (fluorescence) < E (absorption) E (fluorescence) < E (absorption)
(fluorescence) > (fluorescence) > (absorption) (absorption)
[fluorescent light is at longer [fluorescent light is at longer
wavelength than excitation light]wavelength than excitation light]
Spontaneous EmissionSpontaneous Emission
33. . Stimulated EmissionStimulated Emission Must have stimulated emission to have Must have stimulated emission to have
lasing lasing Excited molecules interact with Excited molecules interact with
photons produced by emissionphotons produced by emission Collision causes excited molecules to Collision causes excited molecules to
relax and emit a photon (i. e., emission)relax and emit a photon (i. e., emission) Photon energy of this emission = Photon energy of this emission =
photon energy of collision photon photon energy of collision photon now there are 2 photons with same now there are 2 photons with same energy (in same phase and same energy (in same phase and same direction)direction)
A photon incident on an excited A photon incident on an excited state species causes emission of a state species causes emission of a second photon of the same second photon of the same frequencyfrequency, which travels in , which travels in exactly the same direction, and is exactly the same direction, and is precisely in phase with the first precisely in phase with the first photo.photo.
M* + hM* + hM + 2hM + 2h
Stimulated EmissionStimulated Emission
44. . AbsorptionAbsorption Competes with stimulated Competes with stimulated
emission emission A molecule in the ground state A molecule in the ground state
absorbs photons and is absorbs photons and is promoted to the excited state promoted to the excited state
Same energy level as pumping, Same energy level as pumping, but now the photons that were but now the photons that were produced for lasing are goneproduced for lasing are gone
AbsorptionAbsorption
Population Inversion and Light Population Inversion and Light AmplificationAmplification
To have light amplification in a laser, the To have light amplification in a laser, the number ofnumber of
photons produced by stimulated emission photons produced by stimulated emission must exceedmust exceed
the number lost by absorption. This the number lost by absorption. This condition prevailscondition prevails
only when the number of particles in the only when the number of particles in the higher energyhigher energy
state exceeds the number in the lower; in state exceeds the number in the lower; in other words,other words,
there must be a there must be a population inversion from population inversion from the normalthe normal
distribution of energy states. Population distribution of energy states. Population inversions arcinversions arc
created by pumping.'created by pumping.'
Population Inversion:Population Inversion: Must have population inversion to Must have population inversion to
sustain lasing. sustain lasing. Population of molecules is inverted Population of molecules is inverted
(relative to how the population (relative to how the population normally exists).normally exists).
Normally: there are more molecules Normally: there are more molecules in the ground state than in the in the ground state than in the excited state (need > 50 %).excited state (need > 50 %).
Population inversion: More molecules Population inversion: More molecules in the excited state than in the in the excited state than in the ground state.ground state.
Why is it important?Why is it important? More molecules in the ground More molecules in the ground
state state more molecules that can more molecules that can
absorb photonsabsorb photons
Remember: absorption competes Remember: absorption competes
with stimulated emission with stimulated emission
Light is attenuated rather than Light is attenuated rather than
amplifiedamplified
More molecules in the excited More molecules in the excited
state state net gain in photons net gain in photons
producedproduced
Population Inversion Population Inversion Necessary for AmplificationNecessary for Amplification
Population inversions are obtained by pumping
FIGURE 7-6 FIGURE 7-6 Passage of radiation through (a) a noninverted population Passage of radiation through (a) a noninverted population and (b) an invertedand (b) an inverted
population created by excitation of electrons into virtual states by an population created by excitation of electrons into virtual states by an external energy sourceexternal energy source
(pumping).(pumping).
Three- and Four-Level Three- and Four-Level Laser SystemsLaser Systems
How to achieve population inversion?How to achieve population inversion? Laser systems: 3-level or 4-LevelLaser systems: 3-level or 4-Level 4-level is better 4-level is better easier to sustain easier to sustain
population inversionpopulation inversion 3-level system: lasing transition is 3-level system: lasing transition is
between Ebetween Ey y (excited state) and the (excited state) and the ground stateground state
4-level system: lasing transition is 4-level system: lasing transition is between two energy levels (neither of between two energy levels (neither of which is ground state)which is ground state)
All you need is to have more molecules All you need is to have more molecules in Ein Eyy than E than Exx for population inversion for population inversion (4-level system) (4-level system) easier to achieve easier to achieve than more molecules in Ethan more molecules in Eyy than ground than ground state (3-level system)state (3-level system)
In the In the three-level systemthree-level system, , the transition responsible the transition responsible
for laser radiation is for laser radiation is between an excited state between an excited state Ey Ey
and the ground state E0;and the ground state E0;in in a four-level systema four-level system, on , on
the other hand. radiation is the other hand. radiation is generated by a transition generated by a transition from from Ey to a state Ex that Ey to a state Ex that has has a greater energy than a greater energy than
the ground state.the ground state.
OverallOverall
Easy population inversion
FIGURE 7-7 FIGURE 7-7 Energy level diagrams for two types of laserEnergy level diagrams for two types of lasersystemssystems
Advantages of LasersAdvantages of Lasers• Low Beam Divergence (“Small dot”)Low Beam Divergence (“Small dot”)
• Nearly Monochromatic (“narrow bandwidth”)Nearly Monochromatic (“narrow bandwidth”)
• Coherent (“constructive interference”)Coherent (“constructive interference”)
Types of LasersTypes of Lasers Solid state lasersSolid state lasers
Nd:YAG Nd:YAG neodymium yttrium aluminum garnetneodymium yttrium aluminum garnet 1064 nm1064 nm
Gas lasersGas lasers lines w/ specific lines w/ specific s in UV/vis/IRs in UV/vis/IR
He/NeHe/Ne ArAr++, Kr, Kr++
COCO22
eximers (XeFeximers (XeF++,….),….)
Dye lasersDye lasers limited tunability in the visiblelimited tunability in the visible
Semiconductor diode lasersSemiconductor diode lasers limited tunability in the IR, redlimited tunability in the IR, red
Semiconductor Semiconductor Diode LasersDiode Lasers
An increasingly importantAn increasingly important
source of nearly monochromatic radiation is thesource of nearly monochromatic radiation is the
laser diode. 7 Laser diodes are products of laser diode. 7 Laser diodes are products of modern semiconductormodern semiconductor
technology. We can understand their mechanismtechnology. We can understand their mechanism
of operation by considering the electrical of operation by considering the electrical conductionconduction
characteristics of various materials as illustrated characteristics of various materials as illustrated inin
Figure 7-8.Figure 7-8.
FIGURE 7-9 FIGURE 7-9 A distributed Bragg-reftector laser diode. (From D. W.Nam A distributed Bragg-reftector laser diode. (From D. W.Nam and R. G. Waarts, and R. G. Waarts, LaserLaser
Focus World, 1994, 30 (8),52. Reprinted with permission of Focus World, 1994, 30 (8),52. Reprinted with permission of PennWellPublishing Company.)PennWellPublishing Company.)
Nonlinear Optical Nonlinear Optical Effects with LasersEffects with Lasers
We noted in Section 6B-7 that when an electromagneticWe noted in Section 6B-7 that when an electromagnetic
wave is transmitted through a dielectric· medium,wave is transmitted through a dielectric· medium,
the electromagnetic field of the radiation causesthe electromagnetic field of the radiation causes
momentary distortion, or polarization, of the valencemomentary distortion, or polarization, of the valence
electrons of the molecules that make up the medium.electrons of the molecules that make up the medium.
For ordinary radiation the extent of polarization For ordinary radiation the extent of polarization P isP is
directly' proportional to the magnitude of the electricdirectly' proportional to the magnitude of the electric
field field E of the radiation. Thus, we may writeE of the radiation. Thus, we may write
P=aEP=aE
where" is the proportionality constant.where" is the proportionality constant. observed, and the observed, and the relationship between polarizationrelationship between polarization
and electric field is given byand electric field is given by
P = aE + f3E' + yE' + . . . (7-1)P = aE + f3E' + yE' + . . . (7-1)
FIGURE 7·10 FIGURE 7·10 A frequency-doubling system for convertingA frequency-doubling system for converting975-nm laser output to 490 nm. (From D. W.Nam975-nm laser output to 490 nm. (From D. W.Namand R. G. Waarts, and R. G. Waarts, Laser Focus World, 1994,30 (8),Laser Focus World, 1994,30 (8),
52. Reprinted with permission of PennWeli Pubtishing52. Reprinted with permission of PennWeli PubtishingCompany.)Company.)
WAVELENGTH WAVELENGTH SELECTORSSELECTORS
7 B:7 B:
Wavelength SelectorsWavelength Selectors
Need to select wavelengths (Need to select wavelengths () of light for optical ) of light for optical measurements. The output from a wavelength measurements. The output from a wavelength selector would be a radiation of a single wavelength selector would be a radiation of a single wavelength or frequency. There are two types of wavelength or frequency. There are two types of wavelength selector:selector:
11. . FiltersFilters
22. . Monochromators Monochromators 33. Gratings. Gratings
• 4 4 . . Michelson InterferometerMichelson Interferometer
Wavelength SelectorsWavelength Selectors…..…..
Used to select the wavelength (or wavelength range) of light Used to select the wavelength (or wavelength range) of light that eitherthat either impinges on the sample (fluorescence and phosphorescence)impinges on the sample (fluorescence and phosphorescence) is transmitted through the sample (absorption and emission)is transmitted through the sample (absorption and emission)
This selected wavelength then strikes the detectorThis selected wavelength then strikes the detector the ability to select the wavelength helps you to discriminated the ability to select the wavelength helps you to discriminated
between phenomena caused by your between phenomena caused by your analyteanalyte and that caused by and that caused by interfering or interfering or non-relevantnon-relevant species. species.
Are often combined with a set of Are often combined with a set of SLITSSLITS (discussed later) (discussed later)
Various typesVarious types based on filters (CHEAP COLORED GLASS)based on filters (CHEAP COLORED GLASS) based on prisms (LIMITED APPLICATIONS)based on prisms (LIMITED APPLICATIONS) based on gratings…. (GREAT STUFF)based on gratings…. (GREAT STUFF)
FIGURE 7-11 FIGURE 7-11 Out put of typical wave length selsctor.Out put of typical wave length selsctor.
FILTERSFILTERS
7 C-17 C-1
FiltersFilters Simple, rugged (no moving parts in general)Simple, rugged (no moving parts in general)
Relatively inexpensiveRelatively inexpensive
Can select some broad range of wavelengthsCan select some broad range of wavelengths
Most often used in Most often used in field instrumentsfield instruments simpler instrumentssimpler instruments instruments dedicated to monitoring a single wavelength range.instruments dedicated to monitoring a single wavelength range.
Two types of filters:Two types of filters: Interference filters Interference filters depend on destructive interference of the impinging depend on destructive interference of the impinging
light to allow a limited range of wavelengths to pass through them (more light to allow a limited range of wavelengths to pass through them (more expensive)expensive)
Absorption filters Absorption filters absorbabsorb specific wavelength ranges of light (cheaper, specific wavelength ranges of light (cheaper, more common)...more common)...
Interference filtersInterference filters
Interference FiltersInterference Filters Dielectric layer between two metallic Dielectric layer between two metallic
filmsfilms Radiation hits filter Radiation hits filter some reflected, some reflected,
some transmitted (transmitted light some transmitted (transmitted light reflects off bottom surface)reflects off bottom surface)
If proper radiation If proper radiation reflected light in reflected light in phase w/incoming radiation: other phase w/incoming radiation: other undergo destructive interferenceundergo destructive interference
i.e., i.e., s of interest s of interest constructive constructive interference (transmitted through interference (transmitted through filter); unwanted filter); unwanted s s destructive destructive interference (blocked by filter)interference (blocked by filter)
Result: narrow range of Result: narrow range of s transmitted s transmitted
FIGURE 7-12 FIGURE 7-12 (a) Schematic cross section of an(a) Schematic cross section of aninterference filter.Note that the drawing is not to scaleinterference filter.Note that the drawing is not to scale
and that the three central bands are much narrowerand that the three central bands are much narrowerthan shown. (b) Schematic to show the conditionsthan shown. (b) Schematic to show the conditions
for constructive interference.for constructive interference.
Fabry-Perot Filters (Interference Filters)Fabry-Perot Filters (Interference Filters)
Douglas A. Skoog and James J. Leary, Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.College Publishing, Fort Worth, 1992.
Calcium or Magnesium Calcium or Magnesium Fluoride (FLUORITE!)Fluoride (FLUORITE!)
tFrom 1 to 1’: From 1 to 1’:
For reinforcement to occur at For reinforcement to occur at point 2, point 2,
cos
t
'cos
2
nt
N is order of interference (a small whole number)
A dielectric material is a substance that is a poor conductor of electricity, but an efficient supporter of electrostatic fields.
Fabry-Perot Filters (Interference Filters)Fabry-Perot Filters (Interference Filters)
n
t 2
n
t 2
When When approaches zero approaches zero nn’ = 2t’ = 2t
Snell’s law: Snell’s law: //’’ = = ’/
then then = = ’’
is the wavelength passing the filter and is the refractive index of the dielectric medium
t
Are we missing something?
Interference WedgesInterference Wedges
An interference wedge An interference wedge consists of a pair of consists of a pair of
mirrored,mirrored,partially transparent partially transparent plates separated by a plates separated by a
wedgeshapewedgeshapelayer of a dielectric layer of a dielectric
material.material.
FIGURE 7-13 FIGURE 7-13 Transmission characterics of typical Transmission characterics of typical interference filters. interference filters.
Absorption filtersAbsorption filters
Absorption FiltersAbsorption Filters Colored glass (broader bandwidth: ~50-100 Colored glass (broader bandwidth: ~50-100
nm vs. ~10-nm with interference filters)nm vs. ~10-nm with interference filters) Glass absorbs certain Glass absorbs certain s while transmitting s while transmitting
othersothers TypesTypes
Bandpass: passes 50-100 nmBandpass: passes 50-100 nm Cut-off (e.g., high-pass)Cut-off (e.g., high-pass)
Passes high wavelengths, blocks low Passes high wavelengths, blocks low wavelengthswavelengths
Type of filter could be used for Type of filter could be used for emission or fluorescence (since emission or fluorescence (since excitation light is lower excitation light is lower and should and should be blocked; emission is higher be blocked; emission is higher and and should be collected).should be collected).
Characteristics of Characteristics of absorption filterabsorption filter
Cheaper than interference filterCheaper than interference filter Worse than interference filterWorse than interference filter But widely usedBut widely used
Absorption some spectral rangeAbsorption some spectral range Effective bandwidth = 30 ~ 250 nmEffective bandwidth = 30 ~ 250 nm %T = less than 10%%T = less than 10% Cut-off filterCut-off filter
typestypes Dye suspended in gelatinDye suspended in gelatin Colored glass (stable to heat)Colored glass (stable to heat)
Cut off filteresCut off filteres
Cut off filteres : Cut off filteres : have have transmittances of nearly 100% transmittances of nearly 100% over a over a portion of the visible spectrum but portion of the visible spectrum but
then rapidly decrease to zero then rapidly decrease to zero transmittance over the remainder. A transmittance over the remainder. A narrow spectral band can be isolated narrow spectral band can be isolated
by coupling a cutoff filter with a second by coupling a cutoff filter with a second filter (see Figure 7-15). Figure 7-filter (see Figure 7-15). Figure 7-
1414shows that the performance shows that the performance characteristics characteristics of absorption filters are of absorption filters are
significantly inferior to those of significantly inferior to those of interference-type filters.interference-type filters.
Two basic filter functions….Two basic filter functions…. cutoff cutoff filters absorb light in a specific filters absorb light in a specific
range of wavelengths. They “cutoff” this range of wavelengths. They “cutoff” this range from the detectors (e.g. cutoff for range from the detectors (e.g. cutoff for 550 nm)550 nm)
Absorption filters are cutoffs.Absorption filters are cutoffs.
bandpassbandpass filters absorb light outside of filters absorb light outside of a specific range (e.g. 350-550 nm)a specific range (e.g. 350-550 nm)
Interference filters are bandpass or you can Interference filters are bandpass or you can make a bandpass from a combination of two make a bandpass from a combination of two cutoff filters!cutoff filters!
Comparison of various types of absorptionComparison of various types of absorptionfilters for visible radiation.filters for visible radiation.
MONOCHROMATORSMONOCHROMATORS A)A) PRISM PRISM
MONOCHROMATORS MONOCHROMATORS B)B)GRATING GRATING MONOCHROMATORSMONOCHROMATORS C)C)ECHELLE ECHELLE MONOCHROMATORS.MONOCHROMATORS.
7 C-27 C-2
Monochromators: Monochromators:
For many spectroscopic methods, it is necessary or For many spectroscopic methods, it is necessary or desirabledesirable
to be able to continuously vary the wavelengthto be able to continuously vary the wavelength
of radiation over a broad range. This process is calledof radiation over a broad range. This process is called
scan ing a spectrum. Monochromators are designedscan ing a spectrum. Monochromators are designed
for spectral scanning. Monochromators for ultraviolet,for spectral scanning. Monochromators for ultraviolet,
visible, and infrared radiation arc all similar in visible, and infrared radiation arc all similar in mechanicalmechanical
const ruction in the sense that they use slits,const ruction in the sense that they use slits,
lenses, mirrors, windows, and gratings or prisms. Thelenses, mirrors, windows, and gratings or prisms. The
materials from which these components are fabricatedmaterials from which these components are fabricated
depend on the wavelength region of intended usedepend on the wavelength region of intended use
Components of Components of Monochromators Monochromators Figure 7-18 Figure 7-18
illustrates the optical elements found illustrates the optical elements found in all monochromators, which include in all monochromators, which include the following: (I) an entrance slit the following: (I) an entrance slit that provides a rectangular optical that provides a rectangular optical
image, image, (2) a collimating lens or mirror that (2) a collimating lens or mirror that produces a parallel beam of radiation, produces a parallel beam of radiation, (3) a prism or a grating that disperses (3) a prism or a grating that disperses
the radiation into its component the radiation into its component wavelengths, wavelengths,
(4) a focusing element that reforms the (4) a focusing element that reforms the image of the entrance slit and focuses image of the entrance slit and focuses
it on a planar surface called a it on a planar surface called a focal focal plane, andplane, and
(5) an exit slit in the focal (5) an exit slit in the focal plane that plane that isolates the desired spectral band.isolates the desired spectral band.
FIGURE 7-18 FIGURE 7-18 Two types of monochromators: (a) Czerney-Tumer grating Two types of monochromators: (a) Czerney-Tumer grating monochromator andmonochromator and
(b) Bunsen prism monochromator. (Inboth instances, Al > A,.)(b) Bunsen prism monochromator. (Inboth instances, Al > A,.)
A) A) Prism Prism MonochromatorsMonochromators
Prism MonochromatorsPrism Monochromators
Prisms can be used to disperse ultraviolet, visible, andPrisms can be used to disperse ultraviolet, visible, and
infrared radiation. The material used for their constructioninfrared radiation. The material used for their construction
differs, however, depending on the wavelengthdiffers, however, depending on the wavelength
region (see Figure 7-2b). Figure 7-20 shows the two most common types region (see Figure 7-2b). Figure 7-20 shows the two most common types of prism designs_ The first is a 60° prism, which is usuallyof prism designs_ The first is a 60° prism, which is usually
fabricated from a single block of material. When crystallinefabricated from a single block of material. When crystalline
(but not fused) quartz is the construction material, however, the prism is (but not fused) quartz is the construction material, however, the prism is usually formed by cementing two 30° prisms together, as shown in usually formed by cementing two 30° prisms together, as shown in Figure 7-20a; one is fabricated from right -handed quartz and theFigure 7-20a; one is fabricated from right -handed quartz and the
second from left-handed quartz in this way, the opticallysecond from left-handed quartz in this way, the optically
active quartz causes no net polarization of theactive quartz causes no net polarization of the
emitted radiation; this type of prism is called a emitted radiation; this type of prism is called a Cornu prism. Figure 7-Cornu prism. Figure 7-18b shows a Bunsen monochromator, 18b shows a Bunsen monochromator, which uses a 60° prism, likewise which uses a 60° prism, likewise often made of quartz.often made of quartz.
FIGURE 7-20 FIGURE 7-20 Dispersion by a Dispersion by a prism: prism: (a) (a) quartz Cornu type quartz Cornu type
and and (b)(b) Littrow type. Littrow type.
PrismsPrisms
First type of widely used, “scanning” First type of widely used, “scanning” wavelength selection devices (TURN PRISM)wavelength selection devices (TURN PRISM)
Often made of salts such as sodium chloride, Often made of salts such as sodium chloride, fluorites etc (Remember figure 7-2b).fluorites etc (Remember figure 7-2b).
VERY delicateVERY delicate. Often subject to damage in . Often subject to damage in humidity and wide heat ranges.humidity and wide heat ranges.
Not widely used today in spectroscopy Not widely used today in spectroscopy equipment.equipment. Great demonstration tools for kidsGreat demonstration tools for kids Nice on the cover of a Pink Floyd albumNice on the cover of a Pink Floyd album
PrismsPrisms
Douglas A. Skoog, F. James Holler and Timothy A. Nieman, Principles of Douglas A. Skoog, F. James Holler and Timothy A. Nieman, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia, 1998.Instrumental Analysis, Saunders College Publishing, Philadelphia, 1998.
PrismPrism
B) B) Grating Grating MonochromatorMonochromator
ss
Grating Monochromators (scan a Grating Monochromators (scan a spectrum)spectrum)
Scan spectrum = vary Scan spectrum = vary continuously continuously Materials for construction = Materials for construction = range of range of
interestinterest Components of a grating monochromatorComponents of a grating monochromator
1. Entrance slit (rectangular image)1. Entrance slit (rectangular image)
2. Collimating optic (parallel beam)2. Collimating optic (parallel beam)
3. Grating (disperses light into separate 3. Grating (disperses light into separate s) s)
4. Focusing optic (reforms rectangular 4. Focusing optic (reforms rectangular image)image)
5. Exit slit at focal plane of focusing optic 5. Exit slit at focal plane of focusing optic (isolates desired spectral band) (isolates desired spectral band)
Reflection GratingsReflection Gratings
Widely used in instruments today.Widely used in instruments today.
Light reflected off a surface, and not Light reflected off a surface, and not cancelled out by destructive interference, is cancelled out by destructive interference, is used for selection of wavelengthsused for selection of wavelengths
Constructed of various materials….Constructed of various materials…. Polished glass, silica or polymer substratePolished glass, silica or polymer substrate Grooves milled or laser etched into the surfaceGrooves milled or laser etched into the surface Coated with a reflective material (silvered) such as Coated with a reflective material (silvered) such as
a shiny metala shiny metal
VERY FRAGILE!! VERY FRAGILE!! Sealed inside the instrument. DO NOT TOUCH!Sealed inside the instrument. DO NOT TOUCH!
Reflection (Diffraction) Reflection (Diffraction) Gratings...Gratings...
Widely used in instruments today.Widely used in instruments today. Light reflected off a surface, and not cancelled out by Light reflected off a surface, and not cancelled out by
destructive interference, is used for selection of destructive interference, is used for selection of wavelengthswavelengths
Constructed of various materials….Constructed of various materials…. Polished glass, silica or polymer substratePolished glass, silica or polymer substrate Grooves milled or laser etched into the surfaceGrooves milled or laser etched into the surface Coated with a reflective material (silvered) such as a shiny Coated with a reflective material (silvered) such as a shiny
metalmetal VERY FRAGILE!! VERY FRAGILE!!
Sealed inside the instrument. DO NOT TOUCH!Sealed inside the instrument. DO NOT TOUCH! Laser Cut have 100’s - 1000’s of lines (blazes) per mmLaser Cut have 100’s - 1000’s of lines (blazes) per mm High resolution (<0.01 nm) if neededHigh resolution (<0.01 nm) if needed Most expensive optical part of an instrumentMost expensive optical part of an instrument
Reflection GratingsReflection Gratings
Light hits grating and Light hits grating and
light is dispersedlight is dispersed
Tilt grating to vary which Tilt grating to vary which
is passed at exit slit is passed at exit slit
during the scanduring the scan
1.The 1.The EchelletteGratingEchelletteGrating2 . Concave 2 . Concave Gratings Gratings 3.Holographic 3.Holographic GratingsGratings
Grating Grating MonochromatorsMonochromators
Construction of Gratings…..Construction of Gratings….. The substrate is formed and polished. It is then The substrate is formed and polished. It is then
blazed by one of a number of techniques…blazed by one of a number of techniques… Cut using mechanical tools Cut using mechanical tools
Poor reproducibility in shape and spacing of the blazesPoor reproducibility in shape and spacing of the blazes Etched using chemicalsEtched using chemicals
Better but still not very goodBetter but still not very good Laser etched blazes (aka holographic gratings).Laser etched blazes (aka holographic gratings).
Best method for productionBest method for production Closely spaced blazes (high # of lines/mm) means a greater Closely spaced blazes (high # of lines/mm) means a greater
capacity to separate light into component wavelengthscapacity to separate light into component wavelengths Good reproducibility from blaze to blaze means that the Good reproducibility from blaze to blaze means that the
grating produces fewer “defects” such as double imagesgrating produces fewer “defects” such as double images Most common method todayMost common method today
The substrate is then coated with a very thin (few The substrate is then coated with a very thin (few molecules or atoms thick) film of reflective molecules or atoms thick) film of reflective materialmaterial
The grating is then mounted in a holder and will The grating is then mounted in a holder and will never be touched by anything if correctly cared fornever be touched by anything if correctly cared for
1. echellette -type grating 1. echellette -type grating Figure 7-21 is a schematic representationFigure 7-21 is a schematic representation
of an of an echellette -type grating, which isechellette -type grating, which isgrooved, or grooved, or blazed, such that it has relatively broadblazed, such that it has relatively broad
faces from which reflection occurs and narrow unusedfaces from which reflection occurs and narrow unusedfaces. This geometry provides highly efficient diffractionfaces. This geometry provides highly efficient diffractionof radiation, and the reason for blazing is to concentrateof radiation, and the reason for blazing is to concentrate
the radiation in a preferred direction . Eachthe radiation in a preferred direction . Eachof the broad faces can be considered to be a line sourceof the broad faces can be considered to be a line source
of radiation perpendicular to the plane of the page; thusof radiation perpendicular to the plane of the page; thusinterference among the reflected beams 1,2, and 3 caninterference among the reflected beams 1,2, and 3 can
occur. For the interference to be constructive, it is occur. For the interference to be constructive, it is necessary that the path lengths differ by an integral necessary that the path lengths differ by an integral multiplen of the wavelength A of the incident beam.multiplen of the wavelength A of the incident beam.
echellette -type gratingechellette -type grating
Echelle grating Echelle grating AdvantagesAdvantages
The advantage of an echelle The advantage of an echelle high efficiency and low polarization effects over high efficiency and low polarization effects over
large spectral intervalslarge spectral intervals Together with high dispersion, this leads to Together with high dispersion, this leads to
compact, high-resolution instruments.compact, high-resolution instruments. An important limitation of echelleAn important limitation of echelle
the orders overlap unless separated optically, for the orders overlap unless separated optically, for instance by a cross-dispersing element. instance by a cross-dispersing element.
A prism or echelette grating is often used for this A prism or echelette grating is often used for this purpose. purpose.
For broad spectral range, to use many sucessive For broad spectral range, to use many sucessive ordersorders
http://www.gratinglab.com/library/http://www.gratinglab.com/library/technotes/technote6.asptechnotes/technote6.asp
Example 7-1
Grating with 1450 blazes/mmPolychromatic light at i = 48 deg
L of the monochromatic reflected light at R = +20,+10 and 0 deg?
dd(sin i + sin r) = n(sin i + sin r) = n1) Calculate “d”1) Calculate “d”
d= 1 mm/1450 blazes convert to nm x106 689.7 nm per groove!
dd(sin i + sin r) = n(sin i + sin r) = n2) Calculate “2) Calculate “” for n=1 at +20 ” for n=1 at +20 degdeg
= 689.7 nm ( sin 48 + sin 20)/1 = 748.4 nm!
Grating will give a monochromatic beam of light of 748.4 nm at 20 deg, 632 nm at 10 deg and 513 nm at 0 deg. For n=1!
Eugene Hecht, Eugene Hecht, OpticsOptics, Addison-Wesley, Reading, MA, 1998., Addison-Wesley, Reading, MA, 1998.
ResolutionResolution
New holographic gratings can have up to 64K New holographic gratings can have up to 64K grooves!grooves!
More grooves = better resolving powerMore grooves = better resolving power
R = R = //(1K to 10K)(1K to 10K)
R = nR = nN (N = grooves!)N (N = grooves!)
How well can you focus on two adjacent How well can you focus on two adjacent wavelengths!wavelengths!
2. Concave Gratings.2. Concave Gratings.
Gratings can he formed on a concaveGratings can he formed on a concave
surface in much the same way as on a plane surface.surface in much the same way as on a plane surface.
A concave grating permits the design of a A concave grating permits the design of a monochromatormonochromator
without auxiliary collimating and focusingwithout auxiliary collimating and focusing
mirrors or lenses because the concave surface bothmirrors or lenses because the concave surface both
disperses the radiation and focuses it on the exit slit.disperses the radiation and focuses it on the exit slit.
Such an arrangement is advantageous in terms of cost:Such an arrangement is advantageous in terms of cost:
in addition. the reduction in number of optical surfacesin addition. the reduction in number of optical surfaces
increases the energy throughput of a monochromatorincreases the energy throughput of a monochromator
that contains a concave grating.that contains a concave grating.
3. Holographic 3. Holographic GratingsGratings
. . Holographic gratings are appearingHolographic gratings are appearing
in ever-increasing numbers in modern in ever-increasing numbers in modern optical instruments, even some of the less optical instruments, even some of the less expensive ones.expensive ones.
Grating EquationGrating Equation
Douglas A. Skoog and James J. Leary, Principles of Instrumental Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.Analysis, Saunders College Publishing, Fort Worth, 1992.
Grooved or blazed. Provides highly efficient diffraction (small enough in size compared to wavelength) of radiation. Each broad face is considered as a point source.
d: Spacing between the reflecting surfaces
Beam 2travels a greater distance than beam 1, for constructive interferences to occur,CB + BD = nangle i = CAB, angle r = DAB
CB = dsini, BD = dsinr
n = d(sini + sinr)
d: Spacing between the reflecting surfaces
Performance Characteristics Performance Characteristics the relationship of Grating the relationship of Grating
MonochromatorsMonochromators11.Spectral Purity..Spectral Purity.
22.Dispersion of Grating .Dispersion of Grating Monochromators.Monochromators.
33.Resolving Power of .Resolving Power of Monochromators.Monochromators.
44.light-Gathering power of .light-Gathering power of Monochromators.Monochromators.
The quality of a monochromator depends on The quality of a monochromator depends on the purity of its radiant output, its ability to the purity of its radiant output, its ability to resolve adjacent wavelengths, its light-resolve adjacent wavelengths, its light-gathering power, and its spectral gathering power, and its spectral bandwidth.bandwidth.
1. Spectral Purity1. Spectral Purity
The exit beam of a monochromator The exit beam of a monochromator is usually contaminated with is usually contaminated with small amounts of scattered or small amounts of scattered or stray radiation with wavelengths stray radiation with wavelengths far different from that of the far different from that of the instrument selling.instrument selling.
2. Dispersion of Grating2. Dispersion of GratingDispersion of Grating: Monochromators. The abilityDispersion of Grating: Monochromators. The ability
of a monochromator to separate different wavelengthsof a monochromator to separate different wavelengths
depends on its depends on its dispersion. The angular dispersion isdispersion. The angular dispersion is
given by given by drlddrld λ λ, where dr is the change in the angle of, where dr is the change in the angle of
reflection or refraction with a change in wavelengthreflection or refraction with a change in wavelength
ddλλ..
3. Resolving3. Resolving PowerPower of of Monochromators.Monochromators.
The resolving power R of a monochromator describes the limit of its ability to separate adjacent images that have a slight difference in wavelength.
R=λ/∆λ R=λ/∆λ =nN
4. light-Gathering power 4. light-Gathering power of Monochromatorsof Monochromators
To in crease the signal-to-noise ratio of a To in crease the signal-to-noise ratio of a spectrometer, it is necessary that the spectrometer, it is necessary that the radiant energy that reaches the detector be radiant energy that reaches the detector be as large as possible. The as large as possible. The number F. or number F. or speed speed provides a measure of the ability of provides a measure of the ability of a monochromator To collect the radiation a monochromator To collect the radiation that emerges from the entran slit. The that emerges from the entran slit. The f.number is defined by:f.number is defined by:
CC) ) Echelle Echelle MonochromatorsMonochromators
Echelle monochromatorsEchelle monochromators
contain two dispersing elements arranged in contain two dispersing elements arranged in series. The first of these elements is a special series. The first of these elements is a special type of grating called an type of grating called an echelle grating. The echelle grating. The second, which follows, is usually a second, which follows, is usually a low-low-dispersion prism, or sometimes a grating. The dispersion prism, or sometimes a grating. The echelle grating, which was first described by echelle grating, which was first described by G. R. Harrison in 1949, provides higher G. R. Harrison in 1949, provides higher dispersion and higher resolution than an dispersion and higher resolution than an echellette of the same sizeechellette of the same size..
Echelle grating: Light is reflected Echelle grating: Light is reflected off the short side of the blazes off the short side of the blazes
(grooves) in the grating.(grooves) in the grating.
`
2-D distribution of light and detectionusing an array of transducers (for later)
MONOCHROMATMONOCHROMATOR OR SLITSSLITS
7C-37C-3
Slits = hole in the wallSlits = hole in the wall Control the entrance of light into and out from the Control the entrance of light into and out from the
monochromator. They control quality!monochromator. They control quality!
EntranceEntrance slits control the slits control the intensityintensity of light entering the of light entering the monochromator and help control the range of monochromator and help control the range of wavelengths of light that strike the gratingwavelengths of light that strike the grating Less important than exit slitsLess important than exit slits
Exit Exit slights help select the slights help select the range of wavelengthsrange of wavelengths that that exit the monochromator and strike the detectorexit the monochromator and strike the detector More important than entrance slitsMore important than entrance slits
Can be:Can be: Fixed (just a slot)Fixed (just a slot) Adjustable in Adjustable in widthwidth (effective bandwidth and intensity) (effective bandwidth and intensity) Adjustable in Adjustable in heightheight (intensity of light) (intensity of light)
Monochromator SlitsMonochromator Slits
Good slitsGood slits Two pieces of metal to give sharp Two pieces of metal to give sharp
edgesedges Parallel to one anotherParallel to one another Spacing can be adjusted in some Spacing can be adjusted in some
modelsmodels Entrance slitEntrance slit
Serves as a radiation sourceServes as a radiation source Focusing on the slit planeFocusing on the slit plane
Effect of Slit Width on Effect of Slit Width on ResolutionResolution
Effect of slit width on Effect of slit width on resolutionresolution
BandwidthBandwidth Defined as a span of Defined as a span of
monochromator monochromator setting setting
needed to move the needed to move the image of the image of the entrance slit across entrance slit across the exit slitthe exit slit
Effective bandwidthEffective bandwidth effeff
½½ of the bandwidth of the bandwidth When two slits are When two slits are
identicalidenticalFIGURE 7-24 Illumination of an exit slit by monochromatic radiation λ at various monochromator settings. Exit and entrance slits are identical.
Calculating slit widthCalculating slit width Effective bandwidth(Effective bandwidth(effeff) and D) and D-1-1
DD-1-1 = = yy When When y = w = (slit width)y = w = (slit width) DD-1-1 = = eff eff /w/w
ExampleExample Recpiprocal linear dispersion = 1.2nm/mmRecpiprocal linear dispersion = 1.2nm/mm Sodium lines at 589.0 nm and 589.6 nmSodium lines at 589.0 nm and 589.6 nm Required slit width?Required slit width? eff eff = = ½½ (589.6-589.0) = 0.3 nm (589.6-589.0) = 0.3 nm W = 0.3 nm/(1.2 nm/mm) = 0.25 mmW = 0.3 nm/(1.2 nm/mm) = 0.25 mm Practically, narrower than the theoretical Practically, narrower than the theoretical
values is necessary to achieve a desired values is necessary to achieve a desired resolutionresolution
Wider slits =
greater
intensity,
More signal trading off
withPoorer resolution
Narrow
er slits =
lower intensity,
Less signal trading
off with
Better resolutionFIGURE 7-25 FIGURE 7-25 The effect of the slit width on spectra. The entrance slit is The effect of the slit width on spectra. The entrance slit is
illuminated with A" A"illuminated with A" A"and A 3 only. Entrance and exit slits are identical. Plots on the right show changes in and A 3 only. Entrance and exit slits are identical. Plots on the right show changes in
emittedemittedpower as the setting of monochromator is varied.power as the setting of monochromator is varied.
Wider slits = greater intensity,
Poorer resolution
Narrower slits = lower intensity,
Better resolution
Choice of slit widthsChoice of slit widths
Variable slits for Variable slits for effective bandwidtheffective bandwidth
Narrow spectrumNarrow spectrum Minimal slit widthMinimal slit width Bet decrease in the Bet decrease in the
radiant powerradiant power Quantitative Quantitative
analysisanalysis Wider slit width Wider slit width for for ““moremore”” radiant radiant
powerpower
Effect of Effect of bandwidthbandwidth on on spectral detail for spectral detail for
benzene vaporbenzene vapor
SAMPLE HOLDERS SAMPLE HOLDERS (CELLS)(CELLS)
7D7D
Sample Holders (Cells)Sample Holders (Cells) Must:Must:
contain the sample without chemical interactioncontain the sample without chemical interaction be be more-or-lessmore-or-less transparent to the wavelengths of light in use transparent to the wavelengths of light in use be readily cleaned for reusebe readily cleaned for reuse be designed for the specific instrument of interest….be designed for the specific instrument of interest….
ExamplesExamples quartz is good from about 190-3000 nmquartz is good from about 190-3000 nm glass is a less expensive alternative from about glass is a less expensive alternative from about 300300-900 nm-900 nm NaCl and KBr are good to much higher wavelengths (NaCl and KBr are good to much higher wavelengths (IR rangeIR range))
Cells can be constructed to:Cells can be constructed to: transmit light absorbed at 180 degrees to the incident lighttransmit light absorbed at 180 degrees to the incident light allow emitted light to exit at 90 degrees from the incident lightallow emitted light to exit at 90 degrees from the incident light contain gases (lower concentrations) and have long path contain gases (lower concentrations) and have long path
lengths (1.0 and 10.0 cm cells are most common)lengths (1.0 and 10.0 cm cells are most common)
Sample ContainersSample Containers
The cells or cuvettes that hold the The cells or cuvettes that hold the samples must be made of material that samples must be made of material that is transparent to radiation in the is transparent to radiation in the spectral region of interest. Quartz or spectral region of interest. Quartz or fused silica is required for work in the fused silica is required for work in the ultraviolet region (below 350 nm), both ultraviolet region (below 350 nm), both of these substances are transparent in of these substances are transparent in the visible region. Silicate glasses can the visible region. Silicate glasses can be employed in the region between 350 be employed in the region between 350 and 2000 nm. Plastic containers can be and 2000 nm. Plastic containers can be used in the visible region. Crystalline used in the visible region. Crystalline NaCl is the most common cell windows NaCl is the most common cell windows in the i.r region. in the i.r region.
Absorbance: usually in a matched pair!
Fluorescence, Phosphorescence, Chemiluminescence
Different Shapes and Different Shapes and Sizes of CellsSizes of Cells
RADIATIONRADIATION TRANSDUCERSTRANSDUCERS
7 E7 E
RADIATION RADIATION TRANSDUCERSTRANSDUCERS
7 E-17 E-1
Radiation TransducersRadiation TransducersIntroductionIntroduction
The detectors for early The detectors for early
spectroscopic instruments were spectroscopic instruments were
the human eye or a the human eye or a
photographic plate or film. Now photographic plate or film. Now
a days more modern detectors a days more modern detectors
are in use that convert radiant are in use that convert radiant
energy into electrical signal.energy into electrical signal.
properties of the Ideal properties of the Ideal TransducerTransducer
The ideal transducer would have a high sensitivity, The ideal transducer would have a high sensitivity, a high signal-to-noise ratio, and a constant a high signal-to-noise ratio, and a constant response over a considerable range of response over a considerable range of wavelengths. In addition, it would exhibit a fast wavelengths. In addition, it would exhibit a fast response time and a zero output signal in the response time and a zero output signal in the absence of illumination, Finally, the electrical absence of illumination, Finally, the electrical signal produced by the ideal transducer would be signal produced by the ideal transducer would be directly proportional to the radiant power directly proportional to the radiant power P.P.
Types of Radiation TransducersTypes of Radiation Transducers
As indicated in Figure 7-3b, there arc two generalAs indicated in Figure 7-3b, there arc two general
types of radiation transducers.2o One type responds totypes of radiation transducers.2o One type responds to
photons, the other to heat. All photon transducers (alsophotons, the other to heat. All photon transducers (also
called called photoelectric or quantum detectors) have anphotoelectric or quantum detectors) have an
active surface that absorbs radiation. [n some types, the absorbed active surface that absorbs radiation. [n some types, the absorbed energy causes emission of electrons andenergy causes emission of electrons and
the production of a photocurrent. In others, the radiationthe production of a photocurrent. In others, the radiation
promotes electrons into conduction bands: detectionpromotes electrons into conduction bands: detection
here is based on the resulting enhanced conductivityhere is based on the resulting enhanced conductivity
(photo conduction), Photon transducers are used(photo conduction), Photon transducers are used
largely for measurement of UV, visible, and near infraredlargely for measurement of UV, visible, and near infrared
radiation.radiation.
the relative spectral response ofthe relative spectral response ofthe various kinds of transducers that are the various kinds of transducers that are
useful for UV,useful for UV,visible, and IR spectroscopy.visible, and IR spectroscopy.
PHOTON PHOTON TRANSDUCERSTRANSDUCERS
7E-27E-2
photon transducersphoton transducers
Several types of photon transducers are availableSeveral types of photon transducers are available, including , including (I) (I) photovoltaic cells, in which the radiant energy generates a photovoltaic cells, in which the radiant energy generates a
current at the interface of a semiconductor layer and a metal; current at the interface of a semiconductor layer and a metal; (2) (2) phototubes, in which radiation causes emission of phototubes, in which radiation causes emission of
electrons from a photosensitive solid surface;electrons from a photosensitive solid surface; (3) (3) photomultiplier tubes, which contain a photoemissive photomultiplier tubes, which contain a photoemissive
surface as well as several additional surfaces that emit a cascade of electrons surface as well as several additional surfaces that emit a cascade of electrons when struck by electrons from the photosensitive area;when struck by electrons from the photosensitive area;
(4) (4) photoconductivity transducers in which absorption of radiation by a photoconductivity transducers in which absorption of radiation by a semiconductor produces electrons and holes, thus leading to enhanced semiconductor produces electrons and holes, thus leading to enhanced conductivity;conductivity;
(5) (5) silicon photodiodes. in which photons cause the formation silicon photodiodes. in which photons cause the formation ofelectron-hole pairs and a current across a reversebiasedofelectron-hole pairs and a current across a reversebiased
pn junction; and pn junction; and `̀(6) (6) charge-transfer transducers, charge-transfer transducers, in which the charges developed in a in which the charges developed in a
silicon crystal as a result of absorption of photons are collected and measured.silicon crystal as a result of absorption of photons are collected and measured.
a) Photovolatic cella) Photovolatic cell StructureStructure
metal-semiconductor-metal-semiconductor-metal sandwichesmetal sandwiches
produce voltage when produce voltage when irradiatedirradiated 350-750 nm350-750 nm 550 nm maximum 550 nm maximum
responseresponse 10-100 microA10-100 microA
Barrier-layer cellBarrier-layer cell Low-priceLow-price Amplification difficultyAmplification difficulty Low sensensitivity for Low sensensitivity for
weak radiationweak radiation Fatigue effectFatigue effect
b) Vacuum Phototubeb) Vacuum Phototube StructureStructure
Wire anode and semi Wire anode and semi cylinder cathode in a cylinder cathode in a vacuum tubevacuum tube
Photosensitive materialPhotosensitive material electrons produced by electrons produced by
irradiation of cathode irradiation of cathode travel to anode. travel to anode.
l response depends on l response depends on cathode material (200-cathode material (200-1000 nm)1000 nm) High sensitivityHigh sensitivity Red responseRed response UV responseUV response Flat responseFlat response FIGURE 7-29 A phototube and op amp
readout. Thephotocurrent induced by the radiation causes a voltagedrop across R, which appears as "0 at the output of thecurrent-to-voltage converter. This voltage may be displayedon a meter or acquired by a data-acquisitionsystem.
What do we want in a transducer?What do we want in a transducer? High sensitivityHigh sensitivity High S/NHigh S/N Constant response over many Constant response over many s s
(wide range of wavelength)(wide range of wavelength) Fast response timeFast response time S = 0 if no light presentS = 0 if no light present S S P (where P = radiant power) P (where P = radiant power)
Photon transducers: light Photon transducers: light electrical electrical signalsignal
Thermal transducers: response to Thermal transducers: response to heat heat conduction bands (enhance conduction bands (enhance conductivity)conductivity)
c) Photomultiplier Tube c) Photomultiplier Tube (PMT)(PMT)
Extremely sensitive (use for low light Extremely sensitive (use for low light applications).applications).
Light strikes photocathode (photons strike Light strikes photocathode (photons strike emits electrons); several electrons per photon.emits electrons); several electrons per photon.
Bias voltage applied (several hundred volts) Bias voltage applied (several hundred volts) electrons form current.electrons form current.
Electrons emitted towards a dynode (90 V more Electrons emitted towards a dynode (90 V more positive than photocathode positive than photocathode electrons electrons attracted to it).attracted to it).
Electrons hit dynode Electrons hit dynode each electron causes each electron causes emission of several electrons.emission of several electrons.
These electrons are accelerated towards dynode These electrons are accelerated towards dynode #2 (90 V more positive than dynode # 1) …etc.#2 (90 V more positive than dynode # 1) …etc.
d) Photomultiplier tubes (found in d) Photomultiplier tubes (found in more advanced, scanning UV-VIS and more advanced, scanning UV-VIS and
spectroscopic instruments)spectroscopic instruments) Also function based on the photoelectric effectAlso function based on the photoelectric effect
Additional signal is gained by multiplying the number of electrons produced by the initial reaction in the detector.Additional signal is gained by multiplying the number of electrons produced by the initial reaction in the detector.
Each electron produces as series of photo-electrons, multiplying its signal. Thus the name PMT!Each electron produces as series of photo-electrons, multiplying its signal. Thus the name PMT!
Very sensitive to incoming light.Very sensitive to incoming light. Most sensitive light detector in the UV-VIS range.Most sensitive light detector in the UV-VIS range. VERY rugged. They last a long time.VERY rugged. They last a long time. Sensitive to excessive stray light (room light + powered PMT = DEAD PMT)Sensitive to excessive stray light (room light + powered PMT = DEAD PMT)
Always used with a scanning or moveable wavelength selector (grating) in a monochromatorAlways used with a scanning or moveable wavelength selector (grating) in a monochromator
FIGURE7-31 Photomultiplier tube: (a), photograph of a typical commercial tube; (b), cross:FIGURE7-31 Photomultiplier tube: (a), photograph of a typical commercial tube; (b), cross:sectional view; (c), electrical diagram illustrating dynode polanzatlon and photocurrent measectional view; (c), electrical diagram illustrating dynode polanzatlon and photocurrent mea
surement. Radiation striking the photosensitive cathode (b) gives nse to photoelectrons by thesurement. Radiation striking the photosensitive cathode (b) gives nse to photoelectrons by thehotoelectric effect. Dynode D1 is held at a positive voltage Withrespect to the photocathode.hotoelectric effect. Dynode D1 is held at a positive voltage Withrespect to the photocathode.
~Iectrons emitted by the cathode are attracted to the first dynode and accelerated In the fteld.~Iectrons emitted by the cathode are attracted to the first dynode and accelerated In the fteld.Each electron striking dynode D1 thus gives rise to two to four secondary electrons. TheseEach electron striking dynode D1 thus gives rise to two to four secondary electrons. These
are attracted to dynode D2, which is again positive with respect to dynode D1. The resultingare attracted to dynode D2, which is again positive with respect to dynode D1. The resultingamplification at the anode can be 106 or greater. The exact amplification factor depends onamplification at the anode can be 106 or greater. The exact amplification factor depends on
the number of dynodes and the voltage difference between each. ThiSautomatic Internalthe number of dynodes and the voltage difference between each. ThiSautomatic Internalamplification is one of the major advantages of photomultiplier tubes. With modern Instrumentation,amplification is one of the major advantages of photomultiplier tubes. With modern Instrumentation,
the arrival of individual photocurrent pulses can be detected and counted Insteadthe arrival of individual photocurrent pulses can be detected and counted Insteadof being measured as an average current. This technique, called photon counting, ISof being measured as an average current. This technique, called photon counting, IS
advantageous at very low light levels.advantageous at very low light levels.
Douglas A. Skoog and James J. Leary, Principles of Instrumental Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.Analysis, Saunders College Publishing, Fort Worth, 1992.
8–19 dynodes (9-10 is 8–19 dynodes (9-10 is most common).most common).
Gain (m) is # eGain (m) is # e-- emitted emitted per incident eper incident e-- ( () to the ) to the power of the # of power of the # of dynodes (k).dynodes (k).
m = m = kk
e.g. 5 ee.g. 5 e-- emitted / incident e emitted / incident e--
10 dynodes.10 dynodes.
m = m = kk = 5 = 51010 1 x 10 1 x 1077
Typical Gain = 10Typical Gain = 1044 - 10 - 1077
e) Silicon Diodese) Silicon Diodes
Constructed of charge depleted and charge rich Constructed of charge depleted and charge rich regions of silicon (silicon doped with other ions)regions of silicon (silicon doped with other ions)
Light striking the detector causes charge to be Light striking the detector causes charge to be created between the p and n regions.created between the p and n regions.
The charge collected is then measured as current The charge collected is then measured as current and the array is ‘reset’ for the next collectionand the array is ‘reset’ for the next collection
Used most frequently these days in instruments Used most frequently these days in instruments where the grating is fixed in one position and light where the grating is fixed in one position and light strikes an array of silicon diodes (aka the diode strikes an array of silicon diodes (aka the diode arrayarray Can have thousands of diodes on an arrayCan have thousands of diodes on an array Each diode collects light from a specific wavelength Each diode collects light from a specific wavelength
rangerange The resolution is generally poorer than with a PMTThe resolution is generally poorer than with a PMT However, you can scan literally thousands of times a However, you can scan literally thousands of times a
minute since there are NO moving parts! Then, the minute since there are NO moving parts! Then, the scans are averaged (ensemble or boxcar averaging) to scans are averaged (ensemble or boxcar averaging) to give a resulting spectra.give a resulting spectra.
PhotodiodesPhotodiodes
Douglas A. Skoog and James J. Leary, Principles of Instrumental Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.Analysis, Saunders College Publishing, Fort Worth, 1992.
the relative spectral response ofthe relative spectral response ofthe various kinds of transducers that are the various kinds of transducers that are
useful for UV,useful for UV,visible, and IR spectroscopy.visible, and IR spectroscopy.
Skoog et al. 2007
Forward biasing Reverse biasing
High resistant
e-
MULTICHANNEL MULTICHANNEL PHOTON PHOTON TRANSDUCERSTRANSDUCERS
7E-37E-3
Multichannel photon Multichannel photon transducerstransducers
The first multichannel detector used in spectroscopy was a The first multichannel detector used in spectroscopy was a photographic plate or a film strip that was placed along the photographic plate or a film strip that was placed along the length of the focal plane of a spectrometer so that all the length of the focal plane of a spectrometer so that all the lines in a spectrum could be recorded simultaneously. lines in a spectrum could be recorded simultaneously. Photographic detection is relatively sensitive, with some Photographic detection is relatively sensitive, with some emulsions that respond to as few as 10 to 100 photons. The emulsions that respond to as few as 10 to 100 photons. The primary limitation of this type of detector, however, is the primary limitation of this type of detector, however, is the time required to develop the image of the spectrum and time required to develop the image of the spectrum and convert the blackening of the emulsion to radiant convert the blackening of the emulsion to radiant intensities. Modern multichannel transducers 24 consist of intensities. Modern multichannel transducers 24 consist of an array of an array of small photosensitive elements arranged either small photosensitive elements arranged either linearly or in a two-dimensional pattern on a single linearly or in a two-dimensional pattern on a single semiconductor chip.semiconductor chip.
Multichannel Photon TransducersMultichannel Photon Transducers
Photographic plate or a film stripPhotographic plate or a film stripPlace along the focal plane of a spectrometerPlace along the focal plane of a spectrometer
Photodiode ArraysPhotodiode Arrays
Photodiode ArraysPhotodiode Arrays
In a PDA, the individual photosensitive In a PDA, the individual photosensitive elements are small silicon photodiodes, elements are small silicon photodiodes, each of which consists of a reverse-biased each of which consists of a reverse-biased pn junctionpn junction
Photodiode TransducerPhotodiode Transducer A silicon photodiode transducer A silicon photodiode transducer
consists of a consists of a Reversed Biased Reversed Biased pnpn junctionjunction formed on a silicon chip formed on a silicon chip
A photon promotes an electron from A photon promotes an electron from the valence bthe valence boond (filled orbitals) to the nd (filled orbitals) to the conduction bconduction boond (unfilled orbitals) nd (unfilled orbitals) creating an electron(-) - hole(+) paircreating an electron(-) - hole(+) pair
The concentration of these electron-The concentration of these electron-hole pairs is dependent on the amount hole pairs is dependent on the amount of light striking the semiconductorof light striking the semiconductor
Photodiode ArrayPhotodiode Array
Semiconductors (Silicon and Semiconductors (Silicon and Germanium)Germanium) Group IV elementsGroup IV elements Formation of holes (via thermal Formation of holes (via thermal
agitation/excitation)agitation/excitation) DopingDoping n-type:n-type: Si (or Ge) doped with group V Si (or Ge) doped with group V
element (As, Sb) to add electrons.element (As, Sb) to add electrons.As:As: [Ar]4S[Ar]4S223d3d10104p4p33
p-type:p-type: Doped with group III element (In, Doped with group III element (In, Ga) to added holesGa) to added holesIn: In: [Kr]5S[Kr]5S224d4d10105p5p11
Skoog et al, p43
FIGURE 7-33 FIGURE 7-33 A reverse-biased linear diode-arrayA reverse-biased linear diode-arraydetector: (a)cross section and (b)top view.detector: (a)cross section and (b)top view.
Photodiode Arrays
Charge-Transfer Charge-Transfer DeviceDevice
Charge-Transfer Device Charge-Transfer Device (CTD)(CTD)
Important for multichannel detection Important for multichannel detection (i.e., spatial resolution); 2-dimensional (i.e., spatial resolution); 2-dimensional arrays.arrays.
Sensitivity approaches PMT.Sensitivity approaches PMT. An entire spectrum can be recorded as An entire spectrum can be recorded as
a “snapshot” without scanning.a “snapshot” without scanning. Integrate signal as photon strikes Integrate signal as photon strikes
element. element. Each pixel: two conductive electrodes Each pixel: two conductive electrodes
over an insulating material (e.g., SiOover an insulating material (e.g., SiO22).). Insulator separates electrodes from n-Insulator separates electrodes from n-
doped silicon. doped silicon.
Semiconductor capacitor: stores Semiconductor capacitor: stores charges that are formed when charges that are formed when photons strike the doped silicon. photons strike the doped silicon.
101055 –10 –1066 charges/pixel can be stored charges/pixel can be stored (gain approaches gain of PMT).(gain approaches gain of PMT).
How is amount of charge measured?How is amount of charge measured? Charge-injection device (CID): Charge-injection device (CID):
voltage change that occurs from voltage change that occurs from charge moving between electrodes.charge moving between electrodes.
Charge-coupled device (CCD): Charge-coupled device (CCD): charge is moved to amplifier.charge is moved to amplifier.
PHOTO CONDUCTIVITY PHOTO CONDUCTIVITY TRANSDUCERSTRANSDUCERS
7E-47E-4
Photo conductivity Photo conductivity TransducersTransducers
The most sensitive transducers for The most sensitive transducers for monitoring radiation 10 the near-infrared monitoring radiation 10 the near-infrared region (0.75 to 3 /µm) are semiconductors region (0.75 to 3 /µm) are semiconductors whose resistances decrease when theywhose resistances decrease when they
absorb radiation within this range.absorb radiation within this range.
THERMAL THERMAL TRANSDUCERSTRANSDUCERS
7E-57E-5
Thermal TransducersThermal Transducers
Thermal Transducers are used in Thermal Transducers are used in
infrared spectroscopy. infrared spectroscopy.
Phototransducers are not Phototransducers are not
applicable in infrared because applicable in infrared because
photons in this region lack the photons in this region lack the
energy to cause photoemission of energy to cause photoemission of
electrons. Thermal transducers are electrons. Thermal transducers are
– Thermocouples, Bolometer – Thermocouples, Bolometer
(thermistor(thermistor).).
ThermocouplesThermocouples
In its simplest form, a thermocouple consists of a pairIn its simplest form, a thermocouple consists of a pair
of junctions formed when two pieces of a metal such asof junctions formed when two pieces of a metal such as
copper are fused to each end of a dissimilar metal suchcopper are fused to each end of a dissimilar metal such
as constantan as shown in Figure 3-13. A voltage developsas constantan as shown in Figure 3-13. A voltage develops
between the two junctions that varies with thebetween the two junctions that varies with the
difference in their temperatures. difference in their temperatures. A well- A well-designed thermocouple transducer is capabledesigned thermocouple transducer is capable
of responding to temperature differences ofof responding to temperature differences of
10-6 K. This difference corresponds to a potential difference10-6 K. This difference corresponds to a potential difference
of about 6 to 8 of about 6 to 8 µV/µW.µV/µW.
ThermocouplesThermocouples
bolometerbolometer
A bolometer is a type of resistance A bolometer is a type of resistance thermometer constructed of strips of thermometer constructed of strips of metals, such as platinum or nickel, or of a metals, such as platinum or nickel, or of a semiconductor. Semiconductor bolometers semiconductor. Semiconductor bolometers are often called are often called thermistors .thermistors .
Pyroelectric transducersPyroelectric transducers
Pyroelectric transducers are constructed from Pyroelectric transducers are constructed from single crystalline wafers of single crystalline wafers of pyroelectric pyroelectric materials, which are materials, which are insulators (dielectric insulators (dielectric materials) with very special thermal and materials) with very special thermal and electrical properties. Triglycine sulfate electrical properties. Triglycine sulfate (NH(NH22CHCH22COOH)COOH)33· H· H22SOSO44 (usually deuterated or(usually deuterated or
with a fraction of the glycines replaced with with a fraction of the glycines replaced with alanine), is the most alanine), is the most important pyroelectric important pyroelectric materialmaterial used in the construction of infrared used in the construction of infrared transducers.transducers.
SIGNAL SIGNAL PROCESSORSPROCESSORS
AND AND READOUTSREADOUTS
7F7F
Signal Processors and Signal Processors and ReadoutsReadouts
The signal processor is ordinarily an The signal processor is ordinarily an electronic device that amplifies the electronic device that amplifies the electrical signal from the transducer. In electrical signal from the transducer. In addition, it may alter the signal from dc addition, it may alter the signal from dc to ac (or the reverse), change the phase to ac (or the reverse), change the phase of the signal, and filter it to remove of the signal, and filter it to remove unwanted components. Furthermore, the unwanted components. Furthermore, the signal processor may be called upon to signal processor may be called upon to perform such mathematical operations perform such mathematical operations on the signal as differentiation, on the signal as differentiation, integration, or conversion to a integration, or conversion to a logarithm.logarithm.
PHOTON COUNTINGPHOTON COUNTING7F-17F-1
Photon countingPhoton counting
The output from a photomultiplier tube consists ofThe output from a photomultiplier tube consists of
a pulse of electrons for each photon that reaches the a pulse of electrons for each photon that reaches the detectordetector
surface. This analog signal is often filtered to removesurface. This analog signal is often filtered to remove
undesirable fluctuations due to the random appearanceundesirable fluctuations due to the random appearance
of photons at the photocathode and measuredof photons at the photocathode and measured as a de as a de voltage or eurrent.voltage or eurrent.
FIBER OPTICSFIBER OPTICS
7G7G
Fiber opticsFiber optics
In the late 1960, analytical instruments began to In the late 1960, analytical instruments began to appear on the market that contained fiber optics appear on the market that contained fiber optics for transmiting radiation and images from one for transmiting radiation and images from one component of the instrument to another. Fiber component of the instrument to another. Fiber optics have added a new dimension of utility to optics have added a new dimension of utility to optical instrument designs."optical instrument designs."
Optical FibersOptical Fibers
Used to transmit light waves over non-Used to transmit light waves over non-linear paths.linear paths.
Often used in remote sensing, solution Often used in remote sensing, solution sampling (dipping probes) and field sampling (dipping probes) and field instrumentsinstruments
Based on the fact that light inside a fiber Based on the fact that light inside a fiber can be continuously (totally internally can be continuously (totally internally reflected) if the angle it strikes the fiber reflected) if the angle it strikes the fiber surface at is correct (determines radius of surface at is correct (determines radius of bends, etc.).bends, etc.).
Used in construction of optodes (optical Used in construction of optodes (optical fiber based chemical sensor)fiber based chemical sensor)
PROPERTIES PROPERTIES OFOF OPTICAL OPTICAL
FIBERSFIBERS
7G-17G-1
Properties of Optical Properties of Optical FibersFibers
Optical fibers are fine strands of glass or plastic thatOptical fibers are fine strands of glass or plastic that
transmit radiation for distances of several hundred feettransmit radiation for distances of several hundred feet
or more. The diameter of optical fibers ranges fromor more. The diameter of optical fibers ranges from
0.05 pm to as large as 0.6 cm. Where images are to be0.05 pm to as large as 0.6 cm. Where images are to be
transmitted, bundles of fibers, fused at the ends, aretransmitted, bundles of fibers, fused at the ends, are
used. A major application of these fiber bundles hasused. A major application of these fiber bundles has
been in medical diagnoses, where their flexibility permitsbeen in medical diagnoses, where their flexibility permits
transmission of images of organs through tortuoustransmission of images of organs through tortuous
pathways to the physician. Fiber optics are usedpathways to the physician. Fiber optics are used
not only for observation but also for illumination ofnot only for observation but also for illumination of
objects. In such applications, the ability to illuminateobjects. In such applications, the ability to illuminate
without heating is often very important.without heating is often very important.
Optical FiberOptical Fiber
FIBER-OPTIC SENSORSFIBER-OPTIC SENSORS
7G.27G.2
Fiber-optic sensorsFiber-optic sensors
Fiber-optic sensors, which are sometimes called Fiber-optic sensors, which are sometimes called optrodes, optrodes, consist of a reagent phase immobilized consist of a reagent phase immobilized on the end of a fiber optic. Interaction of the on the end of a fiber optic. Interaction of the analyte with the reagent creates a change in analyte with the reagent creates a change in absorbance, reflectance, fluorescence, or absorbance, reflectance, fluorescence, or luminescence, which is then transmitted to a luminescence, which is then transmitted to a detector via the optical fiber. Fiber optic Sensors detector via the optical fiber. Fiber optic Sensors are generally simple, inexpensive devices that are generally simple, inexpensive devices that are easily miniaturized.are easily miniaturized.
TYPES OF TYPES OF OPTICAL OPTICAL INSTRUMENTINSTRUMENT
7H7H
Types of Optical Types of Optical InstrumentsInstruments
SpectroscopeSpectroscope Optical instrument used for visual identification of Optical instrument used for visual identification of
atomic emission linesatomic emission lines ColorimeterColorimeter
Human eye acts as detector for absorption Human eye acts as detector for absorption measurementsmeasurements
PhotometerPhotometer Contains a filter, no scanning functionContains a filter, no scanning function
FluorometerFluorometer A photometer for fluorescence measurementA photometer for fluorescence measurement
SpectrographSpectrograph Record simultaneously the entire spectrum of a Record simultaneously the entire spectrum of a
dispersed radiation using plate or filmdispersed radiation using plate or film SpectrometerSpectrometer
Provides information about the intensity of Provides information about the intensity of radiaition as a function of wavelength or frequencyradiaition as a function of wavelength or frequency
More……………(confusing……)More……………(confusing……)
types of Optical instrumenttypes of Optical instrument
Spectroscope:Spectroscope:an optical instrument used for thean optical instrument used for the
visual identification of atomic emission lines.visual identification of atomic emission lines. We use the We use the term term colorimeter: colorimeter: to designate an instrument to designate an instrument for absorption for absorption measurements in which the human eye serves as the detector measurements in which the human eye serves as the detector using one or more color-comparison standards. using one or more color-comparison standards. spectro graph: spectro graph: is similar in construction to the two is similar in construction to the two monochromators shown monochromators shown in Figure 7-18 except that the sht arrangement is replaced with in Figure 7-18 except that the sht arrangement is replaced with a large aperture that holds a detector or transducer that is a large aperture that holds a detector or transducer that is continuously exposed tn the entire spectrum of dispersed continuously exposed tn the entire spectrum of dispersed radiation. radiation. spectrometerspectrometer :is an instrument that provides :is an instrument that provides informationinformation
about the intensity of radiation as a function of wavelength or about the intensity of radiation as a function of wavelength or frequency.frequency.
PRINCIPLES OF PRINCIPLES OF FOURIERFOURIERTRANSFORM OPTICALTRANSFORM OPTICALMEASUREMENTSMEASUREMENTS
7I7I
Fourier Transform (FT)Fourier Transform (FT) The instruments we have been talking about work over the The instruments we have been talking about work over the
frequency domain (we are measuring signal vs. frequency or frequency domain (we are measuring signal vs. frequency or wavelength)wavelength)
Fourier transform techniques measure signal vs. time and Fourier transform techniques measure signal vs. time and then convert time to wavelength or frequencythen convert time to wavelength or frequency
FT techniques have much greater resolving power than FT techniques have much greater resolving power than frequency domain techniquesfrequency domain techniques Fewer mechanical partsFewer mechanical parts No “monochromator”No “monochromator” Mathematical deconvolution of the spectrumMathematical deconvolution of the spectrum
FT techniques have higher light throughput because there FT techniques have higher light throughput because there are fewer optical components.are fewer optical components.
Widely used in IR and NMRWidely used in IR and NMR Originally developed to separate out weak IR signals from Originally developed to separate out weak IR signals from
astronomical objects.astronomical objects. An interferometer splits the light beam into two beams and An interferometer splits the light beam into two beams and
then measures the intensity of recombined beamsthen measures the intensity of recombined beams The frequency of these beams is related to the frequency The frequency of these beams is related to the frequency
of the light that caused them…. of the light that caused them….
HistoryHistory In 1950s, astronomyIn 1950s, astronomy
Separate weak signals from noiseSeparate weak signals from noise Late 1960s, FT-NIR & FT-IRLate 1960s, FT-NIR & FT-IR
Fourier transformFourier transform
Resolution of FT Resolution of FT spectrometerspectrometer
Two closely spaced lines only separated Two closely spaced lines only separated if one complete "beat" is recorded. if one complete "beat" is recorded.
As lines get closer together, As lines get closer together, must must increase.increase.
INHERENT ADVANTAGES OF INHERENT ADVANTAGES OF FOURIERFOURIER
TRANSFORM TRANSFORM SPECTROMETRYSPECTROMETRY
7I-17I-1
Advantages of FTAdvantages of FT Throughput / Jaquinot advantageThroughput / Jaquinot advantage
Few optics and slitsFew optics and slits Less dispersion, high intensityLess dispersion, high intensity Usually to improve resolution decrease slit width Usually to improve resolution decrease slit width but less light makes spectrum "noisier" (S/N)but less light makes spectrum "noisier" (S/N)
High ResolutionHigh Resolution = 6 ppm= 6 ppm
Short time scaleShort time scale Simultaneously measure all spectrum at once Simultaneously measure all spectrum at once
saves timesaves time frequency scanning vs. time domain scanningfrequency scanning vs. time domain scanning Fellgett or multiplex advantageFellgett or multiplex advantage
TIME -DOMAIN TIME -DOMAIN SPECTROSCOPYSPECTROSCOPY
7I-27I-2
time -domain time -domain spectroscopyspectroscopy
Conventional spectroscopy can be termed Conventional spectroscopy can be termed frequency domain frequency domain spectroscopy in that spectroscopy in that radiant power data are recorded as a radiant power data are recorded as a function of frequency or the inversely function of frequency or the inversely related wavelength. In contrast, related wavelength. In contrast, time-time-domain spectroscopy, domain spectroscopy, which can be which can be achieved by the Fourier transform, is achieved by the Fourier transform, is concerned with changes in radiant power concerned with changes in radiant power with with time.time.
Time domain Time domain spectroscopyspectroscopy
Unfortunately, no detector can Unfortunately, no detector can respond on 10respond on 10-14-14 s time scale s time scale
Use Michelson interferometerUse Michelson interferometer to to measure signal proportional to measure signal proportional to time varying signaltime varying signal
Freq-domain / time-domainFreq-domain / time-domain
ACQUIRING TIME-DOMAIN ACQUIRING TIME-DOMAIN
SPECTRA WITH A SPECTRA WITH A
MICHELSONMICHELSON INTERFEROMETERINTERFEROMETER
7I-37I-3
modulationmodulation
Velocity of moving Velocity of moving mirror(MM)mirror(MM)
Time to move Time to move /2 /2 cmcm
Bolometer, Bolometer, pyroelectric, pyroelectric, photoconducting photoconducting IR detectors can IR detectors can "see"see““ changes on changes on 1010-4-4 s time scale! s time scale!
This time domain spectrum is made of different wavelengths of light arriving at the detector at different times.
Michelson interferometerMichelson interferometer
Analysis of Analysis of interferograminterferogram
Computer needed to Computer needed to turn complex turn complex interferogram into interferogram into spectrumspectrum
Figure 7-43Figure 7-43 (b) resolved lines(b) resolved lines (c) unresolved lines(c) unresolved lines
FTFT Time -> Frequency Time -> Frequency
inverse FTinverse FT Frequency -> TimeFrequency -> Time
Fourier Transformation of Fourier Transformation of InterferogramsInterferograms
InterferogramInterferogram
retardation retardation Difference in pathlengthDifference in pathlength
interferograminterferogram Plot signal vs. Plot signal vs. cosine wave with frequency cosine wave with frequency
proportional to light frequency but proportional to light frequency but signal varies at much lower signal varies at much lower frequencyfrequency
One full cycle when mirror moves One full cycle when mirror moves distance distance /2 (round-trip = /2 (round-trip = ))
resolutionresolution
resolutionresolution
The resolution of a Fourier transform The resolution of a Fourier transform spectrometer can be described in terms of spectrometer can be described in terms of the difference in wavenumber between the difference in wavenumber between two lines that can be just separated by the two lines that can be just separated by the instrument. That is,instrument. That is,
Semiconductor Diodes Semiconductor Diodes Diode: is a nonlinear device that has Diode: is a nonlinear device that has
greater conductance in one direction greater conductance in one direction than in anotherthan in another
Adjacent n-type and p-type regionsAdjacent n-type and p-type regions pn junction: the interface between pn junction: the interface between
the two regions the two regions
This process continues for 9 dynodesThis process continues for 9 dynodes Result: for each photon that strikes Result: for each photon that strikes
photocathode photocathode ~10 ~1066 –10 –1077 electrons electrons collected at anode.collected at anode.
Is there a drawback? Sensitivity Is there a drawback? Sensitivity usually limited by dark current.usually limited by dark current.
Dark current = current generated by Dark current = current generated by thermal emission of electrons in the thermal emission of electrons in the absence of light.absence of light.
Thermal emission Thermal emission reduce by cooling. reduce by cooling. Under optimal conditions, PMTs can Under optimal conditions, PMTs can
detect single photons.detect single photons. Only used for low-light applications; it Only used for low-light applications; it
is possible to fry the photocathode.is possible to fry the photocathode.