Overview of Spectroscopy - staff.guilan.ac.ir · Originally performed by Young (1801) with light. Subsequently also performed with many types of matter particle (see references)

University of Guilan 12/15/2018

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Overview of Spectroscopy

A. Definition: Interaction of EM Radiation with Matter

We see objects because they remit some part of the light

falling on them from a source.We function as reflection/

transmission spectrometers.

Normally, we see only reflection and transmission of light, but there are other types of interactions.

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Wave Parameters:Period (p) – the time required for the passage of

successive maxima through a fixed point in space.Frequency (v) – the number of oscillations of the field

that occur per second. Equal to 1/p. Determinedby source and remains invariant regardless ofmedia traversed.

Velocity (vi) – the rate at which a wave front movesthrough a medium. Dependent on composition ofmedium and frequency.

Wavelength (λi) – the linear distance betweensuccessive maxima or minima of a wave. Thewavelength must decrease as radiation passes froma vacuum to some other medium.

Wavenumber (υ) – the number of waves percentimeter.

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Effect of change of medium

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B. Types of Interactions

Absorption

Emission

Luminescence

Scattering

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C. Spectroscopic Measurements

AbsorptionEmissionScattering(q, l, t)2. Irradiate with

EM radiation

1. Sample

3. Detector to measure

4. Interpret results. Requires some understanding of the physical basis of the interaction.

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IIA. Classical Description of Light

EM spectrum

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Snell ́s law

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Diffraction of Monochromatic Radiation

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The double-slit interference experiment

Originally performed by Young (1801) with light. Subsequently also performed with many types of matter particle (see references).

D

θd

Detecting screen (scintillators or particle detectors)

Incoming beam of particles (or light)

y

Alternative method of detection: scan a detector across the plane and record arrivals at each point

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Atomic AbsorptionAtoms usually in gaseous state like mercury

vapor generated in a flame absorb light &undergo electronic transition Atomic spectra aresimple line spectra because there are no bonds tovibrate or rotate around, just electrons topromote.

Example – Na vapor has 2 lines 589.0 nm &589.6 nm which come from 3s electronspromoted to 2 possible 3p states of different E

Peak at 285 nm from 3s to 5p = more EUV-vis wavelengths promote outer shell

electronsX-rays promote inner shell e- = much more EMohammad-khah

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In spectroscopy (EM interacts with

matter), the energy of the transition (∆E)

must correspond to the energy of the light

(EM) given by frequency (υ) and Plank’s

constant (h) :

∆E = h υ

This holds for absorption & emission of

radiation.Mohammad-khah

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Theory – The total energy of a molecule can bebroken down into several types of energy ForUV-vis must consider:

electronic energy

vibrational energy

rotational energy

Ignore translational energy

Molecular Absorption – more complex thanatomic absorption because molecules havemany more possible transitions Mohammad-khah

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Electronic energy involves changes inenergy levels of the outer electrons of amolecule

- these changes correspond to the energy of the ultraviolet-visible radiation

- these changes are quantized (i.e. discrete levels exist corresponding to quanta of light)

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Simplified Energy Level Diagram

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In the IR region of the spectrum the

radiation is not energetic enough to

cause electronic transitions

Even less energetic radiation can be used i.e.microwaves and radio waves.

Place sample in magnetic field and canobserve low energy transitions associatedwith changes in spin states e.g. NMR, EPR(ESR)

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Beer Lambert Law

• Can be used for estimating concentrations from spectra

e = molar absorption coefficient [M-1 cm-1]c = concentration [M]l = path length [cm]Absorbance (optical density) = ecl []Dependant on wavelength !

I = I010-ecl

I0 I

l

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Electronic (UV) spectroscopy

• Light absorbed – electron excited to higher molecular orbital

• Transitions occur from HOMO to LUMO- Highest Occupied Molecular Orbital- Lowest Unoccupied Molecular Orbital

DE=hn

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Chromophores

• Chromophores- Part of molecule responsible for absorption

• Auxochromes- Groups that modify absorption of neighboring chromophores- Often have lone pairs, e.g. –OH, -OR, -NR2, -halogen- Bathochromic shift: towards longer wavelength- Hypsochromic shift: towards shorter wavelength

• Typical for unconjugated organic molecules * below 150 nm (vacuum UV)n *, * below 200 nm (quartz UV)n * 200-400nm (quartz UV)

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Effect of conjugation

• Increased conjugation leads to longer absorption wavelengths

• Ex: Butadiene absorbs at 217nm whereas lycopene absorbs at 470nm

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Absorption of Radiation

Selective removal of certain frequencies bytransfer of energy to atoms or molecules.

Particles promoted from lower-energy(ground) states to higherenergy (excited)states.

Energy of exciting photon must exactlymatch the energy difference between theground state and one of the excitedstates of the absorbing species.

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Emission of RadiationElectromagnetic radiation is produced when

excited particles return to lower-energy levels or the ground state.

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Energy-level diagram ofmolecular fluorescence

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General Designs ofOptical Instruments

Stable source of radiant energy. In emission

spectroscopy, sample is radiation source.

A transparent sample container over the

wavelengths used for analysis.

A wavelength selector to isolate region of

spectrum for measurement.

A transducer (detector) to convert radiant

energy data to electrical domain data.

Signal processor and readout. Mohammad-khah

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1) source of radiation

2) device for dispersing radiation intocomponent wavelengths

3) a means of putting sample into theoptical path, i.e., cell

4) Detector to convert the EM to anelectrical signal

5) readout device or circuitry, i.e., meter,computer, recorder, integrator, etc.

Absorption measurements require:

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Block diagram of instrument for absorption

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Block diagram of instrument for absorption

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Emission measurements require:

1) means of exciting emission i.e., way ofpopulating upper energy level whichspontaneously emits

2) device for dispersing radiation intocomponent wavelengths

3) a means of putting sample into the opticalpath, i.e., cell

4) Detector to convert the EM to an electricalsignal

5) readout device or circuitry, i.e., meter,computer, recorder, integrator, etc.Mohammad-khah

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Block diagram of instrument for Fluorescence & Phosphorescence

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Sources – important characteristics

1) Spectral distribution i.e., intensity vs. λ(continuum vs. line sources)

2) Intensity

3) Stability – short term fluctuations

(noise), long term drift

4) Cost

5) Lifetime

6) Geometry – match to dispersion deviceMohammad-khah

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CONTINUUM SOURCES

Thermal radiation (incandescence) –heated solid emits radiation close to thetheoretical “Black Body” radiation i.e.,perfect emitter, perfect absorber Behaviorof Black Body

Total power ~T4 therefore need constanttemperature for stability when usingincandescent sources

Spectral distribution follows Planck’sradiation law Mohammad-khah

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Spectral Distribution Curves of a Tungsten (Black Body) Absorber/Emitter

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Visible Region sources are:

a) Glass enclosed Tungsten (W) filament –

normally operated at ~3000 K with inert atmosphereto prevent oxidation. Useful from 350 nm to 2000 nm,below 350 nm glass envelope absorbs & emission weak

b) Tungsten-Halogen lamps –

can be operated as high as 3500 K. More intense (highflux). Function of halogen is to form volatiletungstenhalide which redeposits W on filament, i.e.,keeps filament from burning out. Requires quartzenvelope to withstand high temps (which alsotransmits down to shorter wavelengths). Fingerprintsare a problem – also car headlights Mohammad-khah

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Gas Discharge Lamps

two electrodes with a current between themin a gas filled tube. Excitation results fromelectrons moving through gas. Electronscollide with gas→ excitation → emission.

At high pressure →“smearing” of energylevels→ spectrum approachescontinuum.

The higher the pressure, the greater theprobability that any given molecule or atomwill be perturbed by its neighbor at themoment of emission. Mohammad-khah

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a) Hydrogen Lamp

most common source

for UV Absorption

measurements H2

emission is from 180

nm to 370 nm limited

by jacket Line

spectrum from 100

watt Hydrogen Lamp

at low pressure in

Pyrex.Mohammad-khah

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b) Deuterium Lamp

same λ distribution as

H2 but with higher

intensity (3 to 5 times)-

D2 is a heavier

molecule & moves

slower so there is less

loss of energy by

collisions High

pressure D2 with

quartz jacket. Mohammad-khah

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c) Xenon Lamp

For higher intensity– Xe at high pressure (10-20 atm)

- high pressure needed to get lots of collisions for broadening leading to continuum

- short life relatively

- arc wander (stabilize)

- need jolt to start

- output = f(time)

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d) High Pressure Mercury Lamp

can’t completely eliminate bands associatedwith particular electronic transitions even atvery high pressures (e.g., 100 atm)

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High Pressure Mercury Spectrum – (e.g., 100 atm)

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For UV-vis absorption spectrophotometry

usually use H2 for UV and tungsten for

visible region (switching mid scan)

Sometimes use D2 instead of H2

For fluorescence spectrophotometry use

xenon arc lamp in scanning instruments

Can use He below 200 nm

Hg at low pressure is used in fixed

wavelength (non scanning) fluorometers

Can use mixture of Hg and XeMohammad-khah

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The Typical UV- VIS Instrumentis Dual-Source

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LINE SOURCES

1) Gas (Vapor) Discharge Lamps at low pressure (i.e., few torr) – minimize collisional interaction so get line spectrum

- most common are Hg and Na

- often used for λ calibration

- Hg pen lamp

- fluorescent lights are another example

- also used UV detectors for HPLC

2) Hollow Cathode Lamps (HCL) – for AA

3) Electrodeless Discharge Lamps (EDL) - AAMohammad-khah

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Wavelength Selection

Three main approaches:

1) Block off unwanted radiation –

optical filters

2) Disperse radiation & select desired band –

monochromator

3) Modulate wavelengths at different frequencies-

interferometerMohammad-khah

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Wavelength Selectors

Spectral Bandwith

Monochromatic light is

really finite bandwidth.

•The spectral bandwidth is the FWHH.

•Lorentzian not Gaussian.

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Absorption – colored glass, colored film,

colored solutions – cheapest way

Absorption filters are also known as bandpassfilters

Usually exhibit low peak transmittance

Typically have a broad peak profile

Can use two or more absorption filterstogether to produce desired transmittancecharacteristics

Generic filters are 2 x 2 inch glass or quartz

Relatively inexpensive

FILTERSAssortment of Glass & Quartz

Optical Filters

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2) Interference filters –usually Fabrey-Perot type

Dielectric material (CaF2or MgF2)

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Condition for constructive interference

If distance (d) is multiple (m) of wavelength (λ)then it won’t be interfered with Concept of Orderconstructive & destructive interference causeswaves with different phase angles to beeliminated except if they are multiples of eachother. Mohammad-khah

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Cut-off filters or sharp-cut filters are alsoavailable such as the 650 nm cut-off filter shownhere Cut-on filters have reverse profile

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Cut-off Filters

cut-off filters havetransmittances of nearly100% over a portion ofthe visible spectrum butthen rapidly decrease tozero transmittance overthe remainder

a narrow spectral bandcan be isolated bycoupling a cut-off filterwith a second filter

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Combining two appropriate cut-off filters producesa bandpass filter. The example shown here comesfrom 3 filters producing bands at 500 & 600 nm.

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FWHM – full width at half maximum

Interference Filter Characteristics and Nomenclature

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Two terms associated with optical filters are:

1) Effective bandwidth measured at ½ peak height

2) Nominal wavelength These filters have nominal wavelengths of 450 & 500 nm

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Transmittance vs. wavelength for typicalFabrey-Perot Interference filter showing firstand second order λ’s (m = 1 & m = 2)

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Components of Optical Instruments

a) Sources

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b) wavelength selectors for spectroscopic instruments.

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c) Construction materials

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(d) detectors for spectroscopic instruments

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Dispersion for Monochromators

Absorption

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MONOCHROMATORSPrism Monochromator

Entrance slit allows source radiation to illuminate the first lenswhich collimates the light spreading it across the face of the prism.Prism disperses radiation into component wavelengths and thesecond lens focuses the spectrum at the focal plane.

An exit sl it selects the band of radiation to reach the detector.

Dispersing element can be a prism or a diffraction grating.

Focusing elements can be lenses or mirrors.Mohammad-khah

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Optical Materials

need optically transparent materials for lenses,

prisms & sample cells

In visible region – can use glass down to 350 nm

In the UV region – quartz is material of choice

In the IR region – NaCl, KBr, etc. The heavier

the atoms of the salt, the farther into the IR

region (i.e., longer λ) before significant absorption

occurs

Problem – sensitivity to moisture Mohammad-khah

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Resolution – ability to distinguish as

separate, nearly identical frequencies;

measured in terms of closest frequencies ∆υ

in a spectrum that are distinguishable

R = υ ∆υ or λ ∆ λ (both dimensionless)

Dispersion – spread of wavelengths in space

Angular Dispersion – angular range dθ over

which waveband dλ is spread dθ dλ in

rad nm

Dispersion پاشندگی

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Dispersion

Linear Dispersion (D) – distance dy over which a

waveband dλ is spread in the focal plane of a

monochromator dy /dλ in mm /nm

Reciprocal Linear Dispersion – range of λ’s

spread over a unit distance in the plane of a

monochromator dλ / dx in nm / mm

Related terms spectral slit width or bandwidth

or bandpass = range of λ’s included in a beam of

radiation measured at half max intensityMohammad-khah

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Wavelength Dispersion and Selection

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Light exiting a monochromator exit slit has a triangular distribution

Optical Efficiency = throughput × resolutionGood criterion for comparing optical systems

Prism < Grating < Interferometer

Monochromator Monochromator Mohammad-khah

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Prisms

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Kinds of PrismsLittrow Prism & Mounting – compact design

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Gratings work on the principles of diffraction & interference

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Gratings

based on diffraction & interferenceTransmission Gratings & Reflection Gratingsconsist of a series of grooves in glass or quartz ora mirror (usual kind)

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Grating Equationn λ = d sin β

Condition for constructive interferenceAC = extra distance light travels for first order = d sin βFor higher orders the distance gets longer Mohammad-khah

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Reflection grating with non-normal incidence

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HOLOGRAPHIC GRATINGS

Dispersion of Grating Monochromator

nλ= d (sin i + sin r)

n dλ= d cos r dr

angular dispersion dr / dλ= n / d cosr

Linear dispersion D= dy / dλ = F dr / dλ

Reciprocal Linear dispersion

D -1= dλ / dy =dλ /(F dr) in Å / mm or nm/mm

Grating D -1= d cosr / n F

r<20° cosr ≈ 1 → D -1 = d / n FMohammad-khah

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Grating Equation

n λ= d (sin i + sin r )

several values of λ exist for a given diffraction

angle r

thus if a first-order line (n = 1) of 900 nm is found at r,second-order (450 nm) and third-order (300 nm) linesalso appear at this angle

ordinarily the first-order line is the most

intense

the higher-order lines can generally be

removed by filters Mohammad-khah

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Grating Dispersion

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Dispersion

ability to separate small wavelengthdifferences

linear dispersion D refers to the variation inwavelength as a function of y, the distancealong the focal planes.

if F is the focal length of themonochromator, the linear dispersion is

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Resolution

Resolution is the separation of wavelengths in a spectrum.

Power

90%

Valley

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Monochromator Slits

Effect of Slit Width on Resolution

Slit width just right! Mohammad-khah

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Monochromator Slits

Slit width too narrow

Slit widths that are too small, do not further narrow spectral features for they are then defined by the natural linewidth of the source

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Monochromator Slits

Slit width too wide

Slit widths that are too large will widen narrow features and diminishtheir peak intensity, degrading the resolving power of the instrument.

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Monochromator Slits

Construction of slits

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Monochromator Slits

Illumination of an exit slit by monochromatic radiation l 2 at various monochromator settings. Exit and entrance slits are identical.

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Monochromator Slits

The effect of the slit

width on spectra. The entrance slit is illuminated with l1, l2, and l3 only. Entrance and exit slits are identical. Plots on the right show changes in emitted power as the setting of monochromator is varied.

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Spectral Bandpass and Slit Function

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Gratings – Czerny-Turner

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Gratings – Ebert Mounting

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Dispersion of Grating vs. prism

Dispersion - almost constant with wavelength for grating (an advantage over prisms) Don’t have to change slits to get constant bandpass Across spectrum

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Methods of reducing stray light :

1) Paint interior black2) Use baffles to obstruct stray radiation3) Use high quality components4) Keep out dust and fumes5) Can also use double monochromator

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Advantages of GratingMonochromators

Linear dispersion of wavelengths.

Fixed dispersion makes it easy to scanan entire spectrum at constantbandwidth after initial adjustment ofslitwidth.

Better dispersion for same size ofdispersing element.

Can disperse radiation in far UV andinfrared regions where absorptionprevents use of prisms. Mohammad-khah

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Disadvantages of GratingMonochromators

Produce great amounts of stray

radiation.

Produce more high-order spectra.

Both of these disadvantages can be minimized withfilters.

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Sample Containers

in common with the optical elements of

monochromators, the cells or cuvettes

that hold the samples must be made of

material that is transparent to radiation

in the spectral region of interest

example, use of quartz in the uv and visible,silicate glasses in visible and near IR

plastic is finding use in the visible region (cheaperand disposable) Mohammad-khah

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Radiation Transducers

Properties of the ideal detector

high sensitivity,

high signal-to-noise ratio

wide wavelength response

fast response time

linear output ( S= kP)

low dark current ( S= kP+ kd)Mohammad-khah

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Radiation Detection

S = kP + kd

where S => electrical response

k => sensitivity of detector

P => power

kd => dark current

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Detector Response

Response is a function of wavelength.

Greater sensitivity, the greater output voltage or current.

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Types of Radiation Transducers

Photon transducers (UV, VIS, NIR): respond to incident photon rate

highly variable spectral response (determined by photosensitive material)

respond quickly (microseconds or faster)

Thermal transducers (IR):

respond to incident energy rate

relatively flat spectral response curves

generally slow (milliseconds or slower)Mohammad-khah

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Photon Transducers

Photovoltaic Cells

Phototubes

Photomultiplier Tubes (PMT)

Photoconductivity Detectors

Photodiode Arrays (PDA)

Charge-coupled Devices (CCD)Mohammad-khah

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Photon Transducers:

Phototube:

– Incident photon causes release of an electron

– Photocurrent Plight

– Not best for low-light scenarios

Covert photon energy to electrical signal (current, voltage, etc.)

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Phototube

Composition

of photocathode determines w

which in turn determines λ

response

photons → electrons → currentUsually need current to voltage converter to display signal as voltage proportional to # of photons

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Photomultiplier Tube (PMT)

Cross Section of a PMT

• each dynode is biased ~90 V more positive than previous

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PMT in use

• Good response to UV/VIS

• Fast response

• Dark current -decrease when cooling

• Housing/dark room condition

• Photon counting

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Photomultiplier Tubes (PMTs)

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Advantages of PMTs

1) Stable except after exposure to high light levels

2) Sensitive

3) Linear over several orders of magnitude

4) Reasonable cost

Simple PMT for visible region = $100

Quartz jacketed PMT for UV & red sensitive tubes for near IR can be more expensive

5) Long lifetime

6) Rapid response (on the order of nanoseconds)Mohammad-khah

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Types of Optical Instruments

a spectroscope is an optical instrument used for thevisual identification of atomic emission lines; itconsists of a monochromator, in which the exit slit isreplaced by an eye piece that can be moved along thefocal plane

a colorimeter is an instrument for absorptionmeasurements in which the human eye serves as thedetector using one or more color-comparisonstandards

a photometer consists of a source, a filter, and aphotoelectric transducer as well as a signal processorand readout photometers designed for fluorescence measurements are also

called fluorometers Mohammad-khah

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a spectrometer is an instrument that providesinformation about the intensity of radiation as afunction of wavelength or frequency

a spectrophotometer is a spectrometer equipped withone or more exit slits and photoelectric transducers thatpermit the determination of the ratio of the power of twobeams as a function of wavelength as in absorptionspectroscopy a spectrophotometer for fluorescence analysis is sometimes called a

spectrofluorometer

Multiplex instrument obtains spectral informationwithout first dispersing or filtering the radiation toprovide wavelengths of interest; the term multiplexcomes from communication theory, where it is used todescribe systems in which many sets of information aretransported simultaneously through a single channel

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Instrument Assemblies

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Absorption: Single Beam

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Schematic diagram of a Double Beam Spectrophotometer

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Absorption: Double Beam

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Documents

Overview of Spectroscopy - staff.guilan.ac.ir · Originally performed by Young (1801) with light. Subsequently also performed with many types of matter particle (see references)