Dong-Sun Lee / cat - lab / SWU 2010 -Fall version

Chapter 25

Instruments for Optical Spectrometry

Components of Various Types of Instrument for Optical Spectroscopy

1) Absorption measurement

2) Fluorescence measurements

3) Emission spectroscopy

Instrumentation for spectrometry

1. Light sources

continuum source

line spectrum

Spectral source types. The spectrum of a continuum source (a) is much broader than that of line source (b).

Spectral distribution of blackbody radiation.

Black Body Radiation

Any object surface can radiate heat to and receive heat from outside, if an object can absorb all the incident radiation, regardless of the frequencies and directions, this object is called Black Body. A ball cavity with a small hole can be regarded as a black body, since any radiation entering the ball cavity can only reflect inside it, thus totally absorbed.

Low pressure mercury arc lamp : 253.7 nm Hg line

Hollow cathode lamps : line sources / AA spectrometry

Laser source

A deuterium lampA tungsten lamp

H2 + Ee H2*

H2* H’ + H’’ + h

Ee = EH2*

= EH’ + EH’’ + h

Light sources.

Intensity of a tungsten filament at 3200K and a deuterium arc lamp.

Laser

Laser is the acronym of Light Amplification by Stimulated Emission of Radiation. A device which produces light with a narrow spectral width. Laser is light of special properties, light is electromagnetic (EM) wave in visible range. Lasers, broadly speaking, are devices that generate or amplify light, just as transistors generate and amplify electronic signals at audio, radio or microwave frequencies. Here light must be understand broadly, since lasers have covered radiation at wavelengths ranging from infrared range to ultraviolet and even soft x-ray range.

A laser is a cavity that has mirrors at the ends and is filled with lasable material such as crystal, glass, liquid, gas, or dye. These materials must have atoms, ions, or molecules capable of being excited to a metastable state by light, electric discharge, or other stimulus. The transition from this metastable state back to the normal ground state is accompanied by the emission of photons which form a coherent beam.

Laser construction 

A laser system generally consists of three important parts:

- An energy source (usually referred to as the pump or pump source);

- A gain medium or laser medium;

- A mirror, or system of mirrors, forming an optical resonator.

Laser cavity. The electromagnetic wave travels back and forth between the mirrors, and the wave is amplified with each pass. The output mirror is partially transparent to allow only a fraction of the beam to pass out of the cavity.

(a) Energy-level diagram illustrating the principle of operation of a laser. (b) Basic components of a laser. The population inversion is created in the lasing medium. Pump energy might be derived from intense lamps or an electric discharge.

Amplification of light

All lasers contain an energized substance that can increase the intensity of light passing through it. This substance is called the amplifying medium or, sometimes, the gain medium, and it can be a solid, a liquid or a gas. Whatever its physical form, the amplifying medium must contain atoms, molecules or ions, a high proportion of which can store energy that is subsequently released as light.

In a neodymium YAG (Nd:YAG) laser, the amplifying medium is a rod of yttrium aluminium garnate (YAG) containing ions of the lanthanide metal neodymium (Nd). In a dye laser, it is a solution of a fluorescent dye in a solvent such as methanol. In a helium-neon laser, it is a mixture of the gases helium and neon. In a laser diode, it is a thin layer of semiconductor material sandwiched between other semiconductor layers. The factor by which the intensity of the light is increased by the amplifying medium is known as the gain. The gain is not a constant for a particular type of medium. It's magnitude depends upon the wavelength of the incoming light, the intensity of the incoming light, the length of the amplifying medium and also upon the extent to which the amplifying medium has been energized. http://members.aol.com/WSRNet/tut/ut1.htm

Schematic of a Nd:YAG laser.

Energizing the amplifying medium

Increasing the intensity of a light beam that passes through an amplifying medium amounts to putting additional energy into the beam. This energy comes from the amplifying medium which must in turn have energy fed into it in some way. In laser terminology, the process of energizing the amplifying medium is known as "pumping".

There are several ways of pumping an amplifying medium. When it is a solid, pumping is usually achieved by irradiating it with intense light. This light is absorbed by atoms or ions within the medium raising them into higher energy states. Xenon-filled flashtubes positioned as shown below are used as a simple source of pumping light. Passing a high voltage electric discharge through the flashtubes causes them to emit an intense flash of white light, some of which is absorbed by the amplifying medium. The assembly of flashtubes is enclosed within a polished metal reflector (not shown in the diagram below) to concentrate as much light as possible on the amplifying medium. A laser that is pumped in this way will have a pulsed output.

Pumping an amplifying medium by irradiating it with intense light is referred to as optical pumping. The source of pumping light can be another laser. Some types of laser that were originally pumped using xenon-filled flashtubes are now pumped by laser diodes.

Gaseous amplifying media have to be contained in some form of enclosure or tube and are often pumped by passing an electric discharge through the medium itself. The mechanism by which this elevates atoms or molecules in the gas to higher energy states depends upon the gas that is being excited and is often complex. In many gas lasers, the end windows of the laser tube are inclined at an angle and they are referred to as brewster windows. Brewster windows are able to transmit a beam that is polarized in the plane of the diagram without losses due to reflection. Such a laser would have an output beam that is polarized.

The diagram illustrates pumping by passing a discharge longitudinally through the gaseous amplifying medium but, in some cases, the discharge takes place transversely from one side of the medium to the other. Many lasers that are pumped by an electric discharge can produce either a pulsed output or a continuous output depending upon whether the discharge is pulsed or continuous.

Various other methods of pumping the amplifying medium in a laser are used. For example, laser diodes are pumped by passing an electric current across the junction where the two types of semiconductor within the diode come together.

Creating a Population Inversion

Finding substances in which a population inversion can be set up is central to the development of new kinds of laser. The first material used was synthetic ruby. Ruby is crystalline alumina (Al2O3) in which a small fraction of the Al3+ ions have been replaced

by chromium ions, Cr3+. It is the chromium ions that give rise to the characteristic pink or red color of ruby and it is in these ions that a population inversion is set up in a ruby laser.

In a ruby laser, a rod of ruby is irradiated with the intense flash of light from xenon-filled flashtubes. Light in the green and blue regions of the spectrum is absorbed by chromium ions, raising the energy of electrons of the ions from the ground state level to the broad F bands of levels. Electrons in the F bands rapidly undergo non-radiative transitions to the two metastable E levels. A non-radiative transition does not result in the emission of light; the energy released in the transition is dissipated as heat in the ruby crystal. The metastable levels are unusual in that they have a relatively long lifetime of about 4 milliseconds (4 x 10-3 s), the major decay process being a transition from the lower level to the ground state. This long lifetime allows a high proportion (more than a half) of the chromium ions to build up in the metastable levels so that a population inversion is set up between these levels and the ground state level. This population inversion is the condition required for stimulated emission to overcome absorption and so give rise to the amplification of light. In an assembly of chromium ions in which a population inversion has been set up, some will decay spontaneously to the ground state level emitting red light of wavelength 694.3 nm in the process. This light can then interact with other chromium ions that are in the metastable levels causing them to emit light of the same wavelength by stimulated emission. As each stimulating photon leads to the emission of two photons, the intensity of the light emitted will build up quickly. This cascade process in which photons emitted from excited chromium ions cause stimulated emission from other excited ions is indicated below:

The ruby laser is often referred to as an example of a three-level system. More than three energy levels are actually involved but they can be put into three categories.These are; the lower level form which pumping takes place, the F levels into which the chromium ions are pumped, and the metastable levels from which stimulated emission occurs. Other types of laser operate on a four level system and , in general, the mechanism of amplification differs for different lasing materials. However, in all cases, it is necessary to set up a population inversion so that stimulated emission occurs more often than absorption.

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Properties of laser light

Monochromatic : one wavelength

Extremely bright : high power at one wavelength

Collimated : parallel rays

Polarized : electric field of waves oscillates in one plane

Coherent : all waves in phase

Coherence can be devided into spatial and temporal coherence. For any em wave, if at time t=0 and t0 the phase diference between two points in space remains the same, we say the em wave has spatial coherence; If at a point P, the em wave at t a

nd t+dt has same phase difference if dt is the same, temporal coherence exists.

Disadvantages of a laser

High maintenance

Limited wavelengths

Common light sources, such as the electric light bulb emit photons in all directions, usually over a wide spectrum of wavelengths. Most light sources are also incoherent, i.e., there is no fixed phase relationship between the photons emitted by the light source.

By contrast, a laser generally emits photons in a narrow, well-defined beam of light. The light is often near-monochromatic, consisting of a single wavelength or color, is highly coherent and is often polarised. Some types of laser, such as dye lasers and vibronic solid-state lasers can produce light over a broad range of wavelengths; this property makes them suitable for the generation of extremely short pulses of light, on the order of a femtosecond (10-15 seconds).

Laser light can be highly intense — able to cut steel and other metals. The beam emitted by a laser often has a very small divergence (i.e. it is highly collimated). A perfectly collimated beam cannot be created, due to the effect of diffraction, but a laser beam will spread much less than a beam of light generated by other means. A beam generated by a small laboratory laser such as a helium-neon (HeNe) laser spreads to approximately 1 mile (1.6 kilometres) in diameter if shone from the Earth's surface to the Moon. Some lasers, especially semiconductor lasers due to their small size, produce very divergent beams. However, such a divergent beam can be transformed into a collimated beam by means of a lens. In contrast, the light from non-laser light sources cannot be collimated.

A laser can also function as an optical amplifier when seeded with light from another source. The amplified signal can be very similar to the input signal in terms wavelength, phase and polarisation; this is particularly important in optical communications.

The output of a laser may be a continuous, constant-amplitude output (known as c.w. or continuous wave), or pulsed, by using the techniques of Q-switching, modelocking or Gain-switching.

The basic physics of lasers centres around the idea of producing a population inversion in a laser medium. The medium may then amplify light by the process of stimulated emission, which if the light is fed back through the medium by means of a cavity resonator, will continue to be amplified into a high-intensity beam. A great deal of quantum mechanics and thermodynamics theory can be applied to laser action, though in fact many laser types were discovered by trial and error.

Population inversion is also the concept behind the maser, which is similar in principle to a laser but works with microwaves. The first maser was built by Charles H. Townes in 1953. Townes later worked with Arthur L. Schawlow to describe the theory of the laser, or optical maser as it was then known. The word laser was coined in 1957 by Gordon Gould, who was also credited with lucrative patent rights in the 1970s, following a protracted legal battle.

The first maser, developed by Townes, was incapable of continuous output. Nikolai Basov and Alexander Prokhorov of the USSR worked independently on the quantum oscillator and solved the problem of continuous output systems by using more than two energy levels. These systems could release stimulated emission without falling to the ground state, thus maintaining a population inversion. In 1964, Charles Townes, Nikolai Basov and Alexandr Prokhorov shared a Nobel Prize in Physics "for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle."

The first working laser was made by Theodore H. Maiman in 1960 at Hughes Research Laboratories in Malibu, California, beating several research teams including those of Townes at Columbia University, and Schawlow at Bell laboratories. Maiman used a solid-state flashlamp-pumped ruby crystal to produce red laser light at 694 nanometeres wavelength.

The verb "to lase" means to give off coherent light or possibly to cut or otherwise treat with coherent light, and is a back-formation of the term laser.http://www.wordiq.com/definition/Laser

Laser (U.S. Air Force)

Laser types

- Gas Laser

HeNe (543 nm and 633 nm)

Argon(-Ion) (458 nm, 488 nm or 514.5 nm)

Carbon dioxide lasers - used in industry for cutting and welding, up to 100 kW possible

Carbon monoxide lasers - must be cooled, but extremly powerful, up to 500 kW possible

- Excimer(excited dimer or trimer) gas lasers, producing ultraviolet light, use

d in semiconductor manufacturing and in LASIK eye surgery;

157 nm (F2)

193 nm (ArF)

222 nm (KrCl)

248 nm (KrF)

308 nm (XeCl)

351 nm (XeF)

- Commonly used laser types for dermatological procedures including removal of tattoos, birthmarks, and hair:

Ruby (694 nm)

Alexandrite (755 nm) Pulsed diode array (810 nm)

Nd:YAG (1064 nm) YAG : yttrium/aluminum garnet

Ho:YAG (2090 nm)

Er:YAG (2940 nm)

- Semiconductor laser diodes,

small: used in laser pointers, laser printers, and CD/DVD players;

bigger: bigger industrial diode laser are available used in the industry for cutting and welding, up to 10 kW possible

- Dye lasers

- Quantum cascade lasers

- Neodymium-doped YAG lasers (Nd:YAG), a high-power laser operating in the infrared, used for cutting, welding and marking of metals and other materials;

- Erbium-doped YAG, 1645 nm

- Thulium-doped YAG, 2015 nm

- Holmium-doped YAG, 2090 nm, a high-power laser operating in the infrared, it is explosively absorbed by water-bearing tissues in sections less than a millimeter thick. It is usually operated in a pulsed mode, and passed through optic fiber surgical devices to resurface joints, remove rot from teeth, vaporize cancers, and to pulverize kidney and gall stones.

- Titanium-doped sapphire

- Erbium-doped fiber lasers, a type of laser formed from a specially made optical fiber, which is used as an amplifier for optical communications.

2. Wavelength selectors (Monochromator)

30 1) Filter a. Absorption filter b. Interference filter

2) Prism a. Transmission prism = Cornu b. Reflection prism = Littrow

Dispersion = d / d

Approximate transmission limits of prism materials Flint glass (contains PbO ) ; 360nm ~ 2 m Quartz(crystalline silica) ; 190nm ~3.3m NaCl or KCl ; 0.3~15 m KBr ; 0.3~30 m LiF ; 0.2~ 5 m CaF 2 ; 0.2~12 m AgCl ; 0.4~25 m CsBr ; 0.3 ~ 50 m

CsI ; 0.3 ~ 70 m KRS-5(TlBr-TlI) ; 1~30 m

3) Diffraction grating

a. Transmission grating b. Reflection grating

o

1 2 1

1

2

Cornu prism 2

o

Aluminizedsurface

Littrow prism

30

Wavelength selectors for spectrometry

Type Wavelength range (nm)

Note

Continuously variable

Grating 100 ~ 40,000 3000 lines/mm for vacuum UV,

50 lines/mm for far IR

Prism 120 ~ 30,000

Discontinuous

Interference filter 200 ~ 14,000

Absorption filter 380 ~ 750

Dispersion of radiation along the focal plane AB of a typical prism(a) and echellette grating (b).

Schematic diagram of diffraction from a grating.

n = (a – b)

d sin = a

– d sin = b

n = d (sin + sin )

Diagram of a Czerny-Turner grating monochromator.

Interference of adjacent waves that are a) 0o , b) 90o and c) 180oout of phase.

Choosing the monochromator bandwidth

Monochromator bandwidth should be as large as possible, but small compared with the width of peak being measured.

Absorbance error introduced by different levels of stray light. Stray light is expressed as a percentage of the irradiance incident on the sample.

stray light

In every instrument, inadvertent stray light (wavelength outside the bandwidth expected from the monochromator) reaches the detector. High qulity spectrometers could have two monochromators in series to reduce stray light.

Output of an exit slit as monochromator is scanned from to

Nominal wavelength

Bandwidths for two types of filters(interference filter vs absorption filter).

Filters

It is frequently necessary to filter (remove) wide bands of radiation from a signal.

(a) Schematic cross section of an interference filter.

(b) Schematic to show the conditions for constructive interference

Transmission spectra of interference filters.

(a) Wide band pass filter has ~90% transmission in the 3- 50 5- m wavelength range but <0.01% transmittance outside this range.

(b) Narrow band-pass filter has a transmission width of 0.1 m centered around 4 m.

Transmittance range for various cell construction materials.

3. Optical Materials and sample containers

CAT-Lab / SWUQuartz cell for UV spectrophotometer

Type Wavelength range(nm)

Photon detectors

Phototubes 150 ~ 1,000

Photomultiplier tubes 150 ~ 1,000

Silicon diodes 350 ~ 1,100

Photoconductive cells 1,000 ~ 50,000

Thermal detectors

Thermocouples 600 ~ 20,000

Bolometers 600 ~ 20,000

Pneumatic cells 600 ~ 40,000

Pyroelectric cells 1,000 ~ 20,000

4. Detectors for spectrometry

A transducer is a type of detector that converts various types of chemical and physical quantities into electrical signals such as electrical charge, current, or voltage.

Response of several different detectors.

The greater the sensitivity, the greater the output (current or voltage) of the detector for a given incident power of photons.

Schematic diagram of photomultiplier with nine dynodes.

Phototube

Comparison of spectra recorded in 5 min by a photomultiplier tube and a charge coupled device.

Absorption spectra of hemoglobin with

identical signal levels but different amount of noise.

Silicon photodiode array

Charge transfer device (CTD)

An operational amplifier current-to-voltage converter used to monitor the current in a solid state photodiode.

Eout = – IR = – kPR = k’P

P= radient power

G = KP + K’

G = electrical response of the detector in units of current, voltage, or charge.

K’ = dark current

Components and materials of spectroscopic instruments.

Instrumentation of UV-visible spectrophotometer

Types of UV-visible spectrophotometer

1) Single beam spectrophotometer

2) Double beam spectrophotometer

Block diagram for a double-beam in-time scanning spectrophotometer .

Block diagram for a diode array spectrometer.

3) Diode-array spectrophotometer

Spectronic 20 spectrophotometer

Absorbance & Transmittance displayPower indicator light

Sample holder

Wavelength selector

Absorbance & Transmittance controlPower switch Zero control

Spectronic 20 spectrophotometer

LED

Scale of spectronic 20 spectrophotometer

Procedure

1) Power on

2) Select wavelength

3) 0% T adjustment

(Calibration)

4) Blank (Reference cell) is inserted into cell holder

5) 100% T adjustment

6) Sample cell is placed in the cell compartment

7) Readout absorbance

8) Power off

Schematic diagram of optical system of Spectronic 20 single beam UV-visible spectrophotometer

The dual-beam design greatly simplifies this process by simultaneously measuring P and Po of the sample and reference cells, respectively. Most spectrometers use a mirrored rotating chopper wheel to alternately direct the light beam through the sample and reference cells. The detection electronics or software program can then manipulate the P and Po values as the wavelength scans to produce the spectrum of absorbance or transmittance as a function of wavelength.

HP8452A diode array UV-visible spectrophotometer

Optical schematic of the Hewlett-Packard HP-8450A diode array UV-visible spectrophotometer.

Q n AThanks

Home page http://www.swu.ac.kr/~dslee

Electronic mail dslee@swu.ac.kr