Instrumentation Laser Optoacoustic Spectroscopy— A New Technique of Gas Analysis L. Β. Kreuzer DIAXCorp. 250 Sobrante Way Sunnyvale, Calif. 94086
Laser Optoacoustic Spec troscopy gas analysis offers a single system which can automatically analyze a mixture of gases with high rejection ratios and sensitiv ity. One particularly prom ising field of application is gaseous air pollution detec tion, where a single LOS System can replace the col lection of single gas analyz ers which is currently re quired to provide a complete analysis of am bient air
discussion of lasers and the properties of the light they generate is presented in the next section. This is followed by sections which describe LOS gas analysis, present experimental re sults, describe the DIAX Model 100 LOS gas analyzer, and discuss appli cations.
and the number of observed emission wavelengths have increased rapidly. Today, it takes 200 pages of the "Handbook of Lasers" (3) to list and describe the more common lasers and their emission wavelengths. Emission wavelengths are now available from the ultraviolet to the far infrared. The current state of laser develop ment suggests that lasers will become increasingly more common as light sources in spectroscopy.
Laser Source of Infrared
Laser action was first demonstrated at 6943 A in ruby by Maiman in 1960. Since that time, the types of lasers
(rnnliniied
Figure 1. Gas laser system. Laser tube is excited by high-voltage discharge between electrodes represented as bulbs at each end (a) When optical cavity consists of two mirrors, laser emission occurs only at lines of high gain (b) Discretely tunable laser results wfien one mirror is replaced by properly ruled and blazed diffraction grating. This configuration generates a single wave'ength at a time. Rotation of grating causes laser to emit both lines of high and low gain
(a) SIMPLE
GAS LASER
HIGH VOLTAGE SUPPLY
The availability of laser sources of infrared light has greatly increased the range of analytical problems which can be solved by infrared spec troscopy. The optoacoustic effect has proved an effective method, in com bination with a laser infrared source, for detecting weak IR absorptions in gases (/, 2). With the combination of a laser source and an optoacoustic method of detecting IR absorption, a new technique of gas analysis called Laser Optoacoustic Spectroscopy ((LOS) has been developed. A proto type gas analyzer using LOS built by the author is able to detect gas con centrations as small as 1 part per bil lion (ppb). It is able to analyze auto matically mixtures of gases and dif ferentiate between gases with over lapping infrared absorption spectra. The purpose of this paper is to de scribe the new technique of LOS gas analysis to potential users. Since many potential users may not have knowledge of laser infrared sources, a
on ραίξρ 240 A)
"RM007.
(b)
GAS LASER WITH DIFFRACTION GRATING WAVELENGTH SELECTION
HIGH VOLTAGE SUPPLY
235 A -DIFFRACTION GRATING COOLING WATER
Gas lasers are particularly well suited as sources for LOS gas analy sis. Lasers such as the CO and CO2 molecular gas lasers provide practical discretely tunable sources. To under stand how these lasers are used as ef fective light sources for LOS, it is not necessary to understand in detail how they operate. It is, however, impor tant to understand those features of laser operation and the characteris tics of laser light produced by these lasers which make them good LOS sources. Gas lasers are often regarded as emitting one wavelength which de pends on the composition of the laser gas. The CO2 laser is commonly said to be a 10.6-μπι laser. This is because the laser configuration most common ly used, consisting of a laser discharge tube and two highly reflecting mirrors (Figure la), generates radiation at 10.6 μπι. If one of the highly reflect ing mirrors is replaced with a proper ly ruled and blazed diffraction grat ing, then the laser can be tuned to emit other wavelengths in addition to 10.6 μπι by rotating the grating (Fig ure lb). The wavelengths that such a laser can be tuned to emit correspond to transitions between energy levels of the C 0 2 laser gas. A properly adjust ed laser of this type will emit a single wavelength for a particular grating orientation. As the grating is rotated from this orientation, the wavelength will remain fixed until the grating has rotated a finite amount, at which point the emission wavelength will jump discontinuously to a new value. This discontinuous tuning property of the CO2 laser is common to many other gas lasers such as the CO and the HeNe laser. At first glance, the tuning charac teristics of gas lasers may seem unde sirable for spectroscopic applications. Their discontinuous tuning character istics do not allow them to be used as light sources to record spectra in the usual manner that IR spectroscopists employ. This discontinuous tuning is a distinct advantage, however, when these sources are used to measure gas absorption in a LOS gas analysis sys tem. This advantage comes from the great degree of frequency stability and reproducibility that this type of laser possesses. The exact wavelength emitted by a gas laser of this type de pends on the energy levels of the laser gas and not on the exact grating ori entation. The grating orientation need only be set exactly enough to se lect the proper laser emission line. This means that it is possible to build sources that will emit a sequence of exactly reproducible and known wavelengths. The value of such a source will become clear in the fol lowing sections which describe LOS and how it is used for gas analysis.
i It
ο Figure 2. Sampling of gas absorption at C 0 2 laser emission wave lengths
3
WAVE NUMBER
Solid c u r v e , representing absorption s p e c t r u m of hypothetical gas, is made up of superposition of a number of collision broadened lines. Tails of these lines are represented by interrupted lines. Vertical lines represent narrow C 0 2 laser emission wavelengths, and their height represents the absorption m e a s u r e d at each w a v e l e n g t h . Near c o i n c i d e n c e between laser emission wavelength and gas absorption wavelength is indicated at point Ρ
Laser Optoacoustic Spectrometer
Infrared absorption properties of gases can be measured with discretely tunable gas lasers as sources by tak ing advantage of the many near coin cidences that exist between laser emission wavelengths and gas absorp tion wavelengths (4-6). The infrared absorption lines which constitute the IR absorption spectrum of a gas are not infinitely narrow. Doppler and collisional effects broaden them to a finite width. The ability to measure gas absorption with a discretely tun able laser source depends on the exis tence of these broadening mecha
nisms. The light beam generated by a discretely tunable laser has an ex tremely narrow spectral spread or line width and may be regarded from a practical point of view as possessing an infinitely narrow line width. Since such a laser tunes discontin uously, it cannot be tuned so that its emission wavelength coincides with the center of an absorption line. For tunately, however, since the absorp tion spectrum of a pure gas has many lines, which will be collisionally broadened at sufficient pressure, laser emission wavelengths are found that fall inside the line width of broadened absorption lines (Figure 2). The abili-
DATA OUTPUT CONTROL COMMANDS INPUT SIGNAL DETECTION ELECTRONICS
TOTAL POWER -
PHASE REFERENCE —
ABSORBED POWER - MICROPHONE
3)
IR. DETECTOR -
--f-
" G A S SAMPLE
VALVE CONTROL
Ϊ Γ ^ FLOW CONTROL VALVES I I TO PUMP SAMPLE IN 24Hz LIGHT CHOPPER
LASER STABILIZATION WAVELENGTH SELECTION
DIFFRACTION j / l-H.V. GRATING Γ/f
rA A
' ^ Ϊ*Γ" C0 2 LASER TUBE
U S E R
0 U T p u r
MIRROR
£
IR. LASER BEAM
Figure 3. Prototype Laser Optoacoustic Spectrometer (LOS) gas analysis system
240 A • ANALYTICAL CHEMISTRY, VOL. 46, NO. 2, FEBRUARY
1974
Ψ
MIRROR TRANSLATOR
ty to use a particular type of laser, for example, a CO2 laser, to measure a selected gas such as ethylene depends on the existence of CO2 laser emis sion wavelengths which are absorbed by ethylene. The 10.5321-μπι line of the CO2 laser is one example of a line which fits this situation. Figure 3 depicts the prototype LOS system built by the author. The in frared source is a C 0 2 laser which can be tuned to any of 50 wavelengths be tween 9.2 and 10.8 μνα. The laser beam, with a power of about 100 mW, is chopped at 24 Hz before it passes through the sample cell. Energy ab sorbed from the beam results in gas molecules making transitions from lower to higher rotation-vibration en ergy levels. These excited molecules decay through nonradiative processes, and the energy absorbed from the laser beam by the gas results in heat ing of the gas. Since the laser beam is interrupted by the light chopper at 24
Hz, this heating of the gas will have a 24-Hz modulation. This periodic tem perature variation results in a period ic pressure variation. Infrared absorption is detected in a LOS system by placing a pressure transducer or microphone in the ab sorption chamber to measure this pe riodic pressure variation. This effect is called the "optoacoustic effect" and was discovered by Bell (7), Tyndall (8), and Rontgen (9) in 1881. It has been used for many years in nondispersive-type IR gas analyzers (10). An optoacoustic effect detector consisting of an absorption chamber and a microphone is an extremely sensitive way to detect weak absorp tions. It is possible to build a micro phone and preamplifier combination which has a low noise level by using modern "state-of-the-art" compo nents. In such a system, the main noise source is the Brownian motion of the microphone diaphragm. This
A Β S Ο R Β A Ν C Ε
10.72
9.24 WAVELENGTH (micrometers)
Figure 4. Laser optoacoustic absorption spectra taken with prototype LOS system at CO2 laser emission wavelengths A. ammonia; B, ethanol; C, ethylene; D. methanol; E, trichloroethylene
Brownian motion sets the lower limit of pressure variation that the micro phone can detect. The power of the laser beam after it has passed through the absorption cell is measured by an infrared detec tor. Signals from the microphone in the cell and the IR detector are de tected with standard "lock-in ampli fier" techniques. These signals are then fed into a small minicomputer via a multiplexer and an analog-todigital converter. The laser is tuned from one emission wavelength to an other by a stepping motor which ro tates the diffraction grating. This motor is controlled by the computer. The gas sample is changed by open ing a set of valves and drawing in a new sample with a suction pump. These valves are also controlled by the computer. Experimental Results
The prototype LOS gas analyzer system described in the last section was built to evaluate the performance of such a system. Particular attention was directed toward high sensitivity, the ability to analyze mixtures of gases, and automatic operations. This section describes the results achieved in these areas and presents a descrip tion of how the apparatus functioned. The prototype LOS system was used to measure the absorption of the 50 wavelengths which could be ob tained from its CO2 laser in a variety of gases and vapors. A sample of the gas or vapor under study was drawn into the sample chamber, and the laser was tuned through each of its emission wavelengths, and the mea sured absorption recorded. Figure 4 shows the absorption properties mea sured for five gases. These data were collected with the gas samples dilut ed to a concentration of about 100 ppm (parts per million) in an artifi cial mixture of nitrogen and oxygen mixed to simulate clean air. The absorption properties of other gases have been measured. In each case, strong absorptions existed at some CO2 laser emission wavelengths if the gas had an absorption band which was overlapped by the range of emission wavelengths of the CO2 laser. The absorption spectra mea sured for gases of this type showed significant absorption at several dif ferent emission wavelengths. Each gas or vapor tested showed a spec trum significantly different from the others tested. These results indicate that LOS spectra of different gases often overlap, but that the spectra of different gases are unique and pro vide a good basis for the identifica tion of components of a gas mixture. The ability of the system to detect small concentrations of gases was
ANALYTICAL CHEMISTRY, VOL. 46. NO. 2, FEBRUARY 1974 • 241 A
the absorbance Ai may be replaced by the sum of the absorbances of each of the components. In Equation 4 the absorbance: 1000
A,~bY,uiKL\
of the#th component is given as the product of the path length b in the sample chamber, the absorbtivity n,K at wavelength ;', and the concentra tion CV Substitution of Equation 4 into 3 gives Equation 5:
m θ α.
I
5 (Ε
(4)
too
ι
Λ
ω
/S
\
g ο
LU Ζ ui
-J μ. ω.
12 (4 16 18 20 22 24 26 28 TIME (MINUTES)
Figure 5. Measurement of ethylene concentration in air samples by use of single C0 2 emission wavelength
This equation is the key to using a LOS system to analyze a gas mixture. It shows that the gas concentrations Cg can be calculated from the mea sured quantities Si and T, by solving a set of simultaneous linear equa tions. The formal solution of Equa tion 5 is given by Equation 6. This equation C
evaluated theoretically and experi mentally. By comparing the strongest absorption in a gas LOS spectrum to the system noise level, it is possible to calculate gas concentrations which would give a unity signal-to-noise ratio. Calculated sensitivities range from 3 to 0.1 ppb. The sensitivity de pends on the absorption strength of the gas. Gases tested include ammo nia, benzene, 1,3-butadiene, 1-butene, 1,2 dichloroethylene, ethanol, ethylene, ethyl ether, methanol, ni tric oxide, nitrogen dioxide, propyl ene, and trichloroethylene. This sen sitivity range applies to the detection of any infrared absorbing gas by a LOS system of this type if the range of laser emission wavelengths over laps a gas absorption band. The ability of the system to detect a small concentration of ethylene in air was tested experimentally. The CO2 laser was tuned to emit at 10.5321 μτα. This wavelength is strongly absorbed by ethylene. This line corresponds to the ethylene ab sorption maximum in Figure 3. A small quantity of ethylene gas was re leased into the air of the laboratory room. Air samples were drawn into the sample cell at 2-min intervals, and the sample absorption was mea sured. The ethylene concentration in the room air decreased exponentially with time because of the forced venti lation of the room by an air condi tioning system and air exhausted by a chemical hood. The ethylene concen tration of each air sample was calcu lated from the measured infrared ab
sorption. These data are recorded in Figure 5. The straight line fit of the data points on a semilog plot indi cates that the room air was well mixed by ventilation and that the LOS system has a linear response. The good fit of the data points at the low concentration readings demon strates the practical achievement of high sensitivity. Mixtures of gases can be analyzed by the LOS prototype system by tak ing advantage of the dependence of the microphone signal on gas absorbance. Suppose that the sample con tainer contains a mixture of gases which has an absorbance A, at laser emission wavelength λι. Then, ac cording to Beer's law, the signal Si, measured by the microphone in the sample container which is proportion al to the power absorbed by the sam ple, is given by Equation 1: (1) e~A·) In Equation 1, P, is the laser beam power at wavelength 1. The beam power ΤΊ, which is transmitted through the sample container, is measured by the IR detector placed behind the sample cell (Figure 3). This is given by Equation 2:
s. = P,( 1
T, = P,e~A-
(2)
Equations 1 and 2 can be combined to solve for sample absorbance: .4 = l o gg((' ^ + 1l )
(3)
If the sample contains Ν component gases, then, according to Beer's law,
242 A • ANALYTICAL CHEMISTRY, VOL. 46, NO. 2. FEBRUARY
1974
1 * τΧα„
_,
/S,
\
l o g ^ + lj (6)
shows that the gas concentrations Cg can be calculated from the measured quantities Si and ΤΊ with the aid of the inverse of the matrix α,κ. The ma trix a,g is composed of the absorption properties of each of the ,V compo nents of the mixture. Determining the composition of a mixture by solving a set of simulta neous equations is a technique that has been used for many years in multicomponent analysis. The effective ness with which Equation 6 can be applied to solve practical problems and analyze gas mixtures depends on the properties of the inverse matrix am1 and on the precision with which Si and Ti can be measured. If (i« ' is "badly behaved," then small mea surement errors in S, and T, may lead to a large uncertainty in the calculat ed gas concentrations. It is important to select the measurement wave lengths so that agi λ is as "diagonal as possible." Each measurement wavelength should be selected to be characteristic of one component and as free as possible from interference from other components. The prototype LOS system ana lyzes gas mixtures by measuring Si and Τ, at selected wavelengths and then calculating the gas concentra tions. The minicomputer memorystores the matrix O/c and the pro grams necessary to solve Equation 6 and calculate gas concentrations. An important characteristic of this proto type system is the ability to detect a small quantity of one gas in the pres ence of a large concentration of an other gas. This ability is described quantitatively by the rejection ratio between pairs of gases. 243 A
Applications of LOS
Table 1. Rejection Ratios II TERFERING COMPONENT COMPONENT BEING MEASURED
Ethanol
Ethanol Methanol
760
Ammonia
1080
Trichloroethylene
200
T h e rejection ratio b e t w e e n a pair of gases is t h e c o n c e n t r a t i o n of t h e first (interfering) gas which will give a signal e q u a l to t h a t p r o d u c e d by a unit c o n c e n t r a t i o n of t h e second gas (gas being m e a s u r e d ) . For e x a m p l e , 1 p p m of e t h a n o l c a n just be d e t e c t e d in t h e presence of 3200 p p m of a m m o n i a . Rejection ratios have been calculated by using m e a s u r e d a b s o r p tion p r o p e r t i e s a n d a s s u m i n g t h a t t h e L O S s y s t e m can m e a s u r e „S\ to an a c curacy of 1%. T h e c a l c u l a t e d rejec tion ratios range from 200 to 10", d e p e n d i n g on t h e degree of s p e c t r a l overlap. T h e prototype LOS system was used to a n a l y z e m i x t u r e s of gases to e v a l u a t e how well t h i s s y s t e m was able to achieve t h e c a l c u l a t e d rejec tion ratios. T h e s e t e s t s were c o n d u c t ed with m i x t u r e s of e t h a n o l , m e t h a nol, a m m o n i a , a n d t r i c h l o r o e t h y l e n e vapor. T h e selected m e a s u r e m e n t w a v e l e n g t h s were 9.25, 10.16, 10.33, and 10.57 μΐη, respectively. T h e mea sured rejection ratios are p r e s e n t e d in T a b l e I. T h e s e v a l u e s are in good a g r e e m e n t with t h e c a l c u l a t e d v a l u e s .
DIAX Model 100 Gas Analyzer T h e DIAX Model 100 gas a n a l y z e r ( Figure 6) will be a v a i l a b l e after J u l y 1974. It utilizes t h e L O S t e c h n i q u e of gas analysis tested in t h e p r o t o t y p e s y s t e m described a b o v e . Unlike t h e p r o t o t y p e s y s t e m , which could only d e t e c t gases t h a t a b s o r b in t h e 9.2 to 10.8-μπι region, t h e Model 100 will be able to detect all infrared a b s o r b i n g gases. T h i s results from t h e i m p r o v e d laser s y s t e m in t h e Model 100 which produces over 200 different w a v e l e n g t h s over t h e range of 2-11 μτη. T h e w a v e l e n g t h s are spread uniformly enough over t h i s region to d e t e c t all IR a b s o r b i n g gases. Like t h e p r o t o t y p e s y s t e m , this system will have two modes of o p e r a t i o n . T h e first, t h e absorption measurement mode, a u t o m a t i c a l l y m e a s u r e s a n d records a s a m p l e ' s a b s o r p t i o n s t r e n g t h at each of t h e laser s y s t e m w a v e l e n g t h s . T h i s m o d e is used to record t h e a b s o r p t i o n properties of a gas s a m p l e c o n t a i n i n g a single c h e m i c a l c o m p o u n d . It produces t h e a b s o r p t i o n reference d a t a n e e d e d for t h e second m o d e of o p e r a t i o n which a n a l y z e s m i x t u r e s of gases. T o a n a l y z e a m i x t u r e of gases, t h e s y s t e m user m u s t first store t h e a b s o r p t i o n p r o p e r t i e s of e a c h
Methanol
Ammonia
270
3200 1900 ... 1000
430 200
Trichloroethylene
16000 300 1080
c o m p o n e n t of t h e m i x t u r e in t h e m i n i c o m p u t e r m e m o r y . T h i s can be d o n e e i t h e r by m e a s u r i n g t h e s e properties in t h e a b s o r p t i o n m e a s u r e m e n t m o d e or by using previously m e a s u r e d L O S s p e c t r a . A library of L O S s p e c t r a is being built u p by DIAX C o r p . T o a n a l y z e a m i x t u r e of gases, t h e s y s t e m is o p e r a t e d in t h e second or analysis mode. O n c e t h e a b s o r p t i o n properties of t h e c o m p o n e n t s of t h e m i x t u r e are stored in t h e c o m p u t e r m e m o r y , t h e system can start a u t o m a t i c m i x t u r e a n a l y s i s . More w a v e l e n g t h s t h a n t h e n u m b e r of gases present in t h e s a m p l e are selected. A s a m p l e is a u t o m a t i c a l l y d r a w n into t h e s a m p l e c h a m b e r , a n d t h e flow valves a r e closed. T h e laser source is t h e n t u n e d in s e q u e n c e to each of t h e selected w a v e l e n g t h s . T h e t u n i n g p a u s e s a t each w a v e l e n g t h to allow s a m p l e a b s o r p t i o n a n d laser power to be m e a s u r e d . At t h e end of t h e wavelength scan, t h e m i n i c o m p u t e r c a l c u l a t e s t h e c o n c e n t r a t i o n of t h e gases p r e s e n t in t h e s a m p l e . T h i s calculation, which is described above, requires a n u m b e r of w a v e l e n g t h s equal to t h e n u m b e r of different c o m p o n e n t gases p r e s e n t in t h e sample. T h e s y s t e m c h e c k s t h e consistency of t h e c a l c u l a t e d gas c o n c e n t r a t i o n s by using m e a s u r e m e n t s a t t h e e x t r a w a v e l e n g t h s also to c a l c u l a t e concentrations. Agreement between these different c a l c u l a t i o n s i n d i c a t e s that the calculations accurately represent the mixture composition. T h i s a u t o m a t i c consistency c h e c k i n g essentially e l i m i n a t e s t h e possibility t h a t a b s o r p t i o n p r o d u c e d by an u n s u s p e c t e d c o m p o n e n t of t h e mixture may cause measurement error. T h e results of t h e c o n c e n t r a t i o n and consistency c a l c u l a t i o n s are t y p e d out on t h e t e l e t y p e u n i t . T h i s information is also a v a i l a b l e a t t h e digital o u t p u t c o n n e c t o r for recording on m a g n e t i c t a p e or t r a n s m i s s i o n to a r e m o t e recording l o c a t i o n . T h i s s y s t e m h a s been designed to a n a l y z e m i x t u r e s of 10 gases w i t h a sensitivity of 1 p p b a n d a cycle t i m e for a c o m p l e t e analysis of 5 m i n . T h e rejection ratio b e t w e e n gases w i t h s i m i l a r a b s o r p t i o n s p e c t r a is g r e a t e r t h a n 200; t h e rejection r a t i o b e t w e e n gases with different s p e c t r a r u n s as high as 10 8 .
ANALYTICAL CHEMISTRY, VOL. 46. NO. 2. FEBRUARY
1974 • 243 A
T h e t e c h n i q u e of L O S gas analysis is e x p e c t e d to find a variety of a p p l i c a t i o n s . It offers a single s y s t e m which can a u t o m a t i c a l l y a n a l y z e a m i x t u r e of gases with high rejection ratios a n d sensitivity. O n e p a r t i c u l a r ly p r o m i s i n g field of a p p l i c a t i o n is gaseous air pollution d e t e c t i o n . A sin gle L O S s y s t e m can replace t h e col lection of single gas a n a l y z e r s which is c u r r e n t l y required to provide a c o m p l e t e analysis of a m b i e n t air. T h e L O S s y s t e m should b e c o m e a useful research tool. It can be a d a p t e d to m a n y specialized research uses byc h a n g i n g t h e c o m p u t e r software. It can be interfaced to a gas c h r o m a t o g r a p h (GC) by feeding t h e effluent gas from t h e c h r o m a t o g r a p h i c c o l u m n into t h e L O S s y s t e m . T h e small s a m p l e v o l u m e t h a t can be used for L O S gas analysis m a k e s this possible. It is possible to d e t e c t 1 p p b of analyte in 1 cc of inactive carrier gas. T h i s m e a n s t h a t p i c o g r a m sensitivity is possible in a c o m b i n e d G C - L O S system. A combined G C - L O S system has s o m e of t h e s a m e properties as a G C - M S (mass spectrometer) combi n a t i o n . However, it m a y offer a d v a n tages of s i m p l i c i t y , sensitivity, and cost. Since most G C carrier gases do not a b s o r b infrared, t h e r e is no need as t h e r e is in a M S , to remove t h e carrier gas. T h e s p e c t r u m recorded in a M S d e p e n d s on t h e m a s s fragmen t a t i o n p a t t e r n of t h e molecule, a n d t h e relative i n t e n s i t y of different p e a k s is n o t r e p r o d u c i b l e from one s y s t e m to a n o t h e r . T h e L O S spec t r u m d e p e n d s o n laser w a v e l e n g t h s a n d IR a b s o r p t i o n properties a n d is highly r e p r o d u c i b l e . It provides an a c c u r a t e m e t h o d for identifying G C p e a k s . I n d u s t r i a l process control is a n o t h e r field of a p p l i c a t i o n . T h e L O S s y s t e m provides real t i m e d a t a in a digital format t h a t c a n easily be in terfaced to a process control c o m p u t er.
References (1) Edwin Kerr and John At wood, Appl. Opt.. 7,915(1968). (2) L. B. Kreuzer, J. Appl. Phvs., 42, 2934 (1971). (3) "Handbook of Lasers," R. J. Pressley, Ed., Chemical Rubber Publ.. Cleveland, Ohio, 1971. (4) L. B. Kreuzer, N. D. Kenvon, and C. Κ. Ν. Patel, Science. 177, 3*47 (1972). (5) P. L. Hanst, Appl. Spectrasc. 24, 161 (1970). (6) R. T. Menzies, N. George, and M. K. Bhaumih, IEEE J. Quantum Electron., QE-6 (December 1970). (7) A. G. Bell, Phil. Mag.. 11,510(1881). (8) J.Tvndall, Proc Ro\. Soc. (London), 31,307(1881). (9) W.C.Rontgen. Phil. Mag.. 11,308 (1881). ( 10) "Infra-Red Physics," .1. T. Houghton and S. D. Smith, pp 276-78, Oxford, England, 1966. 244 A
