Optoacoustic Spectroscopy and Detection

245 Pages·1977·3.377 MB·English

by  Yoh-Han Pao (Eds.)

Contributors
P. C. Claspy
C. Forbes Dewey, Jr.
Jerry A. Gelbwachs
P. L. Kelley
Lloyd B. Kreuzer
M. B. Robin
Allan Rosencwaig
John D. Stettler
Norman M. Witriol
OPTO ACOUSTIC
SPECTROSCOPY
AND
DETECTION
Edited by
YOH-HAN PAO
Department of Electrical Engineering
and Applied Physics
School of Engineering
Case Western Reserve University
Cleveland, Ohio
ACADEMIC PRESS New York San Francisco London 1977
A Subsidiary of Harcourt Brace Jovanovich, Publishers
COPYRIGHT © 1977, BY ACADEMIC PRESS, INC.
ALL RIGHTS RESERVED.
NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR
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ACADEMIC PRESS, INC.
Ill Fifth Avenue, New York, New York 10003
United Kingdom Edition published by
ACADEMIC PRESS, INC. (LONDON) LTD.
24/28 Oval Road, London NW1
Library of Congress Cataloging in Publication Data
Main entry under title:
Optoacoustic spectroscopy and detection.
Includes bibliographies.
,
1. Optoacoustic spectroscopy. I. Pao, Yoh-han.
QD96.06067 535 .84 76-52728
ISBN 0-12-544150-9
PRINTED IN THE UNITED STATES OF AMERICA
List of Contributors
Numbers in parentheses indicate the pages on which the authors' contributions begin.
P. C. Claspy (133), Department of Electrical Engineering and Applied
Physics, Case Western Reserve University, Cleveland, Ohio
C. Forbes Dewey, Jr. (47), Department of Mechanical Engineering, Mas­
sachusetts Institute of Technology, Cambridge, Massachusetts
Jerry A. Gelbwachs (79), Chemistry and Physics Laboratory, The Aerospace
Corporation, El Segundo, California
P.L.Kelley (113), Lincoln Laboratory, Massachusetts Institute of Tech­
nology, Lexington, Massachusetts
Lloyd B. Kreuzer (1), Hewlett-Packard Laboratories, Palo Alto, California
M. B. Robin (167), Bell Laboratories, Murray Hill, New Jersey
Allan Rosencwaig* (193), Bell Laboratories, Murray Hill, New Jersey
John D. Stettler (27), U.S. Army Missile Research, Development, and
Engineering Laboratory, Redstone Arsenal, Alabama
Norman M. Witriol (27), U.S. Army Missile Research, Development, and
Engineering Laboratory, Redstone Arsenal, Alabama
* Present address: Lawrence Livermore Laboratory, Livermore, California.
vii
Preface
This book is an introduction to the principles and practice of
optoacoustic spectroscopy and is intended to be of use in at least the following
three contexts:
(1) as a treatise for self-study by researchers who wish to approach
the subject on a broad intellectual basis;
(2) as a reference, containing many quantitative but simple results,
considerable data, and an extensive bibliography; and finally
(3) as a guide, to serve as a basis for evaluation of the feasibility of using
such techniques in specific instances.
The book consists of eight articles, organized in the form of eight inde­
pendent chapters. These are original articles written by men who have
contributed to the recent rapid development in this field. The articles vary
in the extent to which they are expository or review, depending on the nature
of the material covered.
In any first reading of this treatise, different groupings of chapters
would be of greater interest and relevance, depending on the principal
interest of the reader.
For example, in the early 1970s, the initial resurgence of interest in
optoacoustic spectroscopy was primarily due to the high sensitivity attained
in the use of the method for the detection of trace amounts of gas pollutants
in the earth's atmosphere. The increased sensitivity was primarily due to
the use of lasers as sources of excitation. The spectral region was that of
infrared, and the physical mechanism responsible for the effect was that of
vibrational-translational energy transfer in the gas. These types of results
have been reproduced by many other investigators and the optoacoustic
technique is by now probably the preferred method f7or detecting traces of
gas contaminants at any level less than 1 part in 107 a-nd 1for measuring gas
adsorption coefficients with values less than 10" cm .
In this book, the reader interested primarily in the detection and identi­
fication of gas contaminants would read Chapter 1 by Kreuzer to obtain a
ix
X Preface
general introduction and a physical description of the generation and
measurement of the optoacoustic signal. The description so obtained would
be macroscopic and thermodynamic in nature. Chapter 2 by Stettler and
Witriol provides an understanding of the optoacoustic effect on a molecular
scale, with descriptions of the energy transfer processes and estimates of the
lifetimes of vibrationally excited states. In Chapter 3 Dewey makes known
the options available to the researcher in the choice of optoacoustic system
design. He compares the capabilities and limitations of various optoacoustic
system designs and presents quantitative relations for predicting system
performance. Applications and specific results are discussed by Claspy in
Chapter 6, with emphasis on the use of CO and C0 2 lasers in the 5-6 /im
and 9-12 /im wavelength regions. With these two types of lasers, excitation
may be achieved at a very large number of different discrete wavelengths.
Although such lasers are tunable, they are not continuously tunable and
cannot, for example, be used in the wavelength modulation mode; nor are
they appropriate for molecules that absorb, say, in the 7 /mi region. In the
latter circumstance, in the infrared, either parametric oscillator or tunable
diode lasers may be used and these are discussed by Kelley in Chapter 5.
On the other hand, the researcher interested in solid state optoacoustic
spectroscopy might want to start off by reading Chapter 8 by Allan Rosen-
cwaig. For the visible spectral region, good nonlaser light sources are avail­
able and Rosencwaig shows that very interesting results can be obtained even
with incoherent light sources. In particular, the technique can be used for
exploring the absorption spectra of substances such as powders, gels,
adsorbed films, and even organic tissues. Such substances normally scatter
so much light or are in a physical form such that absorption spectroscopy
techniques cannot be used for investigative and characterization purposes.
Optoacoustic spectroscopy promises to be very useful in this area and may
prove to be even more so once tunable lasers are used for excitation. Laser
sources and especially tunable lasers available for the ultraviolet and visible
wavelength regions are discussed by Gelbwachs in Chapter 4. Photochemists
and organic chemists are likely to find Chapter 7 by Robin on optoacousti-
cally determined electronic spectra to be of particular interest.
To date, the optoacoustic effect has been employed in a wide variety of
experimental situations, each having its particular needs with respect to
sensitivity, accuracy, dynamic range, sample conditioning, and measure­
ment. Accompanying these applications, there have been further significant
advances in techniques, some examples of which are the use of second
harmonic and wavelength modulation for discriminating against window
noise and the use of resonant and/or multipath simple cells for increasing
the signal level.
Preface xi
Heretofore, accounts of these advances and the many other results and
discussions contained in this book could be found only in widely scattered
technical journal literature. One of the purposes of this volume is to make
these available to the researcher in a collected and coordinated format.
1
The Physics of Signal Generation
and Detection
Lloyd B. Kreuzer
Hewlett-Packard Laboratories
Palo Alto, California
I. Introduction 1
II. Absorption of Light 2
III. Excitation of Sound 5
A. Normal Mode Amplitudes 5
B. Calculation of Q 10
C. Thermal Fluctuation Noise 11
IV. Signal Detection 12
A. Microphone Mechanical Model 13
B. Microphone Electrical Model 17
C. Electrical Noise 19
V. Optimum Design of an Optoacoustic System 20
VI. Numerical Example 22
References 25
I. Introduction
In optoacoustic spectroscopy light energy is first converted into
sound and then into an electrical signal (Hill and Powell, 1968). Figure la
represents a simplified experimental arrangement. Light from the laser is
modulated by passing it through a rotating "light chopper." The modulated
beam then passes through a container that holds a gas sample. Energy
absorbed by the gas from the beam heats the gas and causes its pressure to
rise. Since the beam is modulated, this pressure rise is periodic at the beam
1
2 Lloyd B. Kreuzer
GAS CONTAINER
J
MICROPHONE
MODULATOR -
(LIGHT CHOPPER)
AMPLIFIER
(a)
NOISE
I
GENERATION 8 AABBSSOORRPPTTIIOONN EEXXCCIITTAATTIIOONN DETECTION
MMOODDUULLAATTIIOONN OF RADIATION OF SOUND OOFF SSOOUUNNDD AAMMPPLLIIFFIICCAATTIIOONN
OOFF LLIIGGHHTT BBEEAAMM
SIGNAL + NOISE
(b)
Fig. 1. Laser optoacoustic spectrometer, (a) Diagram of the apparatus, (b) The steps in
the generation of the optoacoustic signal.
modulation frequency. It is detected by a microphone and converted into
an electrical signal. The electrical signal is fed into the input of an amplifier
Figure lb represents the stages of this process. At each stage of the process
there will be mechanisms which add noise and degrade the ability to detect
a small signal. The existence of noise sources is included in Fig. lb.
This chapter provides a physical description of the generation and
measurement of the optoacoustic signal. Noise sources that come from
thermal fluctuations and noise at the input of the amplifier are discussed.
The chapter concludes with a short section on optimum design and a
numerical example.
II. Absorption of Light
The first step in the generation of the optoacoustic signal is the absorption
of energy from the modulated light beam. This absorbed energy produces
a periodically varying heat source in the gas that acts as the source of sound.
1. Signal Generation and Detection 3
In this section the production of this heat source and its dependence on
the properties of the incident b-2eam an1d the absorbing gas is described.
Let the intensity in ergs cm sec" of the light beam be given by /(r, t),
where r describes the position and t is the time, and let the heat source
produced by this- 3beam be H(r,t). H has dimensions of power per unit
volume (erg cm sec). In many common experimental situations, H and
/ are related by a simple proportionality constant a, which
H = al - 1 (1)
has the dimensions of reciprocal length (cm ) and is called absorbance,
as measured by the optoacoustic method. Equation (1) is valid when two
conditions are satisfied simultaneously. First, the intensity / is sufficiently
small so that the absorbing transition is not saturated. Second, the time
variation of / is much slower than the rate of transfer of absorbed energy
from the absorbing transition into heat. If the first condition is not satisfied,
then the relationship between H and / becomes nonlinear, and H will contain
frequency components not present in J. For example, if / has a sinusoidal
time dependence of frequency co, then H will have frequency components
at oj and 2co as well as other frequencies with amplitudes related in a non­
linear way to the amplitude of /. On the other hand, if the first condition is
satisfied, but the second is not, then the time variation of H will lag behind
that of /, and the constant a in Eq. (1) is replaced by a term that is frequency
dependent. If the Fourier transforms of /(r, t) and H(r, t) are /(r,co) and
H(r, co), then
H(T,OJ) = a(co)/(r, co). (2)
If both conditions are not satisfied, then the behavior can be very complex
and quantum mechanical coherent effects may be important. Although this
type of effect undoubtedly can be detected by optoacoustic means it has not
been of importance, up to now. It is not discussed in this chapter. In the
absence of quantum coherent effects, a rate equation treatment (Kaiser, 1959;
Kreuzer, 1971) is adequate to connect / and H.
In order to illustrate the rate equation approach, a summary of equations
describing a simple two-level system is given. Let N be the density of ab­
sorbing molecules, n1 the density of absorbing molecules in the excited
state, hv the energy of the transition, Av the linewidth of the transition,
S the line strength of the transition, T r the radiative lifetime, and T c the
collisional decay time of the upper state. The equation describing upper
state population follows in a straightforward manner assuming that the
only paths for decay of the upper state are radiation and collision-induced
decay: