Thermal Spectroscopy
J. Chem. Educ. 1982.59:15. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 01/24/19. For personal use only.
M. R. Fisher and N. S. Nogar1 University of Nebraska-Lincoln, Lincoln, NE 68588
Thermal detectors have long been used to measure the intensity of incident radiation, particularly in the infrared. Traditional infrared absorption spectroscopy is carried out by measuring the attenuation of incident radiation using a heat-sensitive detector. Recently, however, various nontraditional thermal detectors have found increasing use in a form of excitation spectroscopy in which the energy actually absorbed by molecular species, rather than transmitted energy, is measured. This is a technique which we call thermal spectroscopy. In this paper we will describe and characterize the various forms of thermal spectroscopy, with emphasis on the similarities of the various methods. Particular attention will be paid to optoacoustic and thermal lensing spectroscopy, and to applications of these techniques. It is our intention to introduce thermal spectroscopy to those not familiar with the technique, and to indicate pertinent literature references for those with further interest. Generation and Direction of Thermal Signals Excess energy contained in a molecule following absorption of a photon can be dissipated in one of three ways. The molecule may (1) react photochemically, in which case the excess energy is used to break chemical bonds; (2) emit light, via fluorescence or phosphorescence, thereby reducing the internal energy; (3) or relax, in which case the internal (electronic or vibrational) energy of the molecule is converted to translational or thermal energy. It is the latter form of energy that is detected in thermal spectroscopy. Typically, the sample is illuminated with a pulsed or chopped light source, producing periodic fluctuations in the sample pressure, temperature, or density. By examining the magnitude of these fluctuations as a function of wavelength of the incident light, it is possible to obtain a spectrum of the sample under study. The thermal fluctuations produced by relaxation are typically monitored in one of two ways. If the compression-rarefaction pulse produced by the transient temperature jump is measured directly with a pressure transducer, such as a microphone or piezoelectric ceramic, the technique is termed optoacoustic or photoacoustic spectroscopy (1,2). Alternately, the pressure gradient may be probed optically, since the density change will also produce a change in refractive index. Inasmuch as the transient refractive index forms an effective lens, this technique is often called thermal lensing, or thermal blooming spectroscopy ()
~
Kt(v)NP(v)l(dn/dt)
(2)
where l is the path length, (dn/dt) represents the change in refractive index as a function of temperatures, and K reflects the geometry of the light source, position of the thermal lens relative to the detector, radiative lifetime, and wavelength of the exciting light. One feature which should be noted for both optoacoustic and thermal lensing signals is that they are both excitation techniques: that is, the amount of energy absorbed is measured, rather than a decrease in transmitted light. Therefore, short of saturation, the signals will scale linearly with power of the incident light, indicating that high intensity light sources are most useful for thermal lensing spectroscopy. In fact, thermal lensing spectroscopy was virtually unknown until the advent of high power lasers, and optoacoustic spectroscopy has undergone an enormous resurgence since lasers have come into popular usage. In this sense, thermal spectroscopy is quite similar to fluorescence spectroscopy. Other similarities also exist—both thermal and fluorescence spectroscopy are excitation techniques. In fact, they are complementary: thermal spectroscopy measures the photon energy which has been converted to heat, while fluorescence spectroscopy observes re-emitted photons. A typical experimental setup for optoacoustic spectroscopy of liquids or gases is shown in Figure 1. A tunable CW laser is interrupted with a chopper, and the transmitted light passed through the (usually cylindrical) cell. A pressure sensor is located in a sidearm of the cell. This sensor will typically be a commercial electric microphone for gas phase spectroscopy, and a hydrophone for solution work. Alternately, a simple gas phase microphone can be constructed by wrapping a sheet of aluminized mylar around a perforated cylinder (7). The cylinder is inserted in, and colinear with, the sample cell. For solid samples, one of the two detector configurations is typically used. Either the thermal signal can be coupled to a conventional microphone by a buffer gas (as in current cominer-
(1)
where S is the signal strength, e is the absorptivity, N the number density of absorbing material, P is the power (not intensity) of the light source, Q is a factor reflecting the quality of the excited acoustic mode, and K is a collection of constants reflecting among other things, relaxation and spontaneous Present address: University of California, Los Alamos Scientific Laboratory, CNC-2, MS 738, Los Alamos, NM 87545. 1
emission times, heat capacities of the sample components and
responsitivity of the detector. The Q factor will be affected by the geometries of both the cell and the exciting light, and by the chopping (pulse) frequency of the light source; for non-acoustically resonant cells, Q 1. Similarly, while a detailed consideration of thermal lensing signal detection is quite imposing (8, 9), a simplified analysis will reveal the salient features (4). Again, considering an optically thin nonfluorescing, sample,
LOCK-IN
STRIP CHART
Figure 1. Shows typical experimentai setup for optoacoustic spectroscopy. BS is a beam splitter, C a chopper, and the laser is usually a tunable dye laser for spectroscopic applications. Higher sensitivities can be achieved by placing the sample cell inside the laser cavity (15),
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troscopy, ppt detection limits have been reported for both optoacoustic and thermal lensing measurements (4, 13). Thermal spectroscopies have also been used for the detection of other low concentration species, including chemical and spectroscopic transients (14). “Forbidden” spectroscopic transitions have been examined by thermal spectroscopic triplet transitions, symmetry techniques, including singlet forbidden electronic transitions (3), and vibrational overtone and combination bands (5,15). The high sensitivity of thermal detection also has made acoustic detection useful for multiple photon events, including inverse Raman (16), or stimulated Raman spectroscopy, two photon spectroscopy (9), and infrared laser photochemistry (17). Fourier transform instruments have also utilized photoacoustic detection (11). Also, a number of interesting photophysical measurements can be carried out via thermal spectroscopy. Among these are included vibrational relaxation (2,18), fluorescence quantum yield (19), and reaction branching ratio measurements (1,20). Doppler-free spectroscopy (21) has also been carried out using thermal detection. Spectroscopy of solid samples may prove to be one of the most useful applications of optoacoustic spectroscopy (10). In short, thermal methods of detection are a very useful addition to the chemist’s bag of spectroscopic tricks, and deserve consideration for a wide range of photochemical and photophysical measurements. —
Figure 2. Apparatus for dual beam thermal tensing spectroscopy, after reference (12). BS is a beam splitter, C a chopper, L a positive lens, S the sample and P the beam stop containing a pinhole. The probe laser (#1) is typically a iow power fixed frequency laser, such as a HeNe or HeCd, while the heating laser (#2) is typically a tunable dye laser.
cial instruments (if)), or the sample can be attached directly to a piezoelectric transducer (11). The signal from any of these sensor elements typically will be sent into a pre-amplifier, and subsequently to a lock-in amplifier. A spectrum is taken by recording the lock-in output as a function of excitation wavelength. A power meter can be used to normalize the acoustic signal to the laser power. For thermal lensing experiments, a dual beam apparatus is usually most satisfactory (12) (Fig. 2) A fixed wavelength CW probe beam is combined on a beam splitter with the chopped, variable wavelength heating laser. The colinear beams are brought to a focus in front of the sample cell with a positive lens; a simple analysis indicates that optimum positioning for the sample cell is approximately one confocal. parameter from the focus (4). The probe beam then passes through a pinhole situated in front of a photodetector. As the sample is heated, the transient negative lens causes the probe beam to diverge, allowing less light to pass through the pinhole. Again, the signal is typically processed with a lock-in amplifier and displayed on a recorder. Applications of Thermal Spectroscopy The spectroscopic techniques described here are capable of very high sensitivities. This immediately suggests applications in trace analysis, and indeed this was one of the first applications of modern (laser-based) thermal spectroscopy. In gas phase photoacoustic work, sub-ppb detection levels are typically realizable (13), while for condensed phase spec-
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Literature Cited (1) Harshbarger, W. R. and Robin, M, B., Acct. Chem. Res., 6,329 (1973), (2) Pao, Y.-H. “Optoacoustic Spectroscopy and Detection,” Academic Press, New York, 1977; Colies, M. J., et al., Cont. Phys., 20,11 (1979); Rosengren, Lars-Goran, Appl. Opt, 14,1960 (1975). (3) Whinnery, J. R., Acct. Chem. Res., 7, 225 (1974); Hu, C. and Whinnery, J. R., Appl.
Opt., 12,72(1975).
(4) Harris, J. M. and Dovichi, N. J., Anal. Chem., 52,695A (1980); Kliger, D. S., Acct. Chem. Res., 13,129 (1980). (5) Long, M. E., Swofford, R. L., and Albrecht, A. C., Science, 191,183 (1976). (6) Rosencwaig, A. and Gersho, A., J. Appl. Phys., 47,64 (1976); Wrobel, S. and Vala, M., Chem. Phys., 33,93 (1978); Farrow, L. A. and Richton, R. E., J. Appl. Phys., 48,4962 (1977) ; Kerr, E. L. and Atwood, J. G., Appl. Opt., 7,915 (1968). (7) Kreuzer, L. B., J. Appl. Phys., 42, 2934 (1971). (8) Fang, H. L. and Swofford, R. L., J. Appl. Phys., 50, 6609 (1979). (9) Twarowski, A. J. and Kliger, D. S., Chem. Phys,, 20, 253,259 (1977). (10) Blank, R. E. and Wakefield, T., II. Anal. Chem., 51,50 (1979). (11) Farrow, M, M., et al., Appl. Opt., 17,1093 (1978); Farrow, Appl. Phys. Lett., 33, 735
(1978) (12) Swofford, R. L. and Morrell, J. A., J. Appl. Phys., 49, 3667 (1978). (13) Koch, K. P. and Lahmann, W., Appl. Phys, Lett., 32, 289 (1978); W. Lahmann et ah, Anal. Chem., 49, 549 (1977). (14) Patel, C. K. N., Kerl, R. J-, and Burkhardt, E. G., Phys. Rev. Lett., 38, 1204 (1977). (15) Bray, R. G. and Berry, M. J., J, Chem. Phys., 71, 4909 (1979). (16) Siebert, D. R., West, G. A., and Barret, J. J., Appl. Opt,, 19, 53 (1980). (17) Quigley, G. P. Opt Lett., 4,84 (1979). (18) Siebert,D. R., Grabiner, F. R., and Flynn, G. W., J. Chem. Phys., 60,1564 (1974). (19) Rockley, M. G., Chem. Phys. Lett., 50, 427 (1977); Chem. Phys. Lett,, 54, 597 (1978). (20) Hunter, T. F. and Kristjansson, K. S., Chem. Phys. Lett., 58, 291 (1978). (21) Marineto, E. E. and Stuke, M., Opt. Comm., 30,349 (1979). .
