J . Phys. Chem. 1990, 94, 2851-2865 lations of vibrational frequencies. These should help in providing more precise answers to some of the questions raised in this study.
Acknowledgment. We are grateful to Prof. Ernest Eliel for providing several of the compounds used in this study, critical
2857
review of the manuscript, and many enlightening discussions about the data analysis. In addition, we would like to thank GLAXO Inc. and the National Science Foundatin for financial support of this work. We are also grateful to Dr. William Murray for his efforts in the synthesis of several compounds.
TlmeResolved Absorption Changes of Thin CS2 Samples under Shock Compression: Electronic and Chemical Implications C.S . Yoo* and Y. M. Gupta Department of Physics, Washington State University, Pullman, Washington 991 64-281 4 (Received: September 18. 1989)
Electronic and chemical changes in CS2shocked to 12 GPa have been examined by using time-resolved absorption measurements. Experiments were carried out on thin samples (=l pm) to provide good resolution of the absorption spectrum between 280 and 500 nm in pure CS2and to separate pressure, temperature, and time effects. In addition to the V-band existing at ambient conditions, a new band is observed on the red side of the V-band at high pressures. Unlike the V-band, the peak position and intensity of the new band vary substantially with pressure and temperature. The new band shows a "hot" band character and is conjectured to be the T-band resulting from a transition to the 'A, state. The edge shifts of the absorption band, at a given pressure, decrease with decreasing temperature. The time-dependent increase in absorbance and the magnitude of the edge shifts cannot be reconciled with temperature changes and are believed to represent changes in the electronic structure due to diffusive molecular motions. Spectral changes are irreversible above 11-12 GPa due to a shock-induced chemical reaction. The absorption changes from this and previous work suggest that an associative type of chemical reaction occurs in shocked CS2.
I. Introduction The electronic structure of molecules can be altered significantly by increasing pressure and temperature. These electronic changes can either directly or indirectly initiate chemical reactions, particularly in unsaturated CS2 is a good example of a molecule that, under compression, shows large spectral changes and undergoes irreversible chemical reactions. The spectral and chemical changes in CS2 have been studied previously under both and shock c o m p r e ~ s i o n . ~Shock ~ wave studies on CS2 have led to the following developments: measurement of a break in the CS, Hugoniotq6development of an equation of state for CS2,' experiments to establish a chemical reaction in CS2and to measure the kinetic parameters of this reaction,8 and application of time-resolved spectroscopy to examine spectral change^.^ Changes in the transmission spectrum of shocked CS,, in the near-UV and visible region, revealed substantial changes including a shift of the red edge of the 320-nm band toward the red at a rate of approximately 20 nm/GPa.Io Further studiesIi-l3 on the transmission spectrum of shocked CS2 by Duvall and his coworkers have led to a quantification of this shift as a function of pressure and temperature, to the finding that the edge shift is irreversible at pressures greater than 9 GPa, and to a recognition of the difference between the shift observed in shock compressed CS2 and that observed in statically compressed CS2. Recently, we have reported on spectral changes of shocked CS2/hexane mixtures under step wave 10ading.I~ It was evident that absorption band changes with pressure include intensity enhancement, asymmetric broadening of the band, and the red shift of the band, all of which were dependent on the CS2 concentration. A molecular model, based on the staggered-parallel orientation of CS,,was suggested to explain the experimental results in the mixtures. This molecular orientation should involve a diffusive molecular rotation on nanosecond time scale; however, evidence of such motion has not been obtained. Part of the *Author to whom correspondence should be addressed. Present address: Physics Department, Division H, P.O. Box 808, Lawrence Livermore National Laboratory, Livermore, CA 94550.
0022-3654f 90 f 2094-2851$02.50 f 0
difficulty is that, in step wave loading of thick (100-200 pm) samples, it is difficult to separate pressure, time, and temperature effects. The CS2/hexane mixture studyi4 also revealed a new feature, on the red side of the absorption band centered a t 320 nm, for some of the mixtures. This leads to the possibility that CS2 absorption band characteristics at high pressures are different from these at ambient conditions; examination of the band edge alone, as was done in previous studies,"I would not resolve this issue. The work reported here is a continuation of earlier efforts to understand spectral and chemical changes in shocked CS2. Unlike previous studies, we examined the full absorption band of pure CS2,between 280 and 500 nm, using very thin (approximately 1 pm) samples. The present work had three main objectives: (i) to resolve the full absorption band of pure CS2 to achieve an improved understanding of the pressure and temperature effects on the electronic transitions; (ii) to determine temporal changes ( I ) Yakushev, V. V.; Nabatov, S. S.; Yakushev, 0. B. h k 1 . Akad. Nauk USSR 1973, 214, 879. (2) Duvall, G. E. 'Electronic Spectra of Various Liquids under Shock Compressions";ONR Annual Report, No. NOOO14-77C-0232, 1985. (3) Bridgman, P. W. Proc. Am. Acad. Arts Sci. 1942, 74, 399. (4) Whalley, E. Can. J . Chem. 1960, 38, 2105. (5) Agnew, S. F.; Mischke, R. E.; Swanson, B. I. J . Phys. Chem. 1988,92, 4201. (6) Dick, R. D. J . Chem. Phys. 1970, 52,6021. (7) Sheffield, S. A.; Duvall, G. E. J . Chem. Phys. 1983,79,1981. See also: Sheffield, S. A. Shock-Induced Reaction in Carbon Disulfide. Ph.D. Thesis, Washington State University, 1978. (8) Sheffield, S. A. J . Chem. Phys. 1984, 81, 3048. (9) Duvall, G. E.; Ogilvie, K. M.; Wilson, R.; Bellamy, P. M.; Wei, P. S. P. Nature 1982, 296, 846. (10) Ogilvie, K. M.; Duvall, G. E. J . Chem. Phys. 1983, 78, 1077. ( I 1) Duvall, G. E.; Granholm, R. H.; Bellamy, P. M.; Hegland, J. E. Effect of Temperature on the UV-Visible Spectrum of Dynamically Compressed CS2. In Shock Waves in Condensed Matter; Gupta, Y . M., Ed.; Plenum: New York, 1986; p 213. (12) Duvall, G. E. "Optical Spectroscopy of Dynamically Compressed Liquids"; ONR Final Report, No. NOOO14-77C-0232, 1986. (13) Yoo, C. S.; Furrer, J. J.; Duvall, G. E.; Agnew, S. F.; Swanson, B. 1. J . Phys. Chem. 1987, 91. 657. (14) Yoo, C. S.; Duvall, G. E.; Furrer, J. J.; Granholm, R. J . Phys. Chem. 1989, 93, 3012.
0 1990 American Chemical Society
2858
Yo0 and Gupta
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990
L
Collecting Mirror
1
Figure 1. Schematic view of the experimental layout for time-resolved absorption measurements under shock loading. Light from the Xe lamp is transmitted through the sample and is recorded by a system consisting of a 0.3-m spectrograph,electronic streak camera, vidicon detector, and OMA. MI through M5are UV-enhanced front surface mirrors.
in the absorption behavior after the attainment of peak pressure; for the I-pm sample, the peak pressure is attained in a few nanoseconds; (iii) to determine temperature effects by comparing data from thick samples, thin samples, and thin samples subjected to ramp wave loading. Experimental methods related to time-resolved absorption measurements and thin cell experiments are described in section 11. Spectral variation with pressure, temperature, and time are presented in section 111. Discussion of band edge shift, “hot” band formation, time-dependent absorption increase, and possible mechanisms for shock-induced chemical reactions is presented in section IV.
It. Experimental Method A . Time-Resolved Absorption Measurements. The overall experimental set-up for obtaining time-resolved absorption measurements is shown in Figure l , A single-stage gas gun15 which can accelerate projectiles 10.2 cm in diameter and weighing approximately 1 kg to desired velocities was used to generate the shock waves which compress the samples. Further details regarding gas run experiments including the method to measure projectile velocity just before impact have been described elsewhere.8,10*15 Two turning mirrors (M,, M2), an iris, and the impactor are mounted on the projectile. A pulsed Xenon flash lamp (457 A, Xenon Corp., MA) is used as a broad band light source. The lamp is operated at approximately 60% of its explosion energy, so that a structureless broad band with full width at half-maximum (fwhm) greater than 200 nm is obtained. The intensity and spectral shape of the lamp are reproducible from shot to shot within 2% deviation over the spectral range (300-500 nm) examined; the deviation is somewhat larger outside this range. Xe light is collimated to approximately 75 mm in diameter by an off-axis parabolic reflector and is reflected by a turning mirror (M,) which defines the size of the beam on the surface of the projectile. A uniform image, approximately 50 mm by 25 mm, is formed at the I O mm square mirror (M2) placed on the edge of the projectile. In this manner, it is possible to obtain a spectrum with a f30-deg rotation of the projectile without a significant loss of light. The light beam passing through the sample is restricted to the center region by the iris, in the projectile, which has a circular opening 3 mm in diameter. This opening ensures that the sample is in a state of uniaxial strain for the 1 p s of observation. The Xe light is transmitted through the sample and is collected by using a spherical collecting mirror and a turning mirror, M5. A spectrograph (1681 C, SPEX, NJ) and an electronic streak camera (160, Cordin, UT) are used to disperse the light in (15) Fowles, G .R.; Duvall, G. E.; Asay, J.; Bellamy, P.; Feistmann, D.: Gray, T.: Michaels, T.; Mitchell, R. Reo. Sci. Instrum. 1970, 41, 984.
wavelength and in time, respectively. The output image is then recorded by a two-dimensional diode array detector (I252 B, EG&G, NJ) and is read by an optical multichannel analyzer (1460-V, EG&G, NJ). The pulse width of the lamp is approximately 6 p s at halfmaximum intensity and is approximately 3 p s at a steady maximum intensity (within 10% variation). We trigger the lamp approximately 4 p s before impact so that the shock events of interest, approximately 1 p s long after the time of impact, occur at the center of the steady maximum intensity. The electronic camera and the OMA are triggered few hundred nanoseconds prior to impact. Fifty different tracks of each spectrum are continuously sampled for 2.5 p s which is long enough to cover the shock events of interest. The typical resolution of the system is estimated to be approximately 20 ns in time and 2 nm in wavelength, and the spectra are obtained every 50 ns. The optical system described above measures three single beam spectra: a sample spectrum I, which is the transmitted Xe light through pure CS2 and cell; a reference spectrum I , through the empty cell; and a structureless background spectrum Ib which represents the dark count of the system. The spectral characteristics of the reference (sapphire or fused silica windows) spectra in the range between 250 and 600 nm are not changed for the range of pressures and temperatures of interest. Therefore, reference and background spectra are obtained prior to the shot, and only the sample spectra are collected during the shot. An absorption spectrum of CS2 is then calculated as follows:
where t and X represent the track time and wavelength. Precision in measuring the absorbance is dependent on several parameters including spectral width and reproducibility of the Xe light, absorbance of the sample, resolution of the system, dynamic range of the array detector, etc. In the system described here, uncertainty in measuring the absorbance is estimated to be less than 10%at 320 nm. Transmitted light after impact but prior to the shock reaching the CS2 sample provides a good check on the experimental precision. B . Step Wave and Ramp Wave Loading of Thin CS2. Two types of impact experiments, step wave loading (SWL) and ramp wave loading (RWL) of thin CS2 samples, are reported here. Sample holders or cells for these loading conditions are shown in Figure 2. I n the SWL experiments, the CS2 is contained between two z-cut, sapphire disks (Union Carbide, CA) used as optical windows (the front window is 31.75 mm in diameter and 3.18 mm thick, and the back window is 25.4 in diameter and 1.7 mm thick). Compression of the sample results from the shock wave generated when a third sapphire disk (25 mm diameter and 12.7 mm thick) impacts the front window. The forward running shock wave reaches the sample approximately 280 ns after impact and reverberates (SWL) between the boundaries formed by the CS2 and sapphire windows to the peak pressure. For the 1-2 pm thick samples the peak pressure is obtained in a few nanoseconds, which is almost instantaneous for our system time resolution of 20 ns. The thin back sapphire window ensures that the sample is unloaded to ambient pressure while in a state of uniaxial strain (prior to the arrival of edge waves). In two of the S W L experiments, the rear window was thicker (12.7 mm) and no unloading data were obtained. In the RWL experiments, the CS2 is contained between front, fused silica window (Dynasil 1000, Adolf Mellor, RI) that is 46 mm in diameter and 12 mm thick, and a rear, z-cut sapphire window that is 25.4 mm in diameter and 12.7 mm thick. The impactor for the RWL experiments is a fused silica disk (47 mm in diameter and 20 mm thick). Because of the anomalous compression of fused silica, the compression wave propagating through the front window is a ramp wave whose rise time is given by the thickness of the fused ~ i l i c a . ~ ~ ~ ’ ’ ~~~~
~~
~~
(16) Barker, L. M.; Hollenbach, R. E. J . Appl. Phys. 1970, 41, 4208.
Shock Compression of Thin CS2
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The pressure history in the CS2can be obtained by using wave propagation calculations and the equations of state describing the shock response of CS2,' z-cut sapphire,I6 and fused s i l i ~ a . Such ~~?~~ calculations have been reported previously for thicker samples (100-200 pm).9*13Figure 3 shows the results of a similar calculation to obtain pressure histories for l pm thick CS2 subjected to SWL and RWL. In the SWL cell, the shock wave reaches the sample 280 ns after impact and the CS2almost instantaneously rings up to a peak pressure of IO GPa. After 350 ns, the release wave brings the pressure down to zero. In the RWL cell, the compression wave reaches the cell approximately 1.95 ps after impact and the peak value of 4.1 GPa is attained after a 350-ns rise time; the use of fused silica instead of sapphire results in the lower pressure. No unloading data are obtained for ramp wave loading. The accuracy of the calculated CSz pressures has previously been confirmed to 3.5 GPa under step wave loading.I8 In the present work, we are concerned only with the peak pressures that are determined solely by the shock response of sapphire and fused s i l i ~ a . ' ~Because ?'~ the uniaxial strain response of these materials is known to within 1-2%, the calculated pressure values in the present work are very accurate. Temperature calculations, on the other hand, can have considerable uncertainties because there are no reliable experimental methods to measure temperature at these compressions. It is even difficult to estimate the magnitude of the errors. In our calcu(1 9) Gustavsen, R. Time-Resolved Reflection Spectroscopy on Shock lations, we use the equation of state developed by Sheffield and Compressed CS2. Ph.D. Thesis, Washington State University, 1989. Duval17 which is based on most available data on C S 2 . A tem(20) Carslaw, H. S.; Jaeger, J. C. Conduction o j H e a f in Solids; Oxford: University Press: London, 1948; Chapter XI,pp 239-271. (21) Collins, R. "Effect of Thermal Diffusion of Temperature for Thin (17) Comer, M. Shear Wave Measurements to Determine the Nonlinear Cells of CS,"; SDL Internal Report 83-02, Washington State University, 1983. Elastic Response of Fused Silica Under Shock Loading. M.S. Thesis, Washington State University, 1988. ( I 8) Sutherland, G. T.; Gupta, Y.M.; Bellamy, P. M. J . Appl. Phys. 1986, 59, 1141.
(22) Bridgman, P. W. The Thermal Conductivity of Liquids Under Pressure. In Collected Experimental Papers; Harvard Press: Cambridge, MA, 1964; Vol. 3, pp 158-159.
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The Journal of Physical Chemistry, Vol. 94, No. 7, 1990
Yo0 and Gupta
TABLE I: Cell Constants for Thin CS2 Samples and the Thickness Calibration cells” I 2 3 4 5 6
I,,,,~p m
kc: L/mol
2.5 5.0 7.5 10.0 12.5 22.5
0.788 1.602 2.423 2.596 3.869 6.120
t,,d h m 2.90 5.89 8.91 9.54 14.22 22.50
At/t 0.138 0.151 0.158 0.048 0.121 0.000
“The thickness of cell 6 is used as a reference. * t , indicates the thickness measured either by the interference pattern of the cell or a micrometer. C k ,is the cell constant obtained from a plot of absorbance vs concentration. “ t , is the calibrated cell thickness obtained from
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is necessary to determine the cell thickness in situ, after the impact, but before the shock enters the sample. This can be done by measuring the absorbance of the CS2absorption band at 320 nm and using a calibration curve for the cell thickness obtained by the following procedure. Six cells, listed in Table I, were constructed and their thicknesses, t,, were measured initially by a micrometer and/or an interference pattern. Absorbances of 10 CS2/hexane mixture samples were measured in each cell. The cell constant for each cell, k,, is then determined from Beer-Lambert law23 A = ttC = k,C
0
0
2
8
10
(pml
Figure 4. Calibration curve relating the absorbance to the CS2sample thickness at ambient condition. The absorbance shown is the value at peak position of the V-band centered around 320 nm. The solid line represents the second-order polynomial fit. 0 ,
where t and Care the thickness of the cell and CS2concentration, respectively. c is a molar extinction coefficient and is approximately 100 L/(mol cm) at the peak of the V-band near 320 111-n.~~ k , is a cell constant that can be obtained from the slope of the absorbance vs concentration plot. The resulting cell constants for the cells are summarized in Table I. Because of the error in measuring the cell thickness for thin samples, the measured thickness, t,, is corrected as follows
6
4
THICKNESS
I
6
m
U
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f f l ’
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a where tCiand rJef are the corrected cell thicknesses of the ith cell and a reference cell, respectively. Because of its large thickness cell 6 was used as the reference cell. The corrected thicknesses of the mixtures, t,, shown in Table I are then converted to those of pure CS2 samples by using a linear relation of absorbance to concentration (Beer’s law).2s Finally, the absorbance for each mixture is plotted as a function of pure CS2thickness, as shown in Figure 4. The line is a least-squares fit to the points in Figure 4 and is given by A320 nm = 0.207222 0.004t2 ( t in km) D. Spectral Range. Studies in the spectral region below 280 nm are limited by several factors. These include increase in sapphire reflectivity, rapid decrease in Xe-light intensity and decrease in system efficiency, and particularly complicated and strong absorption features in CS2. The absorption features below 280 nm are mainly due to the transitions to three excited states: I&+, Ill,, and 311n.These bands were found at 215, 245, and 275 nm, respectively, in solid CS2.26 These transitions are more strongly allowed than any transition beyond 280 nm. For example, their reported oscillator strength^^^-^' range from approximately 1 for the transition to the state to 0.01 for the transition to the 3rIg state; these values are approximately 5000-50 times stronger than the V-band. Therefore, it is not practical for us to resolve the spectrum below 280 nm without using nanometer-thick cells. In addition, overlap of the bands and appearance
+
(23) Kolthoff, I. M.; Sandell, E. B.; Bruckenstein, S. Quontiratiue Chemical Anulysis, 4th ed.; Macmillan: New York, 1971; Chapter 55 pp 964-983. (24) Stern, E. S.; Timmons, C. J. Electronic Absorption Spectroscopy in Organic Chemistry. 3rd ed.; Edward Arnold: London, 1970; p 205. ( 2 5 ) Beer, A. Ann. Phys. 1952, 86, 7 8 . (26) Monahan, K. M.; Russell, K. L.: Walker, W. C. J . Chem. Phys. 1976, 65, 21 12. (27) Rabalais. J. W.; McDonald, J. M.; Scherr, V.;McGlynn, S.P. Chem Rev. 1971, 71, 7 3 .
N
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440
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Figure 5. Absorption spectra of pure CS2 compressed by step wave loading to 2.6 GPa. The formation of a new band around 360 nm and enhancement of the absorbance with time can be seen.
of the blue edge of the V-band near 300 nm further preclude an examination of these bands using their edge positions. Therefore, we focus only on the spectrum beyond 280 nm. The major absorption feature of CS2 in the region between 280 and 500 nm is the band centered at 320 nm and known as the V-absorption band.28 The V-band is a partially allowed band and has a relatively small extinction coefficient, approximately 100 L/(mol ~ m ) Therefore, . ~ ~ details of this band can be seen in few micrometers thick, or thinner, CS2 layers. 111. Results As indicated above, experimental results reported here are
concerned primarily with shock-induced changes in the absorption spectra of CS2 in the 280-500-nm range. A total of eight experiments were performed and the relevant parameters are summarized in Table 11. Five of the experiments were designed to examine changes in the spectra upon unloading. The absorbance of the V-band after impact but prior to shock compression of the CS2 is used to obtain the sample thickness from the calibration curve shown in Figure 4. Pressure, temperature, and density changes reported in Table I1 are calculated values as discussed in the previous section. The last two columns in Table 11 are ( 2 8 ) Jungen, Ch.; Malm, D. M.: Merer, A. J. Can. J . Phys. 1973,51, 1471
The Journal of Physical Chemistry, Vol. 94, No. 7, I990 2861
Shock Compression of Thin CS2 TABLE 11: Summary of the CS2 Experiments
expt no. 1 2 3 4
5 6 7 8
cell details
type' S S R R R R R ramp
Amax
0.28 0.26 0.41 0.34 0.25 0.58 0.19 0.02
calculated values
impactor velocity,
t,Irm 1.32 1.23 1.89 1.57 1.16 2.66 0.90 0.1 1
km/s 0.1 170 0.1497 0.3366 0.3967 0.4595 0.5293 0.5295 0.4373
edged shift,
P , GPa
T,C K
VIVO
nm
2.55 3.29 7.56 8.96 10.51 12.04 12.05 4.09
515 547 658 725 766 813 813 60 1
0.7054 0.6806 0.6036 0.5890 0.5764 0.5645 0.5644 0.66 19
40 3 45 f 5 93 4 114 f 2 122 5 > 1 7OC > 170' 36 f 2
band reversal
* *
Yes Yes Yes no
no
"S and R represent a standard and a reversible cell, respectively. Ramp represents a ramp wave cell. For details, see Figure 2 in the text. *Amax represents the maximum absorbance of the V-band prior to the shock compression of the CS2. CThecalculated temperatures have not been corrected for thermal conduction. dThe edge position has been defined as the wavelength position where the linear extrapolated line of the red side of the CS2 absorption band meets the base line. 'The undefined shift represents a shift greater than the spectral region covered in the experiment. This large shift is due to a chemical reaction.
discussed in detail in the following subsections. A . Changes in the Absorption Spectrum. Figure 5 shows typical absorption spectra at ambient conditions and at 2.6 GPa attained under step wave loading. The two 2.6-GPa spectra shown are at two different times after the CS2 attains peak pressure. Within experimental precision, no further changes are seen in the spectra after 150 ns. Two major changes are evident from the spectra in Figure 5. (i) A new feature is developed, on the red side of the V-band, near 360 nm. Because the V-band position remains nearly the same, the edge position of the CS2 absorption band is determined by this new feature. This new feature is similar to that observed previously in shocked CS2/hexane mixtures.I4 For example, in the 10% mixture a similar feature appears at 330 nm at 2.4 GPa and it is shifted to approximately 337 nm at 4.3 GPa. (ii) The intensity of the overall absorption band increases with time at a constant pressure. At 2.6 GPa, the peak position of the V-band seems to move toward the red with time; for example, it was at approximately 325 nm initially and is at 340 nm after 150 ns. However, the red edge position of the overall absorption band at 150 ns is nearly constant with, perhaps, a small shift toward the blue. We have tried to simulate the spectrum at 150 ns using two Lorentzian-shaped absorption bands centered at 325 and 360 nm. Unfortunately, we were not successful in describing the various intensity and edge shifts observed in Figure 5 . Further discussion of the observed changes with time is presented in the next subsection. Because of heat conduction, it is likely that the sample temperature decreases with time and the changes observed in Figure 5 are related to temperature changes. To examine this issue further, at least qualitatively, we conducted a ramp wave loading experiment to 4 GPa. Because of the long rise time in attaining peak pressure, the CS2sample for a given peak pressure will have lower temperature under ramp wave loading. Previous studies have shown that the absorption band edge shift of CS2 compressed by ramp wave loading is in good agreement with measurements obtained from static compression in a diamond anvil This suggested that ramp wave loading was close to isothermal compression. Spectral changes associated with ramp wave compression of CS2 are shown in Figure 6. Although the spectra were sampled every 50 ns, we only present the spectra every 100 ns; the calculated pressures corresponding to these spectra are indicated in the figure. Intensity enhancement, red shift of the band, and broadening of the band seen in Figure 6 are similar to changes observed in Figure 5. However, unlike the new feature developing in Figure 5, no new feature is apparent in any spectra in Figure 6. This is likely due to the lower temperature under ramp wave loading, and if so, it appears that the new feature at 360 nm at 2.6 GPa in Figure 5 has a hot band character. It is apparent from Figure 5 and 6 that the V-band is perturbed by the feature developed on its blue side. From the absence of any such feature at ambient condition, it is unlikely that this (29) Agnew, S. F.; Swanson, B. I.; Eckhart, D. G.Spectroscopic Studies of Carbon Disulfide at High Pressure. In Shock Waues in Condensed Matter; Gupta, Y . M . , Ed.; Plenum: New York, 1986; p 221.
\
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400
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WAVELENGTH (nml Figure 6. Absorption spectra of pure CS2 compressed by ramp wave loading to 4.0 GPa. The spectra shown were taken every 100 ns. Although enhancement of absorbance with pressure is observed, no new features are apparent.
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,
,
320
,
,
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440
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(nm)
,
,
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480
Figure 7. Absorption spectra of CS2 compressed by step wave loading to 10.5 GPa. The formation of a new feature and its changes with time
and temperature can be seen. Spectra shown were obtained every 50 ns. Because the ring-up time of the cell takes only a few nanoseconds, all the spectra except the bottom one represent the absorption spectra of CS2 at the final value of 10.5 GPa.
feature is due to a change in sapphire refle~tivity'~ or any experimental artifact. It may represent changes in the CS2 absorption spectrum located between 200 and 280 nm. B. Time-Dependent Absorption Changes. Time-dependent changes of the absorption spectrum, seen in Figure 5 , are substantially enhanced at higher pressures and are observed for longer times. Figure 7 shows time-dependent absorption changes of CS2 compressed to 10.5 GPa. The bottom spectrum is at ambient
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The Journal of Physical Chemistry, Vol. 94, No. 7, I990
Yo0 and Gupta
m
W
0
z
Q
m
a
AT AMBIENT CONDITION UNLOADING
0
m
m Q
I
1 1
260
WAVELENGTH
(nm)
Figure 8. Fitted absorption spectrum of the new feature showing timedependent, nonlinear blue shift of the band and intensity enhancement.
conditions and others are at 10.5 GPa; the spectra are shown every 50 ns. A new feature, similar to the 2.6-GPa experiment in Figure 5, develops as soon as pressure increases to 10.5 GPa. The peak position of this new feature is shifted more toward the red at the higher pressure, for example, 360 nm at 2.6 GPa and 440 nm at 10.5 GPa. The spectra at 10.5 GPa again show time-dependent absorption changes. These include intensity enhancement and blue shift of the new band. It is clear that, due to the blue shift and intensity enhancement, the new band merges to the V-band at late times, and spectral details disappear. At higher pressures, the band edge position is determined by the new band edge, whereas. at ambient condition, it is determined by the V-band edge. The spectra at 10.5 GPa have been fitted by two Lorentzians: one for the V-band and the other for the new feature, centered initially at 440 nm. As expected from the data in Figure 7, the fitted results showed small changes in the V-band intensity, width, and position, whereas substantial changes occur in the new feature. The fitted spectra showed no peak shift of the V-band greater than the experimental resolution of 2 nm. However, small changes in the intensity and width of the V-band were apparent with time. For example, after a 250-11s time period the V-band width increases approximately 15 nm, and its intensity is enhanced nearly 3-fold at the peak position. The Lorentzian fit for the new feature is reproduced in Figure 8. The spectra are separated by 50 ns. The numbers and the dotted line in the figure represent the peak positions of each spectra and shift of the peak positions with time, respectively. Two changes are noteworthy. First, the intensity distribution of the band changes with time and its peak position shifts toward the blue. For example, the peak position is at 440 nm for the first 50 ns but shifts to 398 nm after 250 ns. The rate of the blue shift decreases with time and is, likely, related to the temperature decrease due to thermal conduction. This change supports the idea that this new feature has a hot band character. Second, the bandwidth decreases with time, while the integrated intensity increases with time. Approximately 35% (25 nm) reduction in the band width occurs over 250 ns. The maximum peak intensity increases nearly IO-fold. These changes are similar to those shown in Figure 5. Either cooling or time-dependent molecular behavior of CS2may cause time-dependent absorption changes. While the bandwidth decrease with temperature decreases is likely, it is unlikely that the large absorbance increase for the new band is related to a decrease in temperature. This issue is considered in more detail in the next section. C. Reuersibility of the Absorption Changes. The spectral changes of CS2 reported here are reversible for pressures up to 10.5 GPa and are irreversible at pressures greater than 12 GPa. This is illustrated in Figure 9. The solid line represents a typical spectrum at ambient condition before loading and the broken line represents a typical reversible, unloading spectrum. The spectrum
i I
I
1
,
300
I
I
340
380
WAVELENGTH
I
I
420
(nm)
Figure 9. Typical absorption spectra of CS2 at ambient condition and upon unloading from peak pressure values of 10.5 and 12 GPa. The solid line is a CS2 absorption spectrum just before loading and the broken line is a typical spectrum obtained upon unloading from pressures below 11 GPa. The dotted spectrum represents the spectrum upon unloading from a pressure of 12 GPa. The irreversible change in this spectrum, extending beyond 500 nm, is due to a shock-induced chemical reaction.
at the top is a typical irreversible, unloading spectrum. This irreversibility is due to irreversible chemical reactions in CS2and the reaction threshold is between I 1 and 12 GPa. The irreversible spectrum has an absorbance of approximately 0.8 over a large wavelength region (250-550 nm) and most likely comes from the reaction products; however, it is not clear whether the extinction is due to absorption by the reaction products or is due to scattering of solid particles formed by products under shock compression. There is a small difference in the absorbance between the ambient spectrum and the reversible unloading spectrum. This small difference, less than 0.05 in absorbance, extends over an extremely broad region of wavelength. Appearance of this difference is reproducible for several experiments that range in final pressure between 7 and 1 I GPa. Because the difference is small and structureless, its correlation to the difference either in the final pressure or in the integrated absorbance of the V-band is not apparent. Therefore, it is not clear whether such a difference is due to a partial chemical reaction occurring at pressures lower than 11-12 GPa. It is interesting to note that the reaction threshold pressure, I 1-12 GPa, is higher than the 9-GPa pressure previously observed in thick cell experiments.I2 This result is not surprising if we consider the cooling occurring in the thin layer of CS2. The calculated temperature of a 1 Mum thick CS2 at 12 GPa is approximately 820 K and nearly 37% of this temperature, 300 K, decreases in a 0 . 5 - ~ stime period. If we consider the temperature reduction over typical experimental time, 400-500 ns, for which the pressure remains at the final value, the rate of the reaction threshold pressure change with temperature, A P / A T , becomes approximately 0.02 GPa/K. It is interesting to note that this rate is comparable to the reaction rate (approximately 0.03 GPa/K) observed in various static experiment^.^.^^,^^ Because of the uncertainties in temperature calculations, discussed earlier, little more can be said about this comparison. IV. Analysis and Discussion A . Edge Shift of the CS2 Absorption Band. At ambient condition, the CS2 absorption spectrum between 280 and 500 nm is simple and is mainly due to a transition to the IB2 state: V-band centered near 320 nm.27328331,32However, with pressure and temperature increase, the spectrum becomes complicated due to (30) Butcher, E. G.;Alsop, M.; Weston, J. A,; Gebbie, H. A. Nurure 1969, 199, 156. (31) Bajetna, L.; Gouterman, M.; Meyer, B. J . Phys. Chem. 1971, 75, 2204. (32) Winter, N. W.; Bender, C. F.; Goddard 111, W. A. Cfiem.Pfiys. Left. 1973. 20, 489
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 2863
Shock Compression of Thin CS2
TABLE 111: Parameters for Calculating the CS2 Absorption Cross Section at 300 K O A THICK CELL 0 THIN CELL
654 802
1.85 0.20
For CS2, Em = 30 630 cm-' and r = 450 cm-I. bThe A i s are the displacement of the normal coordinates.
0
2
4
8
8
1
0
1
2
PRESSURE (GPa)
Figure 10. Absorption band edge shift of CS2 for various cells as a function of pressure. For a constant pressure, the shift decrease corresponds to a systematic change in the CS2 temperature. The values for the thick cell and the closed circles for the ramp wave cell are reproduced from ref 1 1 and 12.
the new feature developed on the red side of the V-band. Unlike the V-band, the position and width of the new band depends strongly on pressure and temperature. Therefore, the edge position of the CS2 absorption band at high pressure and temperature is determined by the new band and not by the pressure-shifted edge position of the V-band as was believed in earlier experiments."' Shock-induced shift of the CS2 band edge represents the separation between the V-band edge at ambient condition and the new band edge at high pressure and temperature in the shocked state. Band edge shifts from three different types of experiments on pure CS2 are shown in Figure IO: the triangles represent data from earlier work11J2 for thick cells (approximately 150 pm) subjected to stepwise loading; the squares represent our data for thin cell (approximately 1 pm) subjected to stepwise loading; the circles represent data for thin cells (approximately 1 pm) subjected to ramp wave loading. The filled circles represent data from earlier work;11*12 in previous studies, the data were recorded by using film (see ref 13 for further details regarding film measurements). The small, but systematic, difference between the two ramp wave experiments is due to a difference in how the edge position was determined. The lines in Figure 10 are second-order least-squares fits to the various data:
AA = 20.542P - 0.183P2 for thick cell AA = 15.273P - 0.332P2 for thin cell AA = 11.538P - 0.383P2 for ramp wave cell where AA and P represent the edge shift in nanometers and pressure in GPa, respectively. For a given pressure, the band edge shift for the three types of experiments is given by thick cell > thin cell > ramp wave cell. This result shows a systematic change of the edge shift with CS2 temperature. At 4 GPa, the edge shifts are 79 nm for the thick cell, 56 nm for the thin cell, and 40 nm for the ramp wave cell. In the thick cells, the effect of cooling due to heat conduction in the sapphire boundaries is negligible on the time scale of the experiments and the calculated temperature of 570 K can be used. This is also the initial temperature of the CS2 in the thin cell, but because of heat conduction, there is a significant temperature decrease over 300-500 ns. Also, there is a significant temperature gradient through the sample thickness and it is difficult to assign a single temperature to the CS2. As a convenient method to discuss the data, we used an "average" temperature (an arithmetic mean) for the thin cells. This was done by dividing the sample thickness
after shock compression ( < I pm) into 100 zones and then taking an average over these zones. The average temperature for the thin cell after 300 ns is 425 K and after 500 ns is 387 K. For ramp wave loading of thin cells, the temperature is thought to be close to isothermal condition^.^^ If we make this assumption, then the edge shift values for the 4-GPa pressure correlate quite well with temperature changes for the three types of cells. Despite quantitative uncertainties about temperature calculations and the difficulty in assigning a single temperature to thin cell data, it is clear from the data in Figure I O and the above discussion that the band edge shifts in CS2 increase with temperature as had been calculated before."*I2 The temperatureinduced shift and the emergence of the new band are two results that need to be considered in making comparisons between static and shock compression data. B. Absorption Calculation Based on a Single Electronic Transition. It is now clear that differences in edge shifts between thick cell and ramp wave cell data are due to temperature increase associated with shock compression. However, the mechanism of this temperature-induced shift, approximately 40 nm at 4 GPa, is not clear. It is not known whether the difference is due to thermally induced phonons or vibrational motions that result in band broadening, changes in intensity distribution of the vibrational bands, and a resulting shift of the electronic absorption band, or if it is due to a change in the electronic structure itself. The former can be evaluated by calculating the absorption cross section at several temperatures and at a constant pressure. The absorption cross section at a given pressure can be written in the adiabatic approximation as33 U A ( 4
= CIMI2w,g(w1)
where C i s a constant and M and uI are the electronic transition dipole moment in angstroms and incident photon energy in cm-I, respectively. g(w,) is a band shape function which contains information regarding temperature. Assuming a Lorentzian band shape, g ( w J can be written as
where n mode, ni and nf are total numbers of vibrational modes k , of initial vibrational states li) in the ground electronic state, and of final vibrational states fl in the excited electronic state. The r is the Lorentzian bandwidth. M i ) is a multidimensional Frank-Condon integral and has previously been factorized into one-dimensional integrals in terms of the ratio R of vibrational frequencies between the ground state and the excited state, and the displacement A of their potential minima in dimensionless coordinate^.^^,^^ For evaluating vibrational effects on the absorption band at a given pressure, one can apply the Condon approximation: assuming M to be a constant. The effect of -temperature on the absorption band can be seen from thermal distribution of electrons in the ground state according to the Boltzmann factor B,. Figure 1 1 illustrates temperature effects on the V-absorption band in CS2. The points represent a measured spectrum at ambient condition. The lines are the calculated spectra at 300 K (solid line) and at 1000 K (solid-dot line). Parameters used in the calculation are summarized in Table 111. The vibrational frequencies and the intensities were obtained from a previous (33) Meyer, A. B.; Mathies, R. A,; Tannor, D. J.; Heller, E. J. J . Chem. Phys. 1982, 77, 3857. (34) Manneback, C. Physica 1951, 17, 1001.
2864
Yo0 and Gupta
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990
- A-
Y
-
-1
\
m
POSITION - TEMPERATURE
0
m
2.8 219
3:O
3:l
3:2
WAVENUMBER (cm-'1
3:3
314
3:5
/io4
ml 0
1
I
I
I
50
100
150
200
TIME
, 250
In
300
(nsec)
Figure 11. Experimentally observed CS2absorption spectrum at ambient condition and the calculated spectrum at 300 and 1000 K. Temperature-induced changes are apparent in the intensity distribution of the progressional bands, peak and edge positions, and width of the band.
Figure 12. Peak position of the new feature, near 440 nm, plotted as a function of time. The average temperature of CS2 is also indicated for the same times. Good agreement between the blue shift of the band and the cooling of CS, can be seen.
Raman e ~ p e r i m e n t . ~ Assuming ~ harmonic oscillator wave functions, no changes in normal-mode frequencies, and no Duschinsky rotation, a simple expression relating the Raman intensity Ik and the displacement Ak can be written as36
440 nm at 766 K (10.5 GPa). The maximum band intensity of the T-band arises from u = 2 at 300 K and from u = 3 at 373 K.28 Finally, the peak position of the new feature shifts toward the blue as the average temperature decreases, as illustrated in Figure 12. The peak positions of the new band at 10.5 GPa and the calculated average temperatures are indicated at different times as circles and triangles, respectively. The lines are the fits to each set of points. The scales of the two vertical axes (peak position and temperature) were chosen to provide a good fit between cooling rate and peak shift. These results show that the rate of the blue shift agrees well with the rate of CS2cooling. Therefore, the new feature developed under shock compression in the region between 320 nm and 500 nm is a "hot" band. We suggest that it is likely due to a transition to the IAt state or the T-band. There are other bands, similar to the T-band, in the same spectral region where the new band appears. For example, Rand S - b a n d ~ ~ *appear + ~ ' over a broad region extending from 300 to 450 nm. Similar to the V- and T-bands, these bands also arise from splitting of the A" state into the A2 and B2 states in a bent CS2configuration. However, they are triplet excited states and the transitions are substantially weaker than the transitions of the V- and T-bands. Like the T-band, the intensities of these bands also increase as temperature increases. It is possible that changes of these bands also contribute to changes of the new band reported here, but the effect is expected to be significantly less than that of the V- or T-band. D. Time-Dependent Absorption Changes. Absorbance of the CS2absorption band increases with time at constant pressure. This increase is either due to temperature decrease or due to timedependent molecular behavior. However, it is unlikely that the absorbance increases as temperature decreases. The T-band, for instance, arises from the bent CS2 configuration as a result of large-amplitude molecular b e n d i n g ~therefore, ; ~ ~ ~ ~ its intensity is expected to be increased as temperature increases. We presume that the absorption increase a t constant pressure is due to molecular motions occurring on a nanosecond time scale. Little is known about molecular motions under shock compression. Nevertheless, shock-induced molecular orientation has previously been proposed to explain the experimental observations in CS2/hexane mixture~.'~ Assuming a simple rigid rotor model, the angular velocity of CS2induced by a passage of a shock wave was estimated to be 0.04 rad/ps at 8 GPa. However, liquid CS2 at high pressures and temperatures is not a free rotor as was assumed in the model, and its rotational motions should involve
_I k ---3 k 2 w k ' Ik'
Ak.2Wp3
r was then adjusted for obtaining a best fit of the calculated spectrum to the spectrum obtained experimentally. The calculated spectrum at 300 K agrees well with the experimental spectrum except in the region between 33 300 cm-' (300 nm) and 35 000 cm-I (286 nm). This is due to the feature near 34 500 cm-l (290 nm) which is not due to the absorption of CS2 but is from the Xe flash lamp. Using the same parameters, we then calculated the spectrum at 1000 K shown as a broken line. The resulting spectrum shows a slight broadening 450 cm-I, changes in the relative intensities of the progressional bands, and, thereby, the edge shift of approximately 1 100 cm-' (14 nm) toward the red. However, this shift is less than 35% or 15% of the shift observed at 4 GPa (570 K ) or at 9 GPa (725 K), respectively. This result suggests that temperature-induced absorption band edge shifts in our experiments are caused not only by thermally induced phonon or vibrational motions, but are also due to changes in the electronic structure. This conclusion is not surprising, if we consider the formation of the new band and its temperature dependence. Hence, we suggest that the new band arises from an electronic state different from the IB2state of the V-band. This conclusion raises the questions: what is the nature of transitions resulting in a hot band, and how are these transitions activated under shock compression? C. Hot Band Formation. The V-band, centered near 320 nm, to a bent is due to the transition from a linear ground state (IZg+) excited state IB,. However, there is an additional low-lying singlet excited state 'A2, whose origin in a linear configuration, similar to the IB,, is the ' A ustate. The transition to !A2 has previously been observed on the red side of the V-band between 320 and 340 nm in the vapor phase of CS, and was named as the T-band.28 The intensity distribution of the T-band showed a strong dependence on temperature, suggesting its hot band character. There are similarities between the T-band and the new band observed in Figures 5 and 8. Both bands are located on the red side of the V-band and the intensity distribution depends strongly on the CS2 temperature. For example, the peak position of the band is 360 nm at 547 K (2.6 GPa) and is shifted to the red, near ( 3 5 ) Yoo, C. S.;Gupta. Y . M.; Horn, P. D. Cfiem. ffiys. Lett. 1989, 159, 178. ( 3 6 ) Heller, E. J.; Sundberg, R. L.; Tannor, D.J . ffiys. Cfiem. 1982,86, 1822.
(37) Kleman, B. Con. J . ffiys. 1963,41, 2034. ( 3 8 ) Loguet-Higgins, H. C.; Oepik, U.; Pryce, M. H. L. Proc. R . SOC. London 1958, 244A, 1. (39) Barrow, T.; Dixon, R. N.; Duxbury, G. Mol. Pfiys. 1974, 27, 1217. (40) Jungen, C.; Merer, A. J. Mol. ffiys. 1989, 40, I .
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 2865
Shock Compression of Thin CS, s=C=S
-+
S=C.
+
.S
-b
G r a p h i t e + S u l f u r Polymer
4.
Figure 13. Two simplified, plausible reaction mechanisms for shock-induced chemical reactions of CS2: (a) dissociation and (b) association.
a diffusive barrier. At the present time, we do not know the barrier height; assuming time-dependent absorption changes are due to a similar motion, the rotational time should be approximately 4-5 orders of magnitude greater than the calculated value. There are other observations in CS2 similar to the time-dependent absorption increase reported here. In a previous resonance Raman measurement of CS2,35it was observed that the resonance Raman cross section increases with time at a constant final pressure. This agrees with the time-dependent absorption increase of CS2, because resonance Raman cross section is linearly proportional to the absorption cross section. Phase separation was observed in the CS2/hexane mixtures:' and this should also involve diffusive molecular motions. It seems to suggest that the diffusional relaxation time is not a factor limiting motion of CS2 molecules under shock compression. Such molecular motions can increase the CS2 absorbance directly by increasing the electronic transition dipole moment of the CS2 absorption band, as was discussed in the ref 14. However, it can also increase the CS2 absorbance indirectly by increasing light scattering. At substantially decreased intermolecular distances, the "cage" molecules of CS,, whose sizes are substantially greater than the parent size, can be formed by such molecular motions. The size and number density of the cage molecules will then increase with time, as does the scattering from such particles causing extinction behavior. In addition, incompleteness in returning such particles to the parent molecules with respect to the pressure reversal may result in the small difference that we observed in the reversible unloading spectrum. E . Shock-Induced Chemical Reactions. In light of the results reported here, it is useful to discuss briefly the reaction mechanisms causing the irreversible absorption changes observed in this and previous14 studies. Two reactions, a dissociative reaction to graphite and sulfur polymers and an associative reaction to CS2 multimers, are considered. These reactions are typical of reactions that often occur at high pressure and temperature and have been found in static condition^.^ Simplified, plausible mechanisms for these reactions are shown in Figure 13. An associative type of reaction is favored over a dissociative one for the following reasons: ( i ) Dissociation requires bond scission as the rate-determining step (RDS), whereas association requires a diffusive molecular process, such as packing. The thermal energy produced by step wave loading to the threshold pressure is approximately half that needed for breaking the C=S bond. Therefore, without an inhomogeneous concentration of thermal energy into a few bonds, it is unlikely to dissociate CS, molecules. (ii) CS2 molecules behave cooperatively under shock compre~sion.'~ The threshold pressure increases as concentration decreases and the reaction does not occur at extreme di1~tions.l~ This suggests that the reaction is not initiated directly by bond scission but arises from interactions between neighboring CS, molecules. Delocalization of the r-electrons through C=S bonds of neighboring CS, molecules increases intermolecular bond strength. In addition, the activation barrier for bond scission processes can be lowered by the r-electrons transferring from the intramolecular bond to the intermolecular bond. The parallel orientations of CS2molecules are certainly ideal for such a process. (iii) The absorption band shifts continuously at the reaction threshold pressure (RTP), suggesting that the absorption band of unreacted CS2 at high pressures, near the RTP, is similar to that of reacted ones. This would result from a continuous increase of x-electron delocalization between neighboring CS2 molecules. (41) Yoo, C.
Phys.
S.;Gupta, Y . M. Submitted for publication in J . Chem.
The RTP is then determined by the critical condition for a-electron distribution. Therefore, the reaction expected is a concerted one whose transition state is associated with many CS2 molecules sharing r-electrons. (iv) Finally, the rate of the threshold pressure change with temperature is similar to that observed in several static experiment~,4.~~,'~ where associative type reactions are ubiquitous. Possible products of an associative type reaction are CS, multimers, (CS,),, and/or polymers like Bridgman black polymer, (-S-(C=S)-),. A secondary reaction from the products, if there is any, can further produce mixtures of graphite, sulfur polymer, etc.
V. Conclusions The use of thin CS2 samples has permitted measurements of the full absorption band (between 280 and 500 nm) in shocked CS2. These data show that the CS2 absorption spectrum at high pressures is quite different from that at ambient conditions. At ambient conditions, only the V-band is observed. However, at high pressures, a new band appears on the red side of the V-band; this indicates additional electronic transitions. Band edge shifts are not the edge shifts of V-band as reported earlier;'*'2 instead, they represent the separation between V-band at ambient conditions and the new band edge in the shock state. Temperature dependence of the new band shows a hot band character and we conjecture that this may represent transition to the 'A2 state (T-band).28 Time-dependent increase observed in the absorbance measurements and the magnitude of the edge shifts cannot be reconciled with expected temperature changes and are believed to represent changes in the electronic structure. The present observations support the parallel-alignment molecular model for CS, outlined on the basis of measurements carried out in CS2/ hexane mixture^.'^ A key aspect of this model is molecular rotations involving a diffusive barrier which may result in the delocalization of r-electrons and serve as a precursor to chemical reactions. More direct data than the present work are needed to provide confirmation of this model. The present data do provide evidence for molecular motions on nanosecond time scales. Changes in the CS2 absorption band are irreversible at pressures higher than 12 GPa for the thin cells examined here. This irreversibility is due to a shock-induced chemical reaction and the threshold pressure is dependent on temperature. Previous work on thick samples (100-200 pm) where sample cooling due to heat conduction is not a factor revealed a threshold pressure of approximately 9 GPa. Although temperature calculations remain the most imprecise aspect of the shock experiments, we infer a change in threshold pressure with temperature of 0.02 GPa/K which is comparable to that observed under static conditions. Based on the CS2 absorption changes, this shock-induced chemical reaction is suggested to be an associative type of reaction to CS, multimers, (CS,),, or polymers like (-S-(C=S)-),. The present work, in combination with previous studies in our laboratory, has led to a fairly comprehensive examination of shock-induced changes in CS2. Although considerable understanding of the response to CS, has been developed, a confirmation of postulated molecular mechanisms will require detailed theoretical calculations, and more direct measurements of molecular motion and chemical species using vibrational spectroscopy. In addition, the need for direct temperature measurements cannot be overemphasized. Acknowledgment. Numerous discussions with Professor George Duvall, who initiated the shock studies on CS2, are gratefully acknowledged. Interactions with him on this difficult problem have been most beneficial to us. Several other colleagues in the Shock Dynamics Laboratory are acknowledged for their help: Dr. R. Gustavsen for the thermal conduction calculation and general discussions about CS2; P. Bellamy, J . Thompson, and J. Burt assisted with various experimental efforts. This work was supported by the Office of Naval Research under Contract N0014-86-K-0370, and the enthusiastic interest of Dr. R. S. Miller is acknowledged. Registry No. CS,, 75- 15-0.
