3751
SHOCK-HEATED CS2-Ar REACTIONS
Reactions of Shock-Heated Carbon Disulfide-Argon Mixtures. I.
Light Emission by S. J, Arnold and G. H. Kimbell Canadian Armament Research and Development Establishment, and Centre de Recherches sur lee Atomes et MolBcules, P . 0 . BOX1497, Qudbec, P.Q., Canada (Recdued February g4, 1989)
The light emission from shock-heated CS2-Ar mixtures varying in composition from 1 to 100% CSZhas been measured as a function of wavelength in the range 3750-8250 A. Measurements of the rlrrhenius activation energy indicate that the molecule is excited to the 3&+ state. It was not possible to distinguish between a singlestep excitation process and a more complex mechanism involving vibrational excitation of the ground state.
Introduction When a CS2-Ar mixture, varying in composition from 1 to 100% CSz, is shock-heated, visible light is emitted.1 Previous studies on the decomposition of CS22*3 indicated that the reaction was unimolecular and that the primary step was the collision-induced transition of the CS2 molecule from the ground (’&+) state to a 3Az state. A similar mechanism has been postulated for the C 0 2 m ~ l e c u l e . ~Preliminary results of a study of light emission by shock-heated CS2-Ar mixtures indicated that the intensity was first order in CS2 and zero order in Ar.’ A further study of the light emission in the range 3750-8250 d has been carried out. The activation energy for the primary step, which has been calculated as a function of wavelength, appears to be consistent with shock excitation of CSZ to one of the predicted upper electronic states.
Experimental Section The shock tube was constructed of chromium-plated steel and is 12 i‘t in length from diaphragm to observation station and 14 in.2 in cross-sectional area. The shocks were generated by 1-10 atmospheres of helium in a 3-ft long driver section. All observations were made in the primary shock. Thin-film platinumresistance detectors were used to measure the speed of all shocks fired. I n the first experiments, the signals from the velocity gauges were recorded by photographing an oscilloscope sweep. I n later experiments a microsecond counter was used to measure the time required for the shock to travel between two detectors located on either side of the observation window. Quartz windows, flush-mounted with the inner wall, enabled spectroscopic and spectrophotometric measurements to be made. A Hilger-Watts F/4 quartz-prism spectrograph was used for the spectroscopic experiments and a McPherson Model 235 monochromator for the spectrophotometric experiments. I n both cases the instruments were placed close to the quartz window; no lens was used.
For spectrophotometric measurements in the range 3750-6000 d the monochromator was equipped with a 6OOL/mm grating having a reciprocal dispersion of 34 &mm. The entrance slit wat set at 2 mm allowing a nominal band pass of 68 A. The detector used was an EM1 9514SA photomultiplier which has an “S” type response. For similar measurements in the range 6000-8250 d the monochromator was equipped with a 300L/mm grating having a reciprocal dispersion of 68 d/mm. The entrance slit was reduced to 1 mm thus allowing the same nominal band pass. The detector used in this range was an EM1 9558QB photomultiplier which has an “S-20” response. The photomultiplier output was recorded by photographing a single sweep from a Tektronix 555 oscilloscope triggered by a velocity-gauge signal. A relative calibration of the system was carried out using a tungsten source at a color temperature of 2420°K. For both detection systems the intensity data was normalized to the intensity a t 5000 8. The combined geometric and electronic rise time of the system was 2-3 psec. This depended only on the time taken for the shock to pass the viewed region and on the resistance loading of the photomultiplier. Prior to the firing of each shock, the tube and the ancillary filling system were evacuated to less than mm. The CS2used was Anachemia Reagent Grade which had been dried over “Drierite” and further purified by trap to trap distillation. The argon, of “ultra high purity” grade, containing less than 10 ppm of impurities, was supplied by Matheson. The temperature, pressure, and density of the shocked gas were calculated from the measured shock (1) S. J. Arnold, W. G. Brownlee, and G. H. Kimbell, J . P h y s . Chem., 72,4344 (1968). (2) A. G. Gaydon, H. B. Palmer, and G. H. Kimbell, Proc. Roy. Soc., A279,313 (1964). (3) H. A. Olschewski, J. Troe, and H. G. Wagner, 2. Phys. Chem. (Frankfurt), 45, 329 (1965). (4) T. A. Brabbs, F. E. Belles, S. A. Zlatarich, J . Chem. Phys., 38, 1939 (1963).
Volume 73, Number 11 November 1960
3752
S. J. ARNOLD AND G. H. KIMBELL 6.0
A: 4500
A
5.0 In c .3
6
k
2
L
I
I
0.55
0.65
I
I
0.35
0.45
I
-a"
I
1
0.85
t;"
wavelength, p
4.0
-cn
3.0
Figure 1, hlicrodensitometer traces of the emission obtained from a 20% mixture of CS2in argon shock-heated t o 2000°K.
velocity. The high temperature enthalpy data was obtained from the JANAF Thermochemical Tables; solutions to the hydrodynamic equations were obtained using the CARDE Sigma 7 computer.
Results and Conclusions Xpectral Distribution of the Emission. A HilgerWatts F/4 quartz prism spectrograph was used to photograph the emission from a 20% CSsAr mixture, shock-heated to 2000°K. Figure 1 shows microdensitometer tracings of the emission obtained on Kodak 1-F and 1-N spectroscopic plates. In the case of the 1-F plate, it was necessary to superimpose the radiation from 10 successive shocks, while for the 1-N plate, 30 successive shocks were required. The emission extends from 3600 A in the near ultraviolet to about 9000 in the near infrared and appears to be continuous throughout the whole range. No structure was detectable with a slit width of 0.25 mm. The existence of the two maxima indicated on the 1-N plate appeared to be confirmed by an analysis of the photometric data. Pressure Dependence of the Intensity. oThe peak radiation intensity was mzasured at 250-A intervals over the range 3750-8250 A as a function of the temperature and pressure and the initial mole fraction of CSZ. Values of the temperature and the pressure for typical shock data, calculated assuming frozen chemistry, are shown in Table I. At 4500 A plots of the logarithm of the peak intensity per unit concentration of frozen mixture us. 104/T were linear for mixtures with initial mole fractions of 0.05, 0.10, 0.20, and 1.00 and yielded parallel straight lines. Consequently, over the temperature range 1500-350OoK, the intensity per unit gas concentration is proportional to the initial mole fraction of CS2. When the initial mole fraction was used to scale the ordinate value to the logarithm of the peak intensity per unit concentration of CS2, all these data could be correlated with a The Journal of Physical Chemistry
2.0
3.0
5.0
4.0 104/T,
6.0
OK-'
Figure 2. Plot of the logarithm of the emission intensity per unit concentration of CSi vs. l04/T.
single straight line. Figure 2 shows the resulting plot obtained for the 4500-8 data. Similar plots were obtained a t all other wavelengths measured, although in most cases only mixtures with initial mole fractions of 0.05 and 0.10 were used. The intensity, therefore, is proportional to the concentration of CS2 and independent of the concentration of argon. Kinetics of Excitation. Since the emission intensity is a maximum at the shock front and decays to an equilibrium value (see Figure 3)) the emission must result from excitation of ground state CS2 and not from recombination of its dissociation products CS and S. ( a ) The Single-Step Mechanism. The simplest kinetic scheme which will explain the observed firstorder dependence on CS2 and zero-order dependence on argon is one consisting of collisional excitation and quenching in which the electronic transition occurs in a single step during collision
cs2 + R!t csz*
1
CS2* 2
+M
2-csz + hv
where * represents the electronically excited molecule. If a quasi-stationary state is assumed to exist immediately behind the front with respect to the electronic
SHOF~-HEATED C&Ar REACTIONS
3753
Table I: Sample Calculations of Shack Parameters Mole fnetion
CSI
0.10 0.10 0.10 0.10 0.05 0.05 0.05
0.05 0.05 0.05
velooity. cm/sas X 101
1.81 1.56 1.48 1.50 1.49 1.65 1.61 1.64 1.87 1.88
PI.,,,.,,
PILm.1.
Density ntio (FC0.m
mm
mm
chemistry)
3.0 11.0 8.0 14.5 14.5 10.4 8.4 5.0 3.0 1 .o
136 370 244 456 416 368 284 175 137 45
4.47 4.31 4.25 4.27 3.88 3.99 3.96 3.9R 4.09 4.08
1004
UTdP3
T,. 'K
3033 2320 2137 2197
5.59 3.9R 3.42 3.57 3.57 4.85 4.57 4.97 5.98 6.05
no1 2649 2.i4.3
2624 3320 32%
where E. is the Arrhenius activation energy. More rigorously, kl and kz should be expressed in terms of the third body M resulting in
where m is the mole fraction of CSI. Figure 2 indicates that at any given temperature the intensity per unit concentration of CSz is essentially independent of m. This infera, therefore, (i) the derivative of the squarebracketed term with respect to m in the above equation equals zero and thus
FiKiirp :3. Oscilloscope trace showing the wlocily-gwgc signals (upper trace) mid the intcnsily profile (photomultiplier output, lower trace) Characteristic of B hiahcr temperntrire shock. 1; = 3136"K, P I = 128 mmHg, velocity = 1.X X 10' cm 8ec-I.
excitation process, then the peak intensity a t the front is given by
I n order to simplify the denominator further k s / ( h . [ M I ) must be much less than 1. Under our experimental conditions [ A I ] 'Y lod mol cm+. The rate constant k, is equal to PzZwhere Pzis the fraction of collisions effective in quenching (Pz5 1) and Z, the collision rate, is equal to 10" ern' mol-I sec-'. The rate constant ks for a forbidden transition is 5 I@sec-l. The inequality ka/(kr[AI])< 1 can, therefore, only be fulfilled if Pzis in the interval 0.1 < P, < 1 which infers that the primary quenching mode is with CSz acting &s M (Pzfor argon should be approximately 10-8). Therefore eq a reduces to
(ii) the expression will also be essentially independent of m if klA'
