Vibrational energy transfer rates in CO2–CO mixtures have been measured from 163 to 406 °K using the laser excited vibrational fluorescence method. The near resonant V→V transfer step CO(v=1)+CO2(000)ke⇌ke′ CO(v=0)+CO2(001)+ΔE=−206 cm−1 has been measured in the endothermic direction and after detailed balancing [ke′=ke exp(−ΔE/kT)] is in excellent agreement with data taken in the exothermic direction at and above room temperature. V→R,T quenching of the CO2(001) level by CO2 and CO has also been studied. The CO2 self-relaxation rate reverses in temperature dependence between 250–300 °K with rates becoming progressively faster as temperature decreases. The total quenching rate for CO2 deactivation in CO2–CO collisions behaves similarly to the self-relaxation rate. It was not possible in the present analysis to separate and distinguish the pathways for V→R,T deactivation in equilibrated CO2–CO mixtures. The cross relaxation terms (CO+2–CO) and (CO+–CO 2) are both expected to be important at low temperatures.
Collisional quenching of carbon monoxide by hydrogen and nitrogen has been studied in the 100–650°K temperature range using the laser excited vibrational fluorescence method. The rate constant for CO–H2 deactivation increases smoothly with temperature from 2.6±0.3 sec−1·Torr−1 at 112°K to 170±15 sec−1·Torr−1 at 623°K. The vibration-to-vibration energy transfer results for CO–N2 mixtures (exothermic direction) show only a slight temperature dependence from 103 to 651°K with a broad maximum of 420±30 sec−1·Torr−1 in the temperature range 300–400°K. Comparison of our rates with high temperature shock tubes results show excellent agreement for the CO–H2 V → R,T process and only fair agreement for the CO–N2 V → V exchange process. This latter discrepancy may be partially due to the uncertainties involved in extracting V → V energy transfer rates from shock tube data.
Vibrational deactivation of N2O(001) by N2O, CO, and Ar has been studied from 144 to 405 °K using the laser fluorescence method. The probability of vibration to vibration energy transfer, N2O(001) + CO(v=0) ? N2O(000) + CO(v=1) + ΔE = 81 cm−1, is 0.025 and is independent of temperature from 144 to 405 °K. We have performed theoretical calculations on this rate using the modified Sharma–Brau theory developed by Tam. The present results, together with previous experimental work at higher temperatures, suggest a good theoretical fit above room temperature, but not below. The deactivation of N2O(001) in collisions with N2O, CO, and Ar, N2O(001) + M → N2O(mnl0) + M + ΔE, has been determined to be 102 to 104 times slower than observed for the V→V exchange process. The N2O(001) intramolecular rates become smaller with decreasing temperature with the exception of the N2O self-relaxation rate. Below 250 °K this rate increases rapidly with decreasing temperature.
The laser fluorescence method has been used to measure CO( v = I) collisional transfer rates in several binary and ternary gas mixtures at 296 oK. Excitation of the CO(v = 1) level was achieved using pulsed 4.6 porn radiation from a frequency doubled CO, laser. The relative inertness of CO molecules towards V ~ T deactivation greatly facilitates the study of vibrational relaxation rates of the additive species in selected cases. In this paper, intermolecular V ~ V transfer rates at 296°K are reported for CO(v = I) with N,O, OCS, SO" CS,' C,N" and for CO(v =2) with CO. In addition, the additive deactivation rates were determined for the following collisional processes: N ,0(00 I) with N,O, CO, Ar; OCS(OOI) with OCS, CO, Ar; CS,(OOI) with CS" CO; and C,N,(OOIOO) with C,N, and CO.Precautions taken to assure high gas purity have been reported previously. I All condensable gases were freeze-pumped and outgassed at 77 OK. The gases CzN z , CS z , and OCS were further purified by trap-to-trap distillation. The middle fractions were withdrawn for sample use. Carbon monoxide and Ar were stored under liquid N z several hours prior to sample preparation.
The exceptionally rapid vibrational energy relaxation in ammonia has led to the speculation that the inversion motion is responsible for the higher rate of collisional energy exchange than in related molecules. Exact numerical solution of the scattering equations has been carried out utilizing an atom–atom exponential repulsive potential for one dimensional collisions, with emphasis upon the comparison of models whose internal energy is described by (a) an ordinary harmonic oscillator (noninverting model) and (b) the states that accurately describe the vibrational motion for double minimum potential (inverting model). Although the transition probabilities for the 0+, 0− and 1+, 1− states are high at thermal energies, such transitions do not make a large contribution to the relaxation time found from acoustic experiments. The speed of the observed processes is attributed principally to (a) the ’’acceleration’’ effect which is a consequence of the intermolecular potential of polar molecules and (b) the high impact velocities of rotating molecules with low moments of inertia.
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