Vibrational relaxation in gases

Abstract

The velocity of ultrasonic waves has been measured in a number of gases at 25°C and for values of the ratio, ultrasonic frequency/pressure, ranging from 2 x 10 5 to 2 x 10 7 c s -1 atm -1 . Dispersion, corresponding to a single vibrational relaxation process was shown by acetylene, CD 3 Br and hexafluoro-ethane; and, to a double relaxation process, by ethane. Incipient dispersion was shown by propane, ethyl chloride, ethyl fluoride and dimethyl ether. No dispersion was shown by 1.1-difluoro-ethane, n -butane, iso -butane, neo -pentane and ammonia. Correlation of these with previous results leads to the conclusion that: ( а ) For molecules with a distribution of fundamental frequencies, such that there is only a small gap between the lowest and the remaining frequencies, vibrational activation enters via the lowest mode and spreads rapidly to the other modes, giving rise to a single relaxation process involving the whole of the vibrational energy. The chief factors determining the probability of excitation of the lowest mode are its frequency and the presence or absence of hydrogen atoms in the molecule. Molecules containing two or more hydrogen atoms suffer translational-vibrational energy transfer very much more easily than other molecules. Deuterium has almost the same effect as hydrogen. ( b ) For molecules, in which there is a large gap between the lowest and the remaining fundamental frequencies, a double relaxation process occurs. The complex energy transfer probabilities involved do not fit the same quantitative functional relation with vibrational frequency as in ( a ) above. ( c ) Torsional oscillations due to hindered internal rotation behave similarly to other fundamental modes. For molecules in which there is a large gap between the torsional frequency and the other modes (e. g. ethane) a double relaxation process occurs as in ( b ). Where there is no such gap, vibrational energy enters all modes via the torsional mode as in ( a ).

Coherent Rayleigh–Brillouin scattering measurements of bulk viscosity of polar and nonpolar gases, and kinetic theory

“…The rotation-translation and vibration-translation relaxation times r and vib are shown in nanoseconds at 1 bar and were obtained from Ref. 33, with the exception of the relaxation times of SO 2 , which can be found in Ref. 34, and of ethane, which were obtained from Ref.…”

The kinetics of internal energy randomisation in thermal unimolecular reactions

“…The only signijicant difference in the parameters required to achieve this exact fit is that the second-order randomisation rate constant is about 2+ times as large, which I think is quite reasonable in view of the fact that CD3NC has 6 or 7 times the density of states of CH3NC. The only questionable point is that p for CD3NC appears to be very slightly greater than that for CH3NC, contrary to the general trend found at lower temperatures in ultrasonic relaxation experiments (28).…”

DSMC Modeling of Gasdynamics, Radiation and Fine Particulates in Ionian Volcanic Jets

“…Special effort has been placed on modeling the unique vibrational relaxation process of SO 2 molecules (discussed extensively in [26,27,28]). We assume Z 1 ¤ 3 =1000 for the V-T relaxation of the ν 1 and ν 3 modes: this Z 1 ¤ 3 is expected to provide an acceptable accuracy for the temperatures of the order of several hundred Kelvins.…”