A spectrophone study of vibrational relaxation in methane Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/g158bn06g

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  • The spectrophone method for the study of vibrational relaxation has been claimed to possess a distinct advantage over the more commonly used ultrasonic techniques in that it permits the relaxation of the various infrared-active vibrational modes in a polyatomic molecule to be studied separately. A discussion of the rate equations governing the exchange of energy between several degrees of freedom in polyatomic gases is given which shows that the characteristic relaxation frequencies for such a system are completely independent of the method of excitation, indicating that the information which can be obtained using the spectrophone is essentially the same as that to be found from ultrasonic measurements. An expression is derived for the spectrophone frequency response to be expected from a polyatomic gas which predicts that at sufficiently high light-chopping frequencies an increased loss in the spectrophone signal due to relaxation phenomena should be observed. A study of the spectrophone response in the frequency range where this increased signal loss occurs should-thus afford a means for determining the effective vibrational relaxation time of the gas in the spectrophone cell. An apparatus is described which was used to measure spectrophone signal amplitudes over a range of chopping frequencies from 200 to 1600 cps, at gas pressures from one atmosphere down to a few millimeters. Response curves obtained for methane gas at pressures of one atmosphere and one-half atmosphere exhibit the behavior predicted by the theory, but show no increased signal loss at the higher chopping frequencies. However, the response curves for methane at pressures of 5 mm, 10 mm, and 20 mm do exhibit such a high frequency loss; from these response data a vibrational relaxation time of T= 1.1 microseconds at one atmosphere has been assigned to methane. This value agrees within a factor of two with all similar results reported by other workers. Studies of the spectrophone frequency response obtained for mixtures of methane and carbon dioxide, with carbon dioxide concentrations of one percent and five percent, indicate that the presence of carbon dioxide serves to decrease the vibrational relaxation time in methane. Studies of the spectrophone frequency response obtained for three deuterated methanes, CH₃D, CH₂D₂, and CHD₃, yield values of T= 0.87 microseconds for CH₃D, T= 0.99 microseconds for CH₂D₂, and T= 1.45 microseconds for CHD₃, all at one atmosphere pressure. The longer time for CHD₃ is interpreted in terms of the decreased rotational velocity of the deuterium atoms in this molecule, which reduces the probability of vibrational- rotational energy transfer during molecular collisions.
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