Measurement of vibrational-vibrational exchange rates for excited vibrational levels (2≤v≤4) in hydrogen fluoride gas

Publisher Website
Google Scholar
Abstract
A measurement of the total decay rate (due to both V-V and V-T processes) of the second, third, and fourth vibrational levels in vibrationally cold hydrogen fluoride gas is described. The measured endothermic rate constants are 8×105, 1.6×106, and 1.4×106 sec−1 · torr−1, respectively. It is argued that the major contribution of each of these decay rates is from the process HF(ν = n)+HF(ν = 0)→ HF(ν = n − 1) + HF(ν = 1) where n = 2,3, and 4. The technique used in these measurements is a novel variation of the method of laser induced fluorescence. It involves exciting the first; first and second; or first, second, and third vibrational levels of HF via multiband laser optical pumping. Collisional population of the level of interest then occurs through the reverse of the above vibrational exchange process. An analysis using rate equations shows that the rise rate of the population of a given level is governed by the forward rate of the above process. It is also shown theoretically and experimentally that in the pressure regime where diffusion is unimportant the decay rate of level ν = n is n times that of ν = 1. This result is shown to be a result of the existence of a vibrational exchange equilibrium at a relatively low vibrational temperature. If the pressure is sufficiently low so that the diffusion time across the original radius of excitation is comparable to the decay time of the population of the nth level, it is found that the decay rate of that level is not n times that of ν = 1. This phenomenon which is termed ``vibrational temperature'' diffusion is given a brief experimental and theoretical description.