The time evolution of the velocity distribution of hydrogen atoms in a bath gas

Abstract

In this work we probe an ensemble of energetic hydrogen atoms dilutely dispersed in a bath gas which is at thermal equilibrium. A recently developed laser-induced fluorescence technique allows a quantitative measurement on a nanosecond time scale of the shape of a Doppler-broadened absorption line of hydrogen atoms. A nonequilibrium distribution of H atoms is generated by photodissociation, and the line shape is then measured at a flxed time delay as a function of buffer gas pressure. The buffer gases used are He, Ar, Kr, Xe, O2, and N2. A technique perfected by Shizgal is used to integrate the linearized Boltzmann equation for a hard-sphere potential. The experimental results for the decay of the kinetic energy are fit with cross sections of 10±2, 9±2, 12±2, and 16(+8−4) Å2 for He, Ar, Kr, and Xe, respectively.