Speed and angular relaxation processes induced by collisions with Hē, Ar, Ne, Xe, N2, and O2 for the velocity of superthermal O(1D) photofragments have been studied by measuring the Doppler profiles of O(1D) as a function of the time delay between photolysis and probe laser pulses. The nascent O(1D) atoms, generated in the photodissociation of O2 by linearly polarized light at 157 nm, have a narrow kinetic energy distribution, centered at 9.8 kcal/mol and a large angular anisotropy. The O(1D) atoms are probed with a tunable vacuum-ultraviolet laser at approximately 115.2 nm. The time evolution of the speed distribution is extracted from the Doppler profiles for each bath gas. The anisotropy parameter distribution as a function of the speed at each time delay is also obtained. The distributions obtained indicate that (a) a collision with He reduces the speed of the O(1D) atom efficiently but only changes slightly the direction of its velocity and (b) a collision with Ne or Ar reduces its speed efficiently and almost randomizes the direction of the O( 1D) atom velocity. © 1994 American Institute of Physics.
