Velocity relaxation of fast hydrogen atoms colliding with rare gas atoms, N
-2, O-2, and N2O molecules has been investigated by measuring Doppler broad
ened H atom spectra. The fast hydrogen atoms were generated by photodissoci
ation of H2S at 243.2 nm, and the H atom spectra were measured by two photo
n absorption laser induced Lyman-alpha fluorescence. The H atom spectra sho
w an anisotropic velocity distribution with the anisotropy parameter, beta
= -1.0 +/- 0.05, without any colliding gases. Degradation of the anisotropy
of the angular distribution was measured as a function of pressure of the
colliding gases, He, Ar, Kr, N-2, O-2, and N2O and the collision cross sect
ions for velocity direction change were measured.