Velocity Distribution of Hydrogen Atom Spin Polarization

We present an experimental technique allowing for direct measurement of the velocity dependence of hydrogen atom spin polarization with high resolution and high sensitivity. The strategy is a simple adaptation of the H atom Rydberg time-of-flight approach, here demonstrated for 213 nm photodissociation of HBr. The two coherent contributions to the spin polarization are measured for H atoms produced in conjunction with dissociation to Br( 2 P 3/2 ) and Br*( 2 P 1/2 ); they are found to be negligible for the former channel but substantial for the latter, in qualitative agreement with values inferred from previous theoretical predictions of the Br atom polarization. The ratio of the measured spin polarization anisotropy parameters directly gives the asymptotic scattering phase shift for dissociation along the two potentials for the H + Br* channel. The approach is general and suitable for application to polyatomic molecules or to scattering studies.