Abstract：Experimental ratios of branching in the fine-structure levels of the Rb5P multiplet, as a consequence of an optical collision with He, are reported. The process studied is Rb(5S1/2)+He+hν→Rb(5PJ)+He, where the pulsed laser frequency ν is tuned in the wings of the Rb resonance transitions. The authors defined the detuning Δ to be ν-ν(D2)(Δ＞0) for laser frequency ν highter than that of RbD2 transition and to be ν-ν(D1)(Δ＜0) for ν lower than the RbD1 transition. As RbHe molecular states are correlated to the two 5 2PJ states, there is some likelihood that the molecule will dissociate into 5 2P1/2 or 5P3/2 state. The dissociation results in a nonuniform distribution of atomic Rb5PJ states. The branching ratios are defined as n1/n2, where n1 and n2 are densities of the 5P1/2 and 5P3/2 states dissociated. To determine experimentally the relative cross section for scattering into the two fine-structure states, the relative time-intergrated intensities of the resulting Rb emission lines, I(5P3/2→5P1/2)[I(D2)] and I(5P1/2→5P3/2)[I(D1)] , were measured. The ratios are determined by detunings from about 200 cm-1 in the blue wing to -180 cm-1 in the red wing of the Rb5P multiplet. A rate equation analysis of the pressure dependence was yielded. The branching ratios and cross sections for collisional 5P1/2→5P3/2 transition were obtained from the slope and intercept. The blue-wing branching ratios show a detuning-dependent approach to limit of 0.2. The branching was found to be very large (～40) in the red wing, irrespective of the detuning. Fine structure changing cross section (1.1±0.3)×10-17 cm2 was measured from wing excitation, and the result is consistent with the cross section obtained from resonant excitation of the Rb5PJ state. The measurements show a strong sensitivity to interatomic potentials and to nonadiabatic effects in dissociation dynamics.
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