Slice imaging of H-atom photofragments: effects of the REMPI detection process on the observed velocity distribution

Slice imaging is used to measure the velocity distribution of the H-atom photofragments from the photodissociation of HBr at 243 run. The achievable velocity resolution of our measurements is improved by a factor of 1.5 when (3 + 1) resonance enhanced multiphoton ionization (REMPI) via the 2p-excited state is used rather than the popular (2 + 1) REMPI via the 2s-excited state. This observation is attributed to the large difference in the recoil momentum imparted to the H+ ion by the sibling photoelectron during the ionization process. Although the efficiency of the (3 + 1) REMPI is substantially less than the (2 + 1) REMPI, the line-width of the former transition is sufficiently broad to measure the complete angular distribution of the H-atom products, without scanning the probe laser wavelength. From the H-atom images, the Br((2)p(1/2))/Br(P-2(3/2)) branching ratio is found to be 0.11. (C) 2003 Elsevier B.V. All rights reserved.