Isotope effects in the photofragmentation of symmetric molecules: the branching ratio of OD/OH in water.

With HOD initially in its vibrational ground state, we present a new detailed interpretation of the OD/OH branching ratio (approximately 3) in the photoinduced process D+OH<--HOD-->H+OD, in the first absorption band. Using semiclassical arguments, we show that the branching ratio has little to do with the initial distribution of configurations, but the initial momentum distribution plays a key role in determination of the branching ratio. The formation of D+OH arises from initial situations where OD is stretching, and it stretches faster than OH, whereas all other motions lead to H+OD. This picture is confirmed by quantum wave-packet calculations.