Time-dependent fragment distributions detected via pump-probe ionisation: a theoretical approach

We show how in molecular predissociation a method combining ultrafast pump- probe techniques with a measurement of the relative recoil velocity can map time-dependent neutral fragment distributions into the ionic continuum. Wi th an appropriate probe pulse exciting a resonant transition (such as (1+1) Resonance Enhanced Multiphoton Ionisation, or excitation of ZEKE states), the temporal evolution of fragment distributions can in principle be measur ed. Numerical simulations on NaI predissociation are compared to a simple a pproximate mapping interpretation. The results are discussed in terms of th e interplay between temporal and energetic resolution with respect to curre nt experimental limitations.