Interpreting Pump - Probe Experiments on Dimethyl Methyl Phosphonate (DMMP)

Abstract

Recently ultrafast pump-probe experiments have been performed on Dimethyl Methyl Phosphonate (DMMP) where the pump light was used to adiabatically ionize the molecule and the probe light was used to study the dynamics of the cations produced. There was an oscillatory behavior in the transient cation yield. Also an anti-phase relation was found between the yield of the parent molecular ion (DMMP+ ) and the other cation fragments. In this work a detailed study has been carried out to understand the experimental observations with the help of quantum chemical calculations. Ionization potentials of DMMP and the oscillator strength between the ground and excited electronic states of the cation were calculated. It seems that the probe light excites the DMMP+ to the 2nd and 3rd excited states (D2 and D3) where the molecule gets dissociated further. This happens for certain geometries of DMMP+ where the energies of D2 and D3 become resonant with the probe light. Hence the population of DMMP+ gets depleted giving rise to the dissociated ions. At the DMMP+ ground state geometry this resonance doesn’t occur and hence there is transition causing no depletion of it.