Effect of laser pulse shape on multiphoton excitation of a polyatomic molecule

Abstract

The influence of the laser pulse shape on the multiple photon excitation dynamics of a large molecule like has been studied numerically in the pico- and nanosecond scale. In the theoretical model used multiphoton excitations are described in terms of the optical Bloch equations for the density matrix of four anharmonically shifted discrete vibrational levels coupled by the laser field and undergoing irreversible loss from the topmost level to the quasicontinuum (QC). The excitation in the QC is described by rate equations with loss from the discrete region as input. These equations are solved using the Monte Carlo technique suitably modified for a sharply varying laser intensity profile. The results show that the excitation of the QC and absorption due to a shaped laser pulse is greater than the corresponding quantities for a constant-intensity pulse of the same frequency, fluence and duration around the fundamental. The differences are small when the discrete-level bottleneck is very low (e.g. at the three-photon resonance frequency) or very high (e.g. when the frequency is blue shifted from the fundamental). The time dependence of multiphoton absorption (MPA) and temporal distribution of excited populations, is modified greatly by the pulse shape. The energy distributions of populations in the QC at different times, however, are not affected very much.