A Monte Carlo wave function study of the effect of collisions on the coherent multiphoton excitation of SF6

Abstract

Collisional interruption of coherent excitation of SF6 in the lower discrete region of its laser absorbing mode (ν3) have been studied using the recently developed quantum Monte Carlo wave function (QMCWF) method. The usual pure pump mode description up to 3ν3 and complete mixing of all modes (QC) above it have been assumed. The rotational and anharmonic splitting of the vibrational states upto 3ν3 are taken into account but splitting due to tensor interaction terms are neglected. Excitation to QC is represented by irreversible leakage from the coherent ladder modeled by an imaginary term in the Hamiltonian of the coherently excited vibrational rotational levels. QMCWF study has been carried out at the laser frequency 942.8 cm−1 for which the local time average populations in the intermediate excited vibrational states are found to be negligible. Large leakage occurs only from narrow band of ground rotational states due to 3 photon resonances but their number increases with increasing intensity. Two different collisional energy transfer models, one obeying symmetry imposed restrictions and propensity rule, and the other free from such restrictions except the fact that collisions restore the thermal distribution, have been used. Results show different pressure effects at different temperature and for different intensities. However, the two different models used for collisional rotational transition probabilities give similar enhancement of leakage at high intensities.