EFFECT OF LASER-PULSE SHAPE ON MULTIPHOTON EXCITATION OF A POLYATOMICMOLECULE

The influence of the laser pulse shape on the multiple photon excitati on dynamics of a large molecule like SF6 has been studied numerically in the pico- and nanosecond scale. In the theoretical model used multi photon excitations are described in terms of the optical Bloch equatio ns for the density matrix of four anharmonically shifted discrete vibr ational 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 tech nique 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-l evel bottleneck is very low (e.g, at the three-photon resonance freque ncy) or very high (e.g. when the frequency is blue shifted from the fu ndamental). The time dependence of multiphoton absorption (MPA) and te mporal distribution of excited populations, is modified greatly by the pulse shape. The energy distributions of populations in the QC at dif ferent times, however, are not affected very much.