On the selective multiphoton ionization of sodium by femtosecond laser pulses: A partial-wave analysis

Abstract

Multiphoton ionization of sodium by femtosecond laser pulses of 800nm wavelength in the range of laser peak intensities belonging to the over-the-barrier ionization domain is studied. Photoelectron momentum distributions and the energy spectra are determined numerically by solving the time dependent Schrödinger equation for three values of the laser intensity from this domain. The positions of peaks related to Freeman resonances in the calculated spectra agree with the peak positions in the spectra obtained experimentally by Hart et al. [7]. A partial wave analysis of these peaks revealed that each peak is a superposition of the contributions of photoelectrons produced by the resonantly enhanced multiphoton ionization via different intermediate states. It is demonstrated that at appropriate laser intensities the selective ionization, which occurs predominantly through a single intermediate state, is possible.