Partial-wave analysis of multiphoton ionization of sodium by femtosecond laser pulses of 800 nm wavelength in over-the-barrier ionization regime

Multiphoton ionization of sodium by laser pulses of 800 nm wavelength and 57 fs duration is studied in the range of laser peak intensities belonging to over-the-barrier ionization regime. Photoelectron momentum distributions (PMD) and the energy spectra are determined numerically by solving the time dependent Schr\"odinger equation. The calculated spectra agree well with the spectra obtained experimentally by Hart et al. [Phys. Rev. A 93, 063426 (2016)]. The contributions of photoelectrons with different values of the orbital quantum number in the PMD are determined by expanding the photoelectron wave function in terms of partial waves. Partial wave analysis of the spectral peaks related to Freeman resonances has shown that each peak has photoelectron contributions from different ionization channels which are characterized by different photoelectron energies and different symmetries of released photoelectron wave-packets. These findings are justified by calculating the populations of excited states during the pulse. Our analysis indicates that the contribution of specific ionization channels in the total photoelectron yield might be selectively increased by varying to some extent the values of pulse parameters used here.