  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.
