  Multiphoton ionization of sodium by femtosecond laser pulses of 800 nm
wavelength in the range of laser peak intensities entering over-the-barrier
ionization domain is studied. Photoelectron momentum distributions and the
energy spectra are determined numerically by solving the time dependent
Schroedinger equation for three values of the laser intensity from this domain.
The calculated spectra agree well with the spectra obtained experimentally by
Hart et al (Phys. Rev. A 2016 93 063426). A partial wave analysis of the
spectral peaks related to Freeman resonances has shown 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 specific laser intensities the selective ionization, which
occurs predominantly through a single intermediate state, is possible.
