Theoretical investigation of above-threshold ionization of the sodium atom in weakly coupled plasma

The effects of weakly coupled plasma on above-threshold ionization (ATI) of the sodium atom are theoretically investigated using linearly polarized femtosecond (fs) laser pulses at three different frequencies, ω, 2ω, and 3ω (800, 400, and 267 nm). Within the dipole approximation, the three-dimensional nonrelativistic time-dependent Schrödinger equation is solved numerically using the Crank–Nicolson method in the single-active-electron approximation. The influence of plasma-screening on the ground-state population probability is examined as a function of laser pulse duration for Debye lengths λ = 20 and 40 a.u. Photoelectron spectra (PES) are investigated at λ = 20, 40, and 50 a.u., along with the isolated atom, using the infinite-time surface flux method. As the Debye length decreases, the ATI peak positions in the PES exhibit a blue shift, consistent with earlier studies of ATI in plasma-screened atoms. The momentum-resolved PES shows an increase in the ATI-ring radii in the plasma environment compared with the plasma-free case. The effect of optical cycles (nc = 10, 20, and 30) is also examined for both the plasma-screened and isolated sodium atom, and an enhancement in the differential ionization probability is observed in the plasma environment.