We theoretically study the two-photon ionization of H for the laser frequency range (0.25, 0.5) a.u. by numerically solving the time-dependent Schrödinger equation. The results show that the photoelectron angular distributions (PADs) are strongly dependent on the laser frequency. In the vicinity of the resonant frequency, the photoelectron momentum distributions show a doublet structure. For the inner ring, the population in the parallel direction is larger than the perpendicular direction, and the distribution shows little variation with the laser frequency. For the outer ring, as the laser frequency increases, the electron emissions transfer from the parallel to perpendicular direction. Below the resonant frequency region, the proportion of the perpendicular emissions increases with the laser frequency decreasing. Above the resonant frequency region, the electron emissions transfer from the perpendicular to parallel direction, and finally the dominant emission direction becomes the parallel direction. By separating the contributions from the outgoing s wave, d wave and their interference, the strong dependency of the PADs on the laser frequency can be well understood. These results provide deep insights into the two-photon ionization by ultraviolet laser pulses.
Liu et al. (Sat,) studied this question.