Interference Oscillations in the Angular Distribution of Laser-Ionized Electrons near Ionization Threshold

We analyze the two-dimensional momentum distribution of electrons ionized by few-cycle laser pulses in the transition regime from multiphoton absorption to tunneling by solving the time-dependent Schrödinger equation and by a classical-trajectory Monte-Carlo simulation with tunneling (CTMC-T). We find a complex two-dimensional interference pattern that resembles above threshold ionization (ATI) rings at higher energies and displays Ramsauer-Townsend-type diffraction oscillations in the angular distribution near threshold. CTMC-T calculations provide a semiclassical explanation for the dominance of selected partial waves. While the present calculation pertains to hydrogen, we find surprising qualitative agreement with recent experimental data for rare gases [A. Rudenko, J. Phys. B 37, L407 (2004)].