Mechanisms on the Photoelectron Angular Distributions of Atoms Ionized in Mid-Infrared Laser Fields

By solving the time-dependent Schrodinger equation, we show conclusively that low energy/momenta structure in the photoelectron angular distribution originates from multiple scatterings of the tunnel-ionized electron with the ion. We also show that two conditions must be satisfied simultaneously in order to observe prominent low-energy features. First, multiple scattering of the tunnel-ionized electron wave packet is necessary. Second, tunnel ionization must dominate over multiphoton ionization. While the first condition is generally satisfied for all laser wavelengths, the second condition is satisfied only for longer laser wavelengths.