Effect of Electron Initial Longitudinal Velocity on Low-Energy Structure in Above-Threshold Ionization Spectra

Using a semiclassical model, we investigate the effect of the initial longitudinal velocity of the tunnel-ionized electron on low-energy structure (LES) in above-threshold ionization (ATI) spectra. Our analysis shows that the effect (reduction or enhancement) of the initial longitudinal velocity p_z0 on the LES is dependent on the direction of the initial longitudinal velocity. For the initial longitudinal velocity along the direction of the laser electric field at the moment of tunneling (i.e., p_z0 > 0), the Coulomb interaction between the photoelectron and ion core is enhanced, while for the initial longitudinal velocity in the opposite direction (i.e., p_z0 < 0), the Coulomb effect is reduced. Our work provides solid evidence that the initial electron longitudinal velocity has considerable effect on the photoelectron dynamics in the ATI process.