Semi-classical analysis of fan structure in strong field ionization

We use the semi-classical two-step (SCTS) model to investigate the fan-structure in the photoelectron momentum distribution produced by hydrogen atoms irradiated by linearly polarized near-infrared femtosecond laser pulses. It is shown that the initial tunneling moment and momentum of the fan-structure can be classified into four separated regions corresponding to four trajectory types as direct, forward-deflected, forward-scattering, and backward-scattering. We find a one-to-one mapping between the initial tunneling coordinates and the final momentum for the first three types of trajectories. We show that the phases and yields of the first three types are needed to explain the interference pattern in the fan-structure, rather than two types used normally, emphasizing the important role of the forward-scattering trajectory in formation of the fan-structure close to the field direction. Moreover, with the quiver amplitude fixed, a longer wavelength leads to a further electron tunneling exit, a weaker Coulomb potential effect, and a larger angular momentum.