Nondipole Photoelectron Momentum Shifts In Strong-Field Ionization With Mid-Infrared Laser Pulses Of Long Duration

We consider atomic strong-field ionization in the long-pulse limit for linearly polarized infrared laser fields. We show how nondipole effects in the plane formed by the propagation and polarization directions lead to (i) a shift of the origin of the rings describing the energetically allowed final electron momenta for individual above-threshold ionization (ATI) channels and (ii) a redistribution in the continuum population on each ring. In this manner, the photoelectron energy in a given ATI channel depends on the direction of the ejected electron.