Strong-field ionization of argon: Electron momentum spectra and nondipole effects

We investigate the influence of relativistic nondipole effects on the photoelectron spectra of argon, particularly in the low-kinetic-energy region (0-1 eV). In our experiment, we use intense, linearly polarized 800-nm laser pulses to ionize Ar from a jet and we record photoelectron energy and momentum distributions using a reaction microscope. Our measurements show that nondipole effects can cause an energy-dependent asymmetry along the laser propagation direction in the photoelectron energy and momentum spectra. Model simulation based on the time-dependent Dirac equation can reproduce our measurement results. Moreover, the electron trajectory analysis based on the classical model reveals that the photoelectrons obtain a negative momentum shift in the laser propagation direction due to the interplay between the Lorentz-force-induced radiation pressure during its free propagation in the continuum and rescattering by the Coulomb potential of the parent ion when it is driven back by the laser field.