  We investigate the influence of relativistic nondipole effects on the
photoelectron spectra of argon, particularly in the low kinetic energy region
(0 eV - 5 eV). In our experiment, we use intense linearly polarised 800 nm
laser pulse to ionise Ar from a jet and we record photoelectron energy and
momentum distributions using a reaction microscope (REMI). Our measurements
show that nondipole effect can cause an energy-dependent asymmetry along the
laser propagation direction in the photoelectron energy and momentum spectra.
Model simulation based on time-dependent Dirac equation (TDDE) can reproduce
our measurement results. The electron trajectory analysis based on classical
model reveals that the photoelectron which obtains negative momentum shift
along laser propagation direction is caused by the interplay between the Lorenz
force induced radiation pressure during its free propagation in continuum and
re-scattering by Coulomb potential of the parent ion when it is driven back by
the laser field.
