  The dynamics of single laser-driven electrons and many particle systems with
spin are investigated on the basis of a classical theory. We demonstrate that
the spin forces can alter the electron dynamics in an ultra-relativistic laser
field due to the coupling of the electron's spin degree of freedom to its
kinematic momentum. High-energy electrons can acquire significant
spin-dependent transverse momenta while passing through a counterpropagating
ultra-relativistic infrared laser pulse. Numerical calculations show that the
deflection of the electrons by the laser pulse is determined by the laser
intensity, the pulse duration, and the initial spin orientation of the
electron. We complement our investigation of these dynamical spin effects by
performing particle-in-cell simulations and point out possibilities of an
experimental realization of the predicted effect with available laser
parameters.
