Spin-induced force in intense laser-electron interaction

We investigate analytically the spin-laser coupling of an electron in an ul tra-intense linearly polarized plane laser field. Particular emphasis is pl aced on the reaction of the spin on the electron motion which, in contrast to a recently applied approach, is calculated by means of a quantum mechani cal description. An additional oscillation in the direction of the magnetic field is predicted and the classical results are confirmed up to a numeric al constant. By comparison with Klein-Gordon results we show that this effe ct is doubtless a signature of the spin, and isolate the leading second-ord er correction terms in the ratio of electron and light velocity.