Lorentz force on an electron in a strong plane-wave laser field and the low-frequency limit for ionization

A motion of a classical free charge in an electromagnetic plane wave can be found exactly in a fully relativistic case. I have found an approximate non-parametric form of the suitable equations of motion. In a linearly polarized wave, in the simplest frame of reference, the charge moves along the well-known figure-eight path. I have numerically calculated the Lorentz force acting on the charge as a function of time. By virtue of this, for the low-frequency ionization (or detachment) rate, I discuss a manifestation of nondipole and relativistic effects. When intensity of the plane wave increases, these effects can first appear in angular distributions, then in spectra of outgoing electrons, but have quite little effect on total ionization rates. I try to give an explanation of the latter fact. ©2010 The American Physical Society