Abstract
Starting from a covariant cycle-averaged Lagrangian the relativistic oscillation
center equation of motion of a point charge is deduced and analytical
formulae for the ponderomotive force in a travelling wave of arbitrary strength
are presented. It is further shown that the ponderomotive forces for transverse
and longitudinal waves are different; in the latter, uphill acceleration
can occur. In a standing wave there exists a threshold intensity above which,
owing to transition to chaos, the secular motion can no longer be described
by a regular ponderomotive force.