Mechanism of electron capture by an intense laser beam

In this letter we report the studies of the mechanism which can lead to sub stantial energy exchange between electrons and intense laser beams interact ing in vacuum. It has been found that the wave phase experienced by the ele ctron when it encounters the intense beam is a crucial ingredient for discr iminating the two totally different trajectories: capture and violent accel eration scenario (CAS) or conventional inelastic scattering (IS). When an e lectron is captured, due to the diffraction effect of the optical beam near the focused region, the effective wave-phase velocity along the dynamic tr ajectory of the captured particle is found to be less than c, the speed of light in vacuum. Thus the captured electron can remain in the acceleration phase of the wave for a long time, and gain considerable energy from the la ser field.