Nonadiabatic tunneling in ponderomotive barriers

Localized regions of intense large-scale radiofrequency field are known to act like effective ("ponderomotive") potential barriers, which scatter particles elastically and in the direction determined by the particle initial velocity rather than phase. In smaller-scale fields, transmission through a ponderomotive barrier is probabilistic and resembles tunneling of a quantum particle through a static potential. We derive asymptotic expressions for the phase-averaged transmission coefficient T as a function of the particle energy E0. We show that, unlike for a truly quantum particle, T(E0) is of algebraic form and has a threshold, below which transmission does not occur. We also find a threshold in E0, above which all particles are transmitted regardless of their initial phase.