High-Intensity Kapitza-Dirac Effect

Low-energy (< 20 eV) free electrons produced in above-threshold ionization have been scattered by an intense optical standing wave (the Kapitza-Dirac effect). At intensities of ${10}^{13}$ to ${10}^{14}$ W/${\mathrm{cm}}^2$ there is very high momentum transfer between the standing-wave "lattice" and the electrons ($\Delta p\approx 1000\hslash k$). In this regime, the scattering rate approaches the optical frequency, and the electron motion is most easily analyzed by classical mechanics.