MULTIPHOTON IONIZATION IN CIRCULARLY POLARIZED STANDING WAVES

We derive the wave functions for an electron moving in a quantized circularly polarized standing-wave radiation field. By applying these solutions as the final states, we derive multiphoton ionization transition-rate formulas according to Keldysh-Faisal-Reiss (KFR) theory. Numerical calculations and theoretical analysis show that in the case of two light beams with the same angular momentum, KFR theory does not predict the peak splittings in the photoelectron angular distributions that were observed in an experiment by Bucksbaum, Schumacher, and Bashkansky (BSB) [Phys. Rev. Lett. 61, 1162 (1988)]. A transition-rate formula from a scattering theory developed for the case of single-mode multiphoton ionization by Guo, Aberg, and Crasemann [Phys. Rev. A 40, 4997 (1989)] is extended to the standing-wave case. The theory predicts the peak splitting observed by BSB. Numerical results of the splitting angle from the theory show good agreement with experimental measurements. We also prove that the sine function of the half splitting angle is inversely proportional to the square root of the photoelectron kinetic energy E, i.e., sin(pi/2-theta(max)) is-proportional-to 1/E1/2, where theta(max) is the maximum scattering angle smaller than pi/2. Some predictions are made for future experimental observations, such as quantum effects associated with ponderomotive-energy decay.