Abstract

We report the first study of Multi-Photon Ionization (MPI) using an intense two-color light field whose relative phase is known. The light field was produced by overlapped 80 ps pulses of 1.06 µm and .532 µm at various intensities and the phase difference was measured using non-linear mixing in KD*P. The light was focused into a chamber backfilled with Krypton gas, and the photoelectron energy spectrum was measured. We observed periodic variation of the MPI photoelectron peaks as a function of phase; moreover, the variation was seen to be asymmetric with respect to a change of sign in phase. The degree of asymmetry varied in a regular way as a function of intensity and peak energy. In general, the peak ionization rate was not observed to occur at the phase difference that produced the peak field. These effects were predicted for Hydrogen by Schaffer et al.,¹ and are believed to be general in nature. We also report the effect of adding a small circular polarization component to the light field and the implications for a theory recently advanced by Corkum.²E Finally, we discuss the origin of these effects.

© 1993 Optical Society of America