PHASE DEPENDENCE OF INTENSE-FIELD IONIZATION

Two-color radiation has been used to investigate the physical mechanis m of intense-held ionization. One color was the second harmonic of the other, and there was a well-defined phase between them which was cali brated absolutely. We measured the photoelectron yield from above-thre shold ionization of krypton and xenon as this phase was varied. We sho w that the phase-dependent signature of tunnel ionization is present e ven though the electron spectra are clearly multiphoton in character. We also use the two-color phase as a probe to show evidence for core s cattering during the ionization process.