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Above-threshold ionization (ATI) is investigated in xenon with pulses of light at 616 nm ranging from 15 to 0.4 psec. Significant energy shifts and broadenings of the ATI peaks are observed. For pulse widths less than 1 psec, the individual ATI peaks break up into a narrow fine structure, apparently due to resonance enhancements in the ionization process produced by ponderomotive shifts of states.
Freeman et al. (Mon,) studied this question.