Abstract

The effect of intermediate multiphoton resonances on the threshold and above-threshold ionization probability of rare gas atoms in intense visible and UV laser fields is illustrated with ion and photoelectron spectra. Substructure observable in the electron energy distribution resulting from multiphoton ionization with ultrashort pulses is observed and attributed to intermediate states being shifted into resonance as first described by Freeman et al. Resonant enhancement remains significant at intensities in excess of 10¹³ W/cm². Measurement of the ac Stark shifts and photoionization rates of members of the 4p⁵4d and 4p⁵4d′ manifolds of krypton are presented along with measurements of the multiphoton Rabi rate between the ground and excited states. Measurements of multiphoton ionization cross sections are presented for krypton and xenon and compared with recent perturbative and nonperturbative calculations.

© 1988 Optical Society of America