Rare-gas electron-energy spectra produced by collision-free multiquantum processes

The energy spectra of electrons generated by collision-free multiphoton ionization of Xe, Kr, Ar, Ne, and He irradiated at intensities up to --10/sup 15/ W/cm2 with picosecond 193-nm (6.41-eV) radiation have been studied with an energy resolution of --50 meV. The formation of multiply charged ions by a sequential process of ionization has been directly detected in the electron spectra by the observation of a characteristic pattern of interwoven above-threshold-ionization ladder line series. The appearance and relative intensity of specific electron lines depends strongly on the presence of near-resonances and features of the interaction involving the laser pulse shape, saturation, and the shift of the ionization threshold arising from the influence of the ponderomotive potential. The experimental results are compared qualitatively with data from ion time-of-flight experiments and with differing models of multiphoton ionization