Multi-quantum processes at high field strengths

Subpicosecond ultraviolet laser technology is enabling the exploration of nonlinear atomic interactions with electric field strengths in excess of an atomic unit. As this regime is approached, experiments studying multiple ionization, photoelectron energy spectra, and harmonically produced radiation all exhibit strong nonlinear coupling. Recent findings in connection with the ion spectra produced by such interactions are described, and it is shown that energy transfer from the radiation field to the atom depends importantly on the risetime of the field. The nonlinear atomic interaction is also discussed with respect to high laser intensities (≳1018 W/cm2) in combination with high atomic density (≳1018 cm−3), for which inter‐atomic as well as intraatomic coupling may play a significant role.