Physical Processes at High Field Strengths

The availability of extraordinarily bright femtosecond ultraviolet sources is rapidly extending the study of atomic responses into an unexplored regime for which the electric field strength is considerably in excess of an atomic unit. As this regime is approached, experiments studying multiple ionization, photoelectron spectra, harmonically produced radiation, and fluorescence all exhibit strong nonlinear coupling. Peak total energy transfer rates on the order of ∼ 2 × 10 -4 W/atom have been observed at an intensity of ∼ 10 16 W/cm 2 , and the removal of an electron from an inner principal quantum shell has been demonstrated in xenon. Measurements of the radiation produced by the high field interaction with the rare gases have revealed the presence of both copious harmonic production and fluorescence. The highest harmonic observed was the seventeenth (14.6 nm) in Ne, the shortest wavelength ever produced by that means. Strong fluorescence was seen in Ar, Kr, and Xe with the shortest wavelengths observed being below 10 nm. Furthermore, radiation from inner-shell excited configurations in Xe, specifically the 4 d 9 5 s 5 p → 4 d 10 5 s manifold at ∼ 17.7 nm, was detected. The behaviors of the rare gases with respect to multiquantum ionization, harmonic production, and fluorescence were found to be correlated so that the materials fell into two groups, He and Ne in one and Ar, Kr, and Xe in the other. These experimental findings, in alliance with other studies on innershell decay processes, give evidence for a role of atomic correlations in a direct nonlinear process of inner-shell excitation. It is expected that an understanding of these high-field processes will enable the generation of stimulated emission in the x-ray range.