Studies of multiphoton production of vacuum-ultraviolet radiation in the rare gases

Measurements of the vacuum-ultraviolet radiation produced by intense ultraviolet irradiation of rare gases have revealed the copious presence of both harmonic radiation and fluorescence from excited levels. The highest harmonic observed was the seventeenth in Ne, the shortest wavelength ever produced by that means. Strong fluorescence was seen from ions of Ar, Kr, and Xe, with the shortest wavelengths observed being below 12 nm. Furthermore, radiation from inner-shell excited configurations in Xe, specifically the 4d9 5s 5p to 4d10 5s manifold of Xe(7+) at about 17.7 nm, was detected. These experimental findings, in alliance with other studies concerning multielectron processes, give evidence for a role of electron correlations in a direct nonlinear process of inner-shell excitation.