Multiple Ionization of Rare Gas Atoms Irradiated with Intense VUV Radiation

The interaction of intense vacuum-ultraviolet radiation from a free-electron laser with rare gas atoms is investigated. The ionization products of xenon and argon atomic beams are analyzed with time-of-flight mass spectroscopy. At 98 nm wavelength and ~10¹³ W/cm² multiple charged ions up to Xe⁶⁺ (Ar⁴⁺) are detected. From the intensity dependence of multiple charged ion yields the mechanisms of multiphoton processes were derived. In the range of ~10¹²--10¹³ W/cm² the ionization is attributed to sequential multiphoton processes. The production of multiple charged ions saturates at 5--30 times lower power densities than at 193 and 564 nm wavelength, respectively.