Multiphoton L-shell ionization of H₂S using intense x-ray pulses from a free-electron laser

Sequential multiphoton L-shell ionization of hydrogen sulfide exposed to intense femtosecond pulses of 1.25-keV x rays has been observed via photoelectron, Auger electron, and ion time-of-flight spectroscopies. Monte Carlo simulations based on relativistic Dirac-Hartree-Slater calculations of Auger decay rates in sulfur with single and double L-shell vacancies accurately model the observed spectra. While single-vacancy-only calculations are surprisingly accurate even at the high x-ray intensity used in the experiment, calculations including double-vacancy states improve on yield estimates of highly charged sulfur ions. In the most intense part of the x-ray focal volume, an average molecule absorbs more than five photons, producing multiple L-shell vacancies in 17% of photoionization events according to simulation. For 280-fs pulse duration and ∼1017 Wcm^-2 focal intensity, the yield of S13⁺ is ∼1% of the S3⁺ yield, in good agreement with simulations. An overabundance of S12⁺, and S14⁺ observed in the experimental ion spectra is not predicted by either single-vacancy or double-vacancy calculations.