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 ∼10${}^{17}$ W cm${}^{-2}$ focal intensity, the yield of S${}^{13+}$ is ∼1$\%$ of the S${}^{3+}$ yield, in good agreement with simulations. An overabundance of S${}^{12+}$, and S${}^{14+}$ observed in the experimental ion spectra is not predicted by either single-vacancy or double-vacancy calculations.

• Received 29 July 2012

DOI:https://doi.org/10.1103/PhysRevA.86.053423