Absorption by Some Molecular Gases in the Extreme Ultraviolet

The absorption spectra of several polyatomic molecules (hydrides and fluorides) have been measured in the range of photon energies 100-200 eV. High resolution was obtained by the use of synchrotron radiation and a grazing-incidence spectrometer. Rydberg series were observed with sharp structure converging upon the $L_{\mathrm{I}\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}}$ edges of Si${\mathrm{H}}_4$, P${\mathrm{H}}_3$, ${\mathrm{H}}_2$S, and HCl. These molecules all have argon as a united atom and the observed series correspond to excitation of a $2p$ core electron to $s$- and $d$-like final states. In silane the $d$-like final states show a splitting due to the molecular field. The assignments are confirmed by a molecular-orbital calculation which will be reported elsewhere. The $L_{\mathrm{I}\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}}$ spin-orbit splittings and threshold energies are compared with other experimental and theoretical data.