Vibrationally resolved carbon core excitations in alkane molecules.

Abstract

Core-excitation spectra at the carbon K threshold of methane, ethane, and propane, as well as of the deuterated molecules, were studied by high-resolution photoabsorption using synchrotron radiation from the SX700/II monochromator at BESSY. The spectra contain signals from excitations to C 1${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$np Rydberg states and their vibrational substates, as well as from transitions to C 1${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$ns Rydberg states. The latter excitation is dipole forbidden in methane, but rendered possible by strong vibronic coupling. Transitions to vibrational substates could be identified via the observed isotope effects. The C 1${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$np Rydberg states were found to be split by a Jahn-Teller effect or by molecular-field interaction. C-H and C-C equilibrium distances as well as vibrational frequencies in the core-excited molecular states were derived within the Franck-Condon model from the vibrational fine structures of the C 1${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$3p Rydberg excitations.