Vibronic structure in the carbon1sphotoelectron spectra of HCCH and DCCD

The carbon $1s$ photoelectron spectra of HCCH and DCCD have been measured at a photon energy of 330 eV and an instrumental resolution about half the natural linewidth. The vibrational structure in the spectra has been analyzed in terms of a model in which the parameters are the force constants for carbon-carbon and carbon-hydrogen stretching in the core-ionized molecules and the changes in bond lengths between the core-ionized and neutral molecules. Within this model, three different approaches to core-hole localization have been considered. Treating the core hole as completely localized, with the molecular motion following the diabatic energy surfaces, does not describe the data correctly. Treating the core hole as completely delocalized, with the molecular motion following the adiabatic surfaces, gives a good fit to the spectra but leads to zero-point energies that are completely unreasonable. A fit that takes into account vibronic coupling between the vibrational manifolds of the ${}^2{\Sigma }_u^{+}$ and ${}^2{\Sigma }_g^{+}$ electronic states of the ion gives good agreement with the data and leads to reasonable molecular parameters. Ab initio calculations of the molecular properties of the core-ionized molecule give results that are in excellent agreement with those obtained from this fit. The lifetime width for the carbon $1s$ hole state is $106\pm 2\mathrm{meV},$ significantly higher than for ${\mathrm{CH}}_4\hspace{0.5em}(95\mathrm{}\pm 2\mathrm{meV}).\mathrm{}$ This result is not in accord with predictions based on a one-center model of Auger decay.