Photon energy dependence of the 1 sigma(u)/1 sigma(g) intensity ratio in carbon 1s photoelectron spectroscopy of ethyne

High-resolution measurements of the C Is photoelectron spectrum of ethyne ( acetylene) at phc,ton energies between 297 and 348 eV have been analyzed to give the energy dependence of the relative cross sections for ionization o f the 1 sigma(g), and 1 sigma(u) orbitals. The 1 sigma(u)(-1)/1 sigma(g)(-1 ) intensity ratio is greater than 1 at the lowest photon energies but reach es a minimum of about 0.6 between 315 and 320 eV. These results can be unde rstood in terms of the effect of shape resonance on the ionization cross se ction and provide quantitative information on the position and magnitude of such resonances. [S0031-9007(98)08336-7].