High-resolution measurements of the C $1s$ photoelectron spectrum of ethyne (acetylene) at photon energies between 297 and 348 eV have been analyzed to give the energy dependence of the relative cross sections for ionization of the $1{\sigma }_g$ and $1{\sigma }_u$ orbitals. The $1{\sigma }_u^{-1\mathrm{}}/1\mathrm{}{\sigma }_g^{-1\mathrm{}}$ intensity ratio is greater than 1 at the lowest photon energies but reaches a minimum of about 0.6 between 315 and 320 eV. These results can be understood in terms of the effect of shape resonance on the ionization cross section and provide quantitative information on the position and magnitude of such resonances.

• Received 11 June 1998

DOI:https://doi.org/10.1103/PhysRevLett.82.1120