The C and O $1s$ photoelectron lines of the C${\mathrm{O}}_2$ molecule in the gas phase have been measured with vibrational resolution in the threshold region. The vibrational fine structure on the O $1s$ line is completely dominated by the antisymmetric stretching mode with a frequency of 307 $(\pm 3)\mathrm{meV}$. This mode can be excited only via vibronic coupling, as predicted by Domcke and Cederbaum [Chem. Phys. 25, 189 (1977)], and provides a mechanism for dynamic core-hole localization. Relaxation effects are found to affect strongly the vibrational intensity distribution of the photoelectron line.

• Received 12 November 1996

DOI:https://doi.org/10.1103/PhysRevLett.79.998