Vibronic coupling in the K shell excitation of ethyne

The C 1 s −π ∗ band in the high-resolution photoabsorption spectra of the ethyne isotopomers C2H2, C2D2 and C2HD is found to be dominated by vibronic coupling effects. There is quantitative agreement between experiment and a recent theoretical model which treats vibronic coupling in both the core and valence orbital spaces. The theoretical analysis shows that a strong excitation of the bending modes dominates the observed spectra.