Global Franck-Condon breakdown: nonresonant molecular photoionization processes*

We report photoelectron spectroscopy results of nonresonant Franck-Condon breakdown in the photoionization of CO and ICN. Most importantly, the deviations occur over a surprisingly wide range of energies. For the case of CO$^{+}(X^{2}\Sigma ^{+})$, the v$^{~+~}$=~1/v$^{~+~}$=~0 vibrational branching ratio is found to vary significantly ($>$50{\%}) over a 200~eV range. While it is well understood that resonances can lead to coupling between photoelectron and molecular vibration, there is little information on nonresonant sources of coupling. It appears that Cooper minima may be responsible for the observations. Moreover, for ICN, the vibrationally resolved deviations from Franck-Condon behavior are vibrationally mode-specific. Studies on alternative molecular targets are planned to see whether they exhibit photoelectron dynamics that are geometry-dependent.