The electron photodetachment spectrum of c-C4F8-: A test case for the computation of Franck-Condon factors of highly flexible molecules

The long 20 000 cm−1 vibrational progression observed in the electron photodetachment spectrum of c-C4F8 − anion is analyzed in terms of the Franck-Condon factors between the ground electronic states of the anionic and neutral species. The observed spectrum is correctly reproduced and its vibronic structure is assigned in detail. The very simple structure of the spectrum, consisting of a sequence of evenly spaced peaks, at 355 cm−1 each other, is assigned to a series of overlapping progressions associated with vibrational excitations of two totally symmetric modes rather than to a single mode progression, as originally proposed. The underlying continuum observed in the experimental spectrum is traced back to the excitations of a low frequency ring-puckering vibration, which, in the neutral species, exhibits a double minimum potential energy profile.