Vibrational fine structure of C₅ via anion slow photoelectron velocity-map imaging

High-resolution anion photoelectron spectra of cryogenically cooled C ₅- clusters are reported using slow photoelectron velocity-map imaging spectroscopy. We resolve vibronic transitions to the ν₂ stretching mode and multiply excited ν₅, ν₆, and ν₇ bending modes of neutral C₅ with significantly higher accuracy than previous experiments. Weak transitions to Franck-Condon (FC) forbidden singly excited bending modes are made possible by Herzberg-Teller coupling between electronic states of the neutral cluster. In addition, we resolve vibrational fine structure corresponding to different angular momentum states of multiply excited bending modes. The observation of this multiplet structure, some of which is FC forbidden, is attributed to Renner-Teller coupling between vibrational levels in the C₅- ground electronic state.