Photoelectron Imaging and Photodetachment Spectroscopy for the Cryogenically Cooled Cyanocyclopentadienide Anion

The cyano-cyclopentadiene molecule (CN-C₅H₅) has attracted significant interest since its detection in the interstellar medium, but the radical (CN-C₅H₄) and anionic (CN-C₅H₄^-) forms of cyano-cyclopentadiene have not been studied. The cyano-cyclopentadienyl radical (CN-Cp) has a strong dipole moment, rendering it an ideal system for vibrational and rotational spectroscopy. We report an investigation of the cryogenically cooled cyano-cyclopentadienide anion (CN-Cp^-) using high-resolution photoelectron imaging, photodetachment spectroscopy, and resonant photoelectron imaging. The electron affinity of the CN-Cp radical is measured accurately to be 2.7741 ± 0.0003 eV (22,375 ± 2 cm^-1). A low-lying excited state is observed for the CN-Cp neutral radical at 151 cm^-1 above the ground state. The overlap and vibronic coupling of the ground and low-lying electronic states give rise to complicated and congested photoelectron spectra. A dipole-bound state is observed for the CN-Cp^- anion with a binding energy of 94 cm^-1, along with 15 vibrational Feshbach resonances. Resonant photoelectron spectra via the vibrational resonances yield well-resolved spectra, allowing 26 vibronic levels to be identified for CN-Cp. The rich spectroscopic information will be valuable to compare with theoretical studies to unravel the vibronic coupling and nonadiabatic effects in the CN-Cp radical.