Toward the Spectra of the Cyanocyclopentadienide Anion: Constructing a 27-Dimensional Vibronic Hamiltonian and Performing Full-Dimensional Quantum Dynamics

Abstract The negative-ion photoelectron spectrum of the cyanocyclopentadienide anion is simulated using a multimode vibronic-coupling approach based on a two-state quasi-diabatic Hamiltonian, Hd. This Hamiltonian accurately describes the region near the ab initio equilibrium geometry of the neutral ground state, as well as the minimum-energy crossing point on the conical intersection seam, which lies only approximately 1600 cm–1 above the ground-state minimum. Comparison with the experimental spectra allows the X and A peaks to be assigned to the ground vibrational levels of the 2B1 and 2A2 diabatic states, respectively. Significant mixing of harmonic multimode basis functions induced by the conical intersection is evident even for these low-lying X and A peaks. Moreover, peaks X and A show the strongest enhancement in most of the resonant photoelectron spectra, suggesting that the Δν = −1 propensity rule may extend to multiple electronic states.