Vibronic coupling in the ground and excited states of the pyridine radical cation

Vibronic interactions in the pyridine radical cation ground state, ²A₁, and its lowest excited states, ²A₂ and ²B₁, are studied theoretically. These states originate from the ionization out of the highest occupied orbitals of pyridine, 7a₁ (nσ), 1a₂ (π), and 2b₁ (π), respectively, and give rise to the lowest two photoelectron maxima. According to our previous high-level ab initio calculations [Trofimov et al., J. Chem. Phys. 146, 244307 (2017)], the ²A₂ (π^-1) excited state is very close in energy to the ²A₁ (nσ^-1) ground state, which suggests that these states could be vibronically coupled. Our present calculations confirm that this is indeed the case. Moreover, the next higher excited state, ²B₁ (π^-1), is also involved in the vibronic interaction with the ²A₁ (nσ^-1) and ²A₂ (π^-1) states. The three-state vibronic coupling problem was treated within the framework of a linear vibronic coupling model employing parameters derived from the ionization energies of pyridine computed using the linear response coupled-cluster method accounting for single, double, and triple excitations (CC3). The potential energy surfaces of the ²A₁ and ²A₂ states intersect in the vicinity of the adiabatic minimum of the ²A₂ state, while the surfaces of the ²A₂ and ²B₁ states intersect near the ²B₁ state minimum. The spectrum computed using the multi-configuration time-dependent Hartree (MCTDH) method accounting for 24 normal modes is in good qualitative agreement with the experimental spectrum of pyridine obtained using high-resolution He I photoelectron spectroscopy and allows for some assignment of the observed features.