Construction of Beyond Born-Oppenheimer-Based Diabatic Surfaces of o- C 6 $_{6}$ H 4 $_{4}$ F 2 + $_{2}^{+}$ Radical Cation: Calculation of Photoelectron and Mass-Analyzed Threshold Ionization Spectra.

Theoretically "exact" and numerically "accurate" Beyond Born-Oppenheimer (BBO)-based diabatic Hamiltonian for an aromatic radical cation, namely, o- $C_6$ H ${}_4$ F ${}_2^{+}$ , is constructed theoretically for the first time, where the low-lying five electronic states ( ${\tilde{X}}^2{B}_1$ , ${\tilde{A}}^2{A}_2$ , ${\tilde{B}}^2{B}_1$ , ${\tilde{C}}^2{A}_1$ , and ${\tilde{D}}^2{B}_2$ ) are nonadiabatically coupled with each other, exhibiting conical intersections (CIs)/seams due to accidental Jahn-Teller (JT) couplings as well as depicting pseudo-Jahn-Teller interactions. Once the diabatic PESs and couplings are constructed, those are used to execute multistate multimode nuclear dynamics for generating photoelectron (PE) and mass-analyzed threshold ionization spectra of the neutral analog. The calculated PE spectra for ${\tilde{X}}^2{B}_1$ , ${\tilde{A}}^2{A}_2$ , ${\tilde{B}}^2{B}_1$ , ${\tilde{C}}^2{A}_1$ and ${\tilde{D}}^2{B}_2$ states originated from the BBO-based diabatic Hamiltonian in conjunction with time-dependent discrete variable representation (TDDVR) dynamics show peak-by-peak correspondence with the experimental spectral bands as well as with other theoretical findings.