Vibronic structure in the low lying Rydberg states of hexafluorobenzene and 1,3,5-trifluorobenzene detected by two-photon spectroscopy

An Exhaustive Quantum-Classical Study of C6F6+ Using the Newly Formulated Parallel TDDVR Method

We recently implemented our parallelized quantum-classical dynamical approach, known as the Time-Dependent Discrete Variable Representation (TDDVR) method, which is applied to the spectroscopically important hexafluorobenzene (HFBz) radical cation, where several conical intersections exist in their seven lowest excited electronic states (S11B2u, S21E1g, S31B1u, S41E1u, and S51A2u) considering degeneracy among potential energy surfaces (PESs), to demonstrate their various dynamical aspects. This new parallel version shows almost linear scalability with increasing number of computing processors. To get photoelectron (PE) spectra, Mass-Analyzed Threshold Ionization (MATI) spectra, population dynamics, and many other dynamical observables, the first-principles dynamics is applied at the state-of-the-art level to the corresponding Hamiltonian, where the Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) type interactions are involved in those coupled seven electronic states. The quantum-classical method is used to thoroughly analyze the effects of these couplings on the nuclear dynamics of the involved electronic states, and the findings are compared with those observables obtained from experiments. Intrinsic dynamical properties are explained using the reduced densities of the wave packet (WP) in a coupled electronic manifold. The PE and MATI spectra of HFBz computed using TDDVR are found to be in good agreement with earlier experimental data and other theoretically simulated spectra.

Vibronic coupling in the first six electronic states of pentafluorobenzene radical cation: Radiative emission and nonradiative decay

Nuclear dynamics in the first six vibronically coupled electronic states of pentafluorobenzene radical cation is studied with the aid of the standard vibronic coupling theory and quantum dynamical methods. A model 6 × 6 vibronic Hamiltonian is constructed in a diabatic electronic basis using symmetry selection rules and a Taylor expansion of the elements of the electronic Hamiltonian in terms of the normal coordinate of vibrational modes. Extensive ab initio quantum chemistry calculations are carried out for the adiabatic electronic energies to establish the diabatic potential energy surfaces and their coupling surfaces. Both time-independent and time-dependent quantum mechanical methods are employed to perform nuclear dynamics calculations. The vibronic spectrum of the electronic states is calculated, assigned, and compared with the available experimental results. Internal conversion dynamics of electronic states is examined to assess the impact of various couplings on the nuclear dynamics. The impact of increasing fluorination of the parent benzene radical cation on its radiative emission is examined and discussed.

Observation of a Symmetry-Forbidden Excited Quadrupole-Bound State

We report the observation of a symmetry-forbidden excited quadrupole-bound state (QBS) in the tetracyanobenzene anion (TCNB^-) using both photoelectron and photodetachment spectroscopies of cryogenically-cooled anions. The electron affinity of TCNB is accurately measured as 2.4695 eV. Photodetachment spectroscopy of TCNB^- reveals selected symmetry-allowed vibronic transitions to the QBS, but the ground vibrational state was not observed because the transition from the ground state of TCNB^- (A_u symmetry) to the QBS (A_g symmetry) is triply forbidden by the electric and magnetic dipoles and the electric quadrupole. The binding energy of the QBS is found to be 0.2206 eV, which is unusually large due to strong correlation and polarization effects. A centrifugal barrier is observed for near-threshold autodetachment, as well as relaxations from the QBS vibronic levels to the ground and a valence excited state of TCNB^-. The current study shows a rare example where symmetry selection rules, rather than the Franck-Condon principle, govern vibronic transitions to a nonvalence state in an anion.