Electronic nonadiabatic interactions and ultrafast internal conversion in phenylacetylene radical cation.

Quantum chemistry and dynamics of the ground X (2)B(1) and low-lying excited A (2)A(2), B (2)B(2), and C (2)B(1) electronic states of phenylacetylene radical cation are examined here in striving to understand its photostability, long-lived excited electronic states, and resolved (<10 meV) vibrational energy level spectrum. The electronic potential energy surfaces and their nonadiabatic coupling are computed ab initio. A model Hamiltonian is constructed in a diabatic electronic basis for the nuclear dynamical simulations from first principles. Analysis of electronic structure data reveals the relevance of 24 vibrational degrees of freedom in the quantum dynamics of the X-A-B-C coupled electronic states of the radical cation. The complex vibrational energy level spectrum of this coupled electronic manifold is calculated and assigned. Theoretical results are in excellent accord with the experimental photoelectron spectroscopy data. The agreements and discrepancies of the theoretical results are also recorded and discussed with the mass-analyzed threshold ionization and photoinduced Rydberg ionization and photodissociation spectroscopy results of the X and C electronic states, respectively. The lifetimes of the excited electronic states of phenylacetylene radical cation are estimated from the decay of electronic population and are discussed in relation to the available experimental data.