Pyrene radical cation and the diffuse interstellar band at 4430 Å: A theoretical survey

Abstract

Multistate and multimode vibronic dynamics on the first six coupled (X˜2B1g - A˜2B2g - B˜2B3u - C˜2Au - D˜2B3u - E˜2B3g ) electronic states of pyrene radical cation are investigated with the aid of ab initio electronic structure calculations of potential energy surfaces (PESs) and first principle quantum dynamical methods. Several low-lying conical intersections among various electronic states are found and their impact on the nuclear dynamics is discussed. The simulation of nuclear motion on the underlying coupled electronic states of pyrene radical cation is examined by both time-independent and time-dependent quantum mechanical methods. The combination of time-independent and time-dependent quantum mechanical simulations render a satisfactory explaination of a large part of the complex vibronic spectrum of Py+ which is in very good agreement with the photoelectron spectroscopic data. Furthermore, this study also establishes that polycyclic aromatic hydrocarbons can be the molecular carrier of diffused interstellar bands and ultrafast non-radiative deactivation of their excited electronic states makes them photostable against strong UV irradiation in the interstellar medium.