Accessing Polyromantic Radicals From Below: A Showcase for a Harmonic Model in Franck Condon Simulation of Photoelectron Spectra of Polyaromatic Radical Anions
As one of the most abundant forms of carbon in the universe, polyaromatic hydrocarbons (PAH) play a central role in the field of Biochemistry and Astrochemisty. In this work, we present a Franck–Condon analysis of the vibrational structure in the negative ion photoelectron spectra of naphthalene as a prototype for analysis of a photoelectron spectra of a system containing a rigid ring structure. The Franck–Condon profiles of the anion photoelectron spectra are simulated using the PESCAL program that uses the theoretical geometries, normal mode vectors, and vibrational frequencies of the anion and neutral states calculated using the GAUSSIAN 09 software package. These simulations are based on a harmonic oscillator approximation model that utilizes the Duschinsky rotation between the normal mode vectors of the anion and neutral. This harmonic approach very accurately simulates electronic spectra that involve relatively small displacements of equilibrium geometries of the neutral relative to the anion of a large cyclic system like naphthalene.