Zero kinetic energy photoelectron spectroscopic investigations of hydroquinone isomers

Abstract

In this paper, we characterize the vibrational levels of the cis and trans isomers of hydroquinone (HYQ) cations and determine their adiabatic ionization potentials (IP) using zero kinetic energy (ZEKE) photoelectron spectroscopy. The Franck–Condon activity was observed in the modes 1, 3, 6a, 6b, 9a, 9b, 15 for both the isomers. For a few modes such as 15, 9b, and 6b, the assignments were justified based on the symmetry arguments. The IPs for the cis and trans isomers were determined to be 64055 and 64008 cm–1, respectively. The energy minimized structures and the normal mode frequencies for both the isomers in the cationic ground state (D0) and ground electronic state (S0) were calculated ab initio at various levels with the 6-31G** basis set. The optimized energies and frequencies of normal modes for the two isomers in the first electronic excited state (S1) were calculated at the configuration interaction with singlet excitation (CIS) and CASSCF level using the same basis set.