The electronic states of nitrobenzene: electron-energy-loss spectroscopy and CASPT2 calculations

Abstract

The electronic excitations of thin solid films of nitrobenzene have been investigated by low-energy electron-energy-loss (EEL) spectroscopy with the aim of locating triplet states not previously observed. The spectra represent the first results for the solid phase. At low residual energy optically forbidden bands are detected. CASPT2 calculations suggest that the intensity in the energy range from 3.0 to 4.0 eV is due to several overlapping transitions whereas between 5.0 and 5.9 eV only one ¹ππ* and two ³ππ* excitations are located. The broadness of the band in the latter range is therefore not satisfactorily explained as a mere effect of band congestion but probably relates to a flat potential energy surface or large geometry deformation of the respective states. Crystalline films of nitrobenzene exhibit a specular enhancement of dipole transitions. This effect is observed for the first time and can be exploited for the analysis of electronic low-energy EEL spectra recorded on solid samples.