Electron energy loss spectroscopy and anion formation in gas phase coronene

Abstract

Electron collisions with gas phase coronene have been studied in the energy range 0–80 eV to investigate electronic and pure vibrational excitations, as well as the anion formation near zero energy. Quantum chemical calculations using time dependent density functional theory (TDDFT) have been performed for an improved data analysis. Results have been obtained on both dipole allowed and forbidden transitions to low lying electronic excited states up to 8 eV, by varying the incident energy and scattering observation angle. Calculated values for excitation energies are in good agreement with experiment and with previous data. Pure vibrational excitation, performed with an energy resolution of 0.025 eV, is presented at incident energies from 1.5 to 9 eV. Five groups of excited vibrational modes are observed with energy losses ranging from 0.065 to 0.375 eV. Several resonant states appear to contribute to these excitations. Calculations of the harmonic vibrational frequencies of coronene, and determination of their main character, are used for the analysis of the results. The vertical electron affinity of coronene is calculated to be 0.45 eV in very good agreement with previous measured or calculated results. The parent anion (C₂₄H₁₂)⁻ yield versus electron energy presents 2 peaks at zero and 0.30 eV (within 0.030 eV), in contrast with previous results where only one peak at zero was reported [Denifl et al., J. Chem. Phys., 2005, 123, 104308].