ELECTRONIC AND VIBRATIONAL-SPECTRA OF MATRIX-ISOLATED ANTHRACENE RADICAL CATIONS - EXPERIMENTAL AND THEORETICAL ASPECTS

Radical cations of anthracene have been formed by vapor phase electron impact followed by trapping in an argon matrix at 12 K. Visible/ultraviolet and infrared absorption spectra of the anthracene cations in an argon matrix have been run. Significant differences in the infrared band intensities between neutral and cationic anthracene have been observed. The effects of photolysis and added CCl4 have been studied and their influence on the infrared band intensities correlated with known visible bands attributable to the anthracene cation. Theoretical calculations using Pariser-Parr-Pople and intermediate neglect of differential overlap methodologies with high level multireference perturbation configuration interaction, specifically modified for spectroscopic applications, have been performed. Both approaches predict the previously observed photoelectron spectrum well. For the optical absorption, the match with the experimental spectrum is also good, but there are notable differences between the two predictions. Ab initio restricted open shell Hartree-Fock calculations for the cation vibrational frequencies show excellent correspondence with the observed infrared bands. The theoretical infrared intensities show some remarkable differences between the neutral and ionized species. For example, the CH in-plane bending modes and CC in-plane stretching vibrations are predicted to increase by several orders of magnitude upon ionization. Experimental infrared intensities have been estimated and certain bands do show a significant increase in the cation band intensities over the neutrals. The implication of these findings for the hypothesis that polycyclic aromatic hydrocarbon cations are responsible for the unidentified infrared emission bands from interstellar space is discussed briefly.