HIGH-RESOLUTION ONE-PHOTON AND 2-PHOTON SPECTRA OF MATRIX-ISOLATED ANTHRACENE

Single-site fluorescence and one- and two-photon fluorescence excitation spectra of anthracene in an argon matrix at about 12 K are presented. The spectra exhibit a resolution of less than 4 cm-1 and show almost no site contamination. The excitation spectra are discussed with respect to the transitions to the first and second excited singlet state (1L(a) and 1L(b)). The spectra confirm the results of earlier investigations that the onset of the hidden 1L(b) transition to the second excited singlet state lies at about 28000 cm-1 in nonpolar solvents. By choosing different sites for the excitation spectra the solvent shifts for the two transitions were measured. Extrapolation of these shifts leads to an estimate of the separation between the origins of the two transitions in the free molecule of less than 500 cm-1. For one-photon absorption a two-dimensional fluorescence/fluorescence excitation spectrum is presented. This spectrum shows that in the one-photon case none of the bands observed up to an energy of about 1600 cm-1 above the origin of the 1L(a) band can be assigned to the 1L(b) transition with any confidence. All observed long axis polarized vibrational bands are false origins of the 1L(a) transition.