A STUDY OF ANTHRACENE-AR-N (N=0-5) IN THE GROUND CATIONIC STATE BY LASER THRESHOLD PHOTOELECTRON-SPECTROSCOPY - SELECTIVE IONIZATION OF COMPLEX ISOMERS FORMED IN THE FREE JET EXPANSION

The van der Waals complexes formed between anthracene and argon in a free jet expansion are studied using laser resonance enhanced multiphoton ionization (REMPI) threshold photoelectron spectroscopy with the aim of selectively ionizing specific isomers of small- to medium-sized clusters for which discrete absorption peaks exist in the excitation spectrum. Two-color (1+1') REMPI threshold photoelectron spectra of a number of isomers of anthracene-Ar-n, (n=1-5) have been recorded in addition to that of anthracene itself, The following adiabatic ionization energies (I-a) have been obtained to within +/-5 cm(-1): 59 872 (n=0), 59 807 and 59 825 (n=1), 59 757 and 59 774 (n=1), 59 695 (n=3), 59 606 and 59 660 (n=4), and 59 565 cm(-1) (n=5). For n=1-3, detailed van der Waals cation vibrational structure was observed, showing progressions in both bending and stretching mode vibrations. The resulting vibrational information together with the ionization energy red shifts has helped in assigning bands observed in the threshold photoelectron spectra to particular geometric isomers some of which were not necessarily observed in the excitation spectrum.