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 f ree jet expansion are studied using laser resonance enhanced multiphot on ionization (REMPI) threshold photoelectron spectroscopy with the ai m of selectively ionizing specific isomers of small- to medium-sized c lusters for which discrete absorption peaks exist in the excitation sp ectrum. Two-color (1+1') REMPI threshold photoelectron spectra of a nu mber of isomers of anthracene-Ar-n, (n=1-5) have been recorded in addi tion 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 60 6 and 59 660 (n=4), and 59 565 cm(-1) (n=5). For n=1-3, detailed van d er Waals cation vibrational structure was observed, showing progressio ns in both bending and stretching mode vibrations. The resulting vibra tional information together with the ionization energy red shifts has helped in assigning bands observed in the threshold photoelectron spec tra to particular geometric isomers some of which were not necessarily observed in the excitation spectrum.