PYRENE RARE-GAS CLUSTERS - ANOMALOUS SPECTRAL SHIFTS AND VAN-DER-WAALS MODE INTENSITIES

S-0-->S-1 electronic spectra of pyrene.R(n) van der Waals complexes and clusters with R = Ne, Ar, Kr, and Xe and n = 1-5 were measured in pulsed supersonic jet expansions by mass-selective absorption spectroscopy employing fragmentation-free two-color resonant two-photon ionization (2C-R2PI) and time-of-flight mass separation. Electronic spectra were recorded for n = 1 and 2 for Ne and Xe and n = 1-5 for Ar and Kr. The spectral shifts of the n = 1 and n 2 (1 + 1 isomer) complexes are anomalously small, being 10-15 times less than for most other aromatic substrates. Also, the intensity pattern of the van der Waals vibrational modes is very unusual. These observations imply a considerable flattening of the intermolecular potential energy surface along the long axis of pyrene upon S-0 --> S-1 excitation. As a consequence, the red shift induced by a rare-gas atom is minimal for positions above the pyrene center of mass and maximal for a displacement along the pyrene long axis by about +/-2 Angstrom. These results are in qualitative agreement with recent predictions using a second-order perturbation theoretical approach of Shalev et al. [J. Chem. Phys. 1992, 96, 1848]. Ionization energies of the above-mentioned clusters were determined and are discussed.