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Rotational coherence spectroscopy (RCS) implemented by time-resolved ionization depletion (TRID) was used to examine the structure of naphthalene trimer. Results provide evidence for a C3h structure in which three symmetrically equivalent naphthalene moieties are oriented with their long axes parallel. Results also show that the sensitivity of ionization detection coupled with the selectivity of mass analysis available through RCS-TRID permit high-resolution analysis not obtainable by other methods.

Coherent Raman scattering (CRS), stimulated Raman loss (gain) (SRL(G)), mass-selective ionization-detected stimulated Raman spectroscopy (IDSRS) and their Fourier-transform versions are nonlinear Raman studies that help evaluate weakly bound clusters and complexes in molecular beams. SRL and CRS help study various characteristics of large homogeneous clusters such as condensation dynamics in free-jet-expansions, cluster phases, cluster temperatures and low-frequency collective modes in ground states. Mass-selective IDSRS enables the study of small-to-medium sized species by elucidating structure- and dynamics-related properties and their correlation with size and solution status.

The intramolecular vibrational redistribution (IVR)-assisted ionization-gain stimulated Raman spectroscopy (IGSRS) and the jet-cooled fluorene are described. The fluorene spectra has shown extensive vibrational coupling interactions involving both the aliphatic and aromatic CH-stretching first excited states with nearby background states.

The first excited state (S1) intermolecular potential energy surface for the fluorobenzene-argon van der Waals complex is evaluated. Results are compared to the experimental data available and to those found in previous theoretical papers on ground state potentials for similar complexes.

The first excited state ([S.sub.1]) intermolecular potential energy surface for the p-difluorobenzene-Ar van der Walls complex was evaluated using the coupled-cluster method and the augmented-cc-pVDZ-3321 basis set. The results when compared to the benzene- and fuorobenzene-AR [S.sub.0] intermolecular potential energy surfaces show stronger interaction in the excited state, the dissociation energy being larger and the bond distance shorter.

The ground state intermolecular potential energy surface for the p-difluorobenzene-Ar van der Waals complex is evaluated using the coupled cluster singles and doubles including connected triple excitations [CCSD(T)] model and the augmented correlation consistent polarized valence double-zeta basis set extended with a set of 3s3p2d1f1g midbond functions. The results clearly improve previously available data and show the importance of a good correlation method and basis set when dealing with van der Waals complexes.

The structural properties of aromatic-aromatic van der Waals complexes of the form M-X, where M = perylene or fluorene and X = benzene or toluene, were investigated using rotational coherence spectroscopy. The structure of aromatic-aliphatic hydrocarbon dimers of the form M-Y, where M = perylene or fluorene and Y = cyclohexane or methylcyclohexane, was also characterized. Experimental results showed that the rotational constants of the perylene complexes are in good agreement with centrally bound, parallel-stacked structures in which the monomer planes are separated by a distance of 3.5-4.3 A.