The van der Waals vibrations of aniline–, phenol–, fluorobenzene–, and chlorobenzene–Ar1 complexes have been measured using one-color resonance enhanced multiphoton ionization spectroscopy, together with time-of-flight mass spectrometry, in a skimmed supersonic molecular beam. A delayed ionization extraction technique is used to suppress contributions to the spectra from dissociating complexes. The S1–S0 electronic origins for the van der Waals complexes are found to be shifted towards lower energy (red shift) relative to the parent molecule electronic origin for all the Ar1 complexes. The red shifts increase in magnitude in the order: fluorobenzene, chlorobenzene, phenol, aniline. Progressions, overtones and combination transitions involving the low frequency van der Waals vibrations, i.e., the symmetric bend (bx), the asymmetric bend (by) and the stretch (sz) are observed clearly in the S1←S0 excitation spectra. Intensity profiles are found to deviate substantially from those expected on the basis of harmonic Franck–Condon factors. A model involving stretch–bend anharmonic coupling via cubic terms in the vibrational potential is found to account for the observed spectral features and intensity anomalies.

Vibrational structure associated with van der Waals modes of the aniline–(argon)2 complex has been observed in the region near the origin of the S1←S0 electronic transition of the complex using resonance enhanced, multiphoton ionization (REMPI) spectroscopy. The aniline–Ar2 spectrum in this region displays only a few discrete bands built on an intense electronic origin. The dominant vibrational band, associated principally with the symmetric van der Waals stretching motion of the two argon atoms against the aromatic frame, occurs at 38.5 cm−1 displacement relative to the 000 band, with weaker transitions at 15 and 30 cm−1 displacement. A simple model for the van der Waals vibrations of aromatics bound to one and two rare gas atoms is developed and allows allow us to explain the aniline–Ar2 spectrum, using van der Waals bond parameters determined from the previously measured An–Ar1 spectrum. The agreement between the predicted and observed aniline–Ar2 spectrum confirms the view that van der Waals stretching vibrations are coupled anharmonically to near resonant bending vibrations in aniline–Ar1 and aniline–Ar2.

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Rydberg excited iodine-argon van der Waals complexes studied by resonance enhanced multiphoton ionization spectroscopy High resolution threshold photoelectron spectroscopy of aniline and aniline van der Waals complexes Suppression of fragment contributions to massselected resonance enhanced multiphoton ionization spectra of van der Waals clustersThe origin region of the SI +-So transitions of the aniline-Ar 3 , aniline-Ar 4 , and aniline-Ars molecules have been measured using mass selected resonance enhanced, multi photon ionization (REMPI) spectroscopy. The aniline-Ar 3 spectrum exhibits two distinct groups of peaks. The more prominent group displays a regular vibrational progression, with five obvious members and a spacing of ~ 16 cm -I. Vibrational structure in the other group is less distinctive. On the basis of cluster potential calculations described in this paper, we believe that two stable aniline-(argon) 3 isomers exist in the supersonic expansion and that the two groups of peaks correspond to absorption by these two isomers. Spectra recorded at masses corresponding to aniline-(argon)4 and aniline-(argon)s display broadened structure that probably reflects contributions from larger aniline-(argon) n clusters which fragment upon ionization. There is, however, some evidence for a progression with a spacing of ~ 16 cm -I in the aniline-(argon)4 spectrum. Dispersed fluorescence spectra from relatively small aniline-Ar n clusters (4 < n < 10) indicate that vibrational redistribution from Franck-Condon active van der Waals modes occurs with rates of at least 5 X 10'1 S -I.

Si('D2) has been detected by atomic laser-induced fluorescence following photoexcitation of SiH2 into high bending vibrational levels of the Á'B, state. The Si('D2) + H2 channel appears to open between v{ = 6 and 7, establishing Mff°(SiH2) = 65.4 ±1.6 kcal mol"1. SiH2 appears to dissociate preferentially from high rotational levels of the A, v2' > 6 states.