Stretch–bend coupling between van der Waals modes in the S1 state of substituted benzene–Ar1 complexes

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Abstract
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.

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