Infrared spectroscopy and dimer formation at the surface of medium-large argon clusters

Abstract

The infrared photodissociation spectra of medium-large argon clusters (n⩽ 10³) containing one (or two) active chromophore molecule(s) have been studied using both the conventional seeded expansion method and a ‘pick-up’ technique for surface deposition of a guest molecule onto the cluster surface. The species are spectroscopically characterized by infrared laser photoevaporation with line-tunable CO₂ lasers and off-axis bolometric detection.

The positions of the chromophore vibrational levels are interpreted in terms of the host–chromophore interaction and the dynamical state of the cluster. By comparison with the IR spectrum of SF₆ in liquid Ar, the presence of a previously unassigned peak in the SF₆/Ar_n spectra is attributed to clusters which are either liquid or which have an amorphous structure with the same density as the bulk-liquid Ar. This result has implications for the analysis of other experiments, like electron diffraction where neither liquid Ar clusters nor large amorphous Ar clusters are observed. Results of the study of the spectroscopy of CF₃Cl/Ar_n clusters produced by both techniques reveal behaviour markedly different from the SF₆/Ar clusters and, finally, the study of argon clusters with pick-up of CF₃Cl, as a function of the chromophore density in the pick-up region, have led to the first identification of a dimerization reaction at the cluster surface.