Infrared photodissociation spectroscopy of mass-selected Al+(CO2)n and Al+(CO2)nAr clusters

Weakly bound Al+(CO2)(n) and Al+(CO2)(n)Ar complexes are produced by laser vaporization in a pulsed supersonic expansion. The clusters are mass-selected and studied by laser photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer. The excitation laser is an OPO/OPA system that produces tunable infrared light near the asymmetric stretch of CO2. Al+(CO2)(n) clusters fragment by the loss of CO2, while Al+(CO2)(n)Ar clusters fragment by the loss of argon. Dissociation is more efficient on resonance, and thus monitoring the fragmentation as a function of wavelength produces the infrared resonance-enhanced photodissociation (IR-REPD) spectrum of the complex of interest. The spectra show a blue-shift of the asymmetric CO2 stretch which decreases as the size of the cluster increases. Al+(CO2)(n)Ar transitions appear at essentially the same frequencies as those for the pure CO2 analogues but with significantly narrower line widths. The observed infrared bands are compared to the predictions of theory and specific structures are proposed for the smaller clusters. Band positions in the larger clusters provide insight into the effects of solvation.