Infrared photodissociation spectroscopy of Mg+(CO2)n and Mg+(CO2)nAr clusters

Mg+(CO2)(n) and Mg+(CO2)(n)Ar ion-molecule complexes are produced by laser vaporization in a pulsed-nozzle source and studied with mass-selected infrared photodissociation spectroscopy. Photodissociation of Mg+(CO2)(n) occurs by elimination of one or more intact CO2 molecules, while Mg+(CO2)(n)Ar species eliminate argon. Infrared resonance-enhanced photodissociation (IR-REPD) spectra are obtained by measuring the fragment yield versus energy near the asymmetric stretch vibration Of CO2 (2349 cm(-1)). Structured vibrational bands are measured that are mostly blue shifted with respect to the vibration in the free molecule. In Mg+- (CO2), complexes, higher laser powers are required to achieve photodissociation, and the resonances are broad. In Mg+(CO2)(n)Ar complexes, fragmentation is more efficient and the bands are sharper. For the smaller complexes Mg+(CO2)(n=1-3), the number of IR bands and their positions are in good agreement with theoretical predictions. The results provide strong evidence for a linear structure for Mg+-CO2, a bent configuration for Mg+(CO2)(2), and a trigonal pyramidal structure for Mg+(CO2)(3). Larger complexes have vibrational bands near the free-CO2 asymmetric stretch, indicating the presence Of CO2 ligands not attached to the metal.