Infrared Photodissociation Spectroscopy of Mg+(CO2)n and Mg+(CO2)nAr Clusters

Mg + (CO 2 ) n and Mg + (CO 2 ) 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 + (CO 2 ) n occurs by elimination of one or more intact CO 2 molecules, while Mg + (CO 2 ) 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 CO 2 (2349 cm -1 ). Structured vibrational bands are measured that are mostly blue shifted with respect to the vibration in the free molecule. In Mg + (CO 2 ) n complexes, higher laser powers are required to achieve photodissociation, and the resonances are broad. In Mg + (CO 2 ) n Ar complexes, fragmentation is more efficient and the bands are sharper. For the smaller complexes Mg + (CO 2 ) 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 + −CO 2, a bent configuration for Mg + (CO 2 ) 2, and a trigonal pyramidal structure for Mg + (CO 2 ) 3 . Larger complexes have vibrational bands near the free-CO 2 asymmetric stretch, indicating the presence of CO 2 ligands not attached to the metal.