Infrared spectroscopy to probe structure and growth dynamics in Fe+-(CO2)n clusters

Fe+-(CO2)(n) ion-molecule complexes are produced by laser vaporization in a pulsed-nozzle cluster source. These species are analyzed and mass-selected using a specially designed reflectron time-of-flight mass spectrometer. Infrared photodissociation of these complexes is investigated with an optical parametric oscillator/amplifier system using wavelengths near the CO2 asymmetric stretch vibration (2349 cm(-1)). Dissociation occurs by successive elimination of CO2 molecules. Tunable laser experiments obtain infrared resonance-enhanced photodissociation spectra for these complexes. Small complexes have CO2 asymmetric stretch resonances shifted to higher frequency than the free CO2 mode. The blueshift decreases initially with cluster size, but becomes nearly constant after the n=4 cluster. Argon-tagged complexes, e.g., Fe+-(CO2)(n).Ar-m, photodissociate via the same CO2 resonances by elimination of argon. Except for the n=1 complex, bands for the tagged complexes occur at the same frequency as those for the corresponding CO2 complex without argon. Larger complexes exhibit additional resonances near the free CO2 asymmetric stretch indicating "surface" molecules not attached to the metal. Blueshifted resonances also persist in these complexes attributed to "core" ligands attached to the metal ion. In the largest clusters studied (n=9-14), additional resonances with an intermediate blueshift are measured associated with "caged" CO2 molecules not attached to the metal. These measurements demonstrate that infrared photodissociation spectroscopy has exciting potential to study clustering structures and dynamics around metal ions. (C) 2002 American Institute of Physics.