High resolution infrared spectra of a carbon dioxide molecule solvated with helium atoms

Infrared spectra of He-N-CO2 clusters with N up to about 20 have been studied in the region of the CO2 nu(3) fundamental band (similar to2350 cm-1) using a tunable diode laser spectrometer and pulsed supersonic jet source with cooled (>-150 degreesC) pinhole or slit nozzles and high backing pressures (<40 atm). Compared to previous studies of He-N-OCS and -N2O clusters, the higher symmetry of CO2 results in simpler spectra but less information content. Discrete rotation-vibration transitions have been assigned for N=3-17, and their analysis yields the variation of the vibrational band origin and B rotational constant over this size range. The band origin variation is similar to He-N-OCS, with an initial blueshift up to N=5, followed by a monotonic redshift, consistent with a model where the first five He atoms fill a ring around the equator of the molecule, forcing subsequent He atom density to locate closer to the ends. The B value initially drops as expected for a normal molecule, reaching a minimum for N=5. Its subsequent rise for N=6 to 11 can be interpreted as the transition from a normal (though floppy) molecule to a quantum solvation regime, where the CO2 molecule starts to rotate separately from the He atoms. For N>13, the B value becomes approximately constant with a value about 17% larger than that measured in much larger helium nanodroplets. (C) 2004 American Institute of Physics.