HIGH RESOLUTION INFRARED SPECTRA OF CARBON DIOXIDE SOLVATED WITH HELIUM ATOMS

Infrared spectra of $He_{N}-CO_{2}$ clusters with N up to about 20 have been studied in the region of the $CO_{2} \\nu_{3}$ fundamental band $(2350 cm^{-1})$ using a tunable diode laser spectrometer and pulsed supersonic jet source with cooled $(> -150 C)$ pinhole or slit nozzles and high backing pressures $(< 40 atm)$. Compared to previous studies of $He_{N}-OCS$ [1] and $-N_{2}O$ [2] clusters, the higher symmetry of $CO_{2}$ results in simpler spectra but less information content. The binary complex, $He-CO_{2}$, was studied previously by Weida et al. [3]. With increasing cluster size, $N = 2$ to 17, we observe discrete rotation-vibration transitions $(R(0), P(2), R(2))$ whose analysis yields the variation of the band origin and B rotational constant over this size range. The vibrational origin variation is very similar to $He_{N}-OCS$, with an initial blue shift up to $N = 5$, followed by a monotonic red shift, consistent with a model where the first 5 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 $CO_{2}$ molecule starts to rotate separately from the He atoms. For $N > 13$, the B value is approximately constant with a value about 17% larger than that measured in much larger helium nanodroplets [4]. Very recent quantum Monte Carlo simulations by Mezzacapo and Moroni are in excellent agreement with these experimental results [5].