HIGH RESOLUTION INFRARED SPECTRA OF CARBON DIOXIDE SOLVATED WITH HELIUM ATOMS
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Date
2004
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Ohio State University
Abstract
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].
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[1] J. Tang, Y. Xu, A.R.W. McKellar, and W. J$aUger, Science, 297, 2030 (2002). [2] Y. Xu, W. J$aUger, J. Tang, and A.R.W. McKellar, Phys. Rev. Lett., 91, 163401 (2003). [3] M.J. Weida, J.M. Sperhac, D.J. Nesbitt, and J.M. Hutson, J. Chem. Phys., 101, 8351 (1994). [4] K. Nauta and R.E. Miller, J. Chem. Phys., 115, 10254 (2001). [5] J. Tang, A.R.W. McKellar, F. Mezzacapo, and S. Moroni, Phys. Rev. Lett., in press (2004).
Author Institution: Steacie Institute for Molecular Sciences, National Research Council of Canada
Author Institution: Steacie Institute for Molecular Sciences, National Research Council of Canada