			15min:

HIGH RESOLUTION INFRARED SPECTRA OF CARBON DIOXIDE SOLVATED WITH HELIUM ATOMS.

JIAN TANG AND A. R. W. MCKELLAR, Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario K1A 0R6, Canada.

Infrared spectra of He_N-CO₂ clusters with N up to about 20 have been studied in the region of the CO₂ 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 -N₂O [2] clusters, the higher symmetry of CO₂ results in simpler spectra but less information content. The binary complex, He-CO₂, 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 CO₂ 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].

\vskip 0.3 truecm \centerline References \parindent=0pt

[1] J. Tang, Y. Xu, A.R.W. McKellar, and W. Jäger, Science , \textbf297, 2030 (2002).

[2] Y. Xu, W. Jäger, J. Tang, and A.R.W. McKellar, Phys. Rev. Lett. , \textbf91, 163401 (2003).

[3] M.J. Weida, J.M. Sperhac, D.J. Nesbitt, and J.M. Hutson, J. Chem. Phys. , \textbf101, 8351 (1994).

[4] K. Nauta and R.E. Miller, J. Chem. Phys. , \textbf115, 10254 (2001).

[5] J. Tang, A.R.W. McKellar, F. Mezzacapo, and S. Moroni, Phys. Rev. Lett. , in press (2004).