15min:

HUI LI, PIERRE-NICHOLAS ROY AND ROBERT J. LE ROY, Guelph-Waterloo Centre for Graduate Work in Chemistry and Biochemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.

Spectroscopic studies of molecules embedded in helium droplets provide a unique opportunity to investigate a superfluid. High-resolution infrared spectra of CO₂-(He)_N clusters ranging in size from `small' (N\! eq \!1-17) to `intermediate' (N\! 60) have been reported recently by McKellar and co-workers.\footnote J. Tang, A.R.W. McKellar, F. Mezzacapo and S. Moroni Phys. Rev. Lett. 92, 145503 (2004);~ A.R.W. McKellar, J. Chem. Phys. 128, 044308 (2008).~ However, they have not yet been able to extend this range to clusters large enough that the observed vibrational band origins shifts ₃ and rotational constants B₀ and D₀ approached the nanodroplet limit values.\footnote K. Nauta and R.E. Miller, J. Chem. Phys. 115, 10254 (2001);~ R. Lehnig and W. J\" ager, Chem. Phys. Lett. 424, 146 (2006). This paper describes the use of modeling techniques to explore the domain between intermediate and `larger' (N\! \!10²-10³) clusters, seeking to characterize the onset of nanodroplet behaviour.

Path integral Monte Carlo (PIMC) simulations using the worm algorithm can in principle provide reliable quantitative results for ``larger'' clusters approaching the nanodroplet limit. We have recently determined a three-dimensional analytical `Morse/Long-Range' potential energy surface for CO₂-He, which explicitly depends on the Q₃ asymmetric-stretch vibrational motion of CO₂, and takes account of change in the average value of Q₁ when ₃ is excited.^, Using this potential, predicted vibrational frequency shifts have been found to be in excellent agreement with experiment across the range N\!=\!1-40,^d so it is expected to provides a good description of larger clusters. This paper describes results obtained on using this potential in PIMC simulations to predict the effective rotational constant B₀ and the ₃ band-origin shift for CO₂ in (He)_N clusters with N > 10². The importance of identical-particle exchange effects will be examined by comparing results obtained using both Boltzmann and Bose-Einstein statistics.