Size and isomer dependence of HF vibrational frequency shift in ArnHF van der Waals clusters with n=1-14

This article reviews our multidimensional quantum calculations of the size and isomer dependence of HF vibrational frequency shift (red shift) of ArnHF clusters, n = 1-14. The cluster size range considered encompasses the formation and closing of the first solvent shell around HF, for n = 12, as well as the change of the preferred HF location from the surface to the interior of the Ar-n microcluster, which occurs for n = 9. Our theoretical treatment, employing pairwise additive potential-energy surfaces constructed from highly accurate Ar-HF and Ar-Ar pair potentials, reproduces quantitatively the experimental results available for ArnHF, n = 1-4. The size dependence of the HF red shift is revealed to be strongly non-monotonic for the clusters with n = 1-12. In addition, we predict that the red shift of ArnHF clusters is highly isomer specific, providing distinct spectroscopic signatures for different cluster isomers. The variations of HF red shift with the cluster size and isomeric structure are readily understood by considering the changes in the number of Ar atoms in direct contact with HF.