Doped helium clusters analyzed through quantum chemistry methods

A quantum-chemistry-like methodology developed recently to study molecules solvated in atomic clusters is reviewed, and the results of its application to diatomic molecules immersed in helium clusters are presented and discussed. The methodology is based on treating the atoms of the solvent cluster as the 'electrons' and the solvated molecule ('BC') as a structured 'nucleus' of the combined solvent-solute system. The 'electron'-'electron' and 'electron' -'nucleus' interactions are represented by parametrized two-body and ab initio three-body potentials, respectively. The 'intranuclear' (intramolecular) energy is mimicked by a parametrized pair potential energy function. The methodology furnishes the wave functions, and thereby it allows for computation of the infrared or Raman spectra of the solvated molecules. The computed spectra are then compared with the measured ones. In agreement with the experimental observations, the computed spectra of the solvated molecule show considerable differences depending on whether the solvent cluster is comprised of pure bosonic (He-4), pure fermionic (He-3), or both bosonic and fermionic helium atoms. The differences in the spectra are explained in terms of the differences in the spin-statistics of the solvent clusters. The bosonic vs fermionic nature of the solvent is also reflected in the selection rules. In the case of a polar molecule, the Q-branch of the spectrum is forbidden when the molecule is solvated in a bosonic cluster, and it becomes allowed when the solvent is a fermionic cluster. (C) 2007 Wiley Periodicals, Inc.