VIBRATIONAL FREQUENCY-SHIFT OF HF IN HELIUM CLUSTERS - QUANTUM SIMULATION AND EXPERIMENT

We report accurate variational and diffusion quantum Monte Carlo calcu lations for the size dependence of the vibrational frequency shift of HF molecules embedded in helium clusters with up to n = 198 helium ato ms,The frequency shift exhibits a strong initial size dependence and s aturates at a redshift of about 2.7 +/- 0.1 cm(-1) for clusters with o ver 100 atoms, This value is in good agreement with our experimental r edshift of 2.65 +/- 0.15 cm(-1) for clusters with over 1000 atoms, The helium cluster is found to undergo significant structural changes upo n embedding of HF, The density in the nearest neighbor shell exceeds: the bulk helium density by a factor of two, A second nearest neighbor density maximum and a peripheral density plateau very close to the bul k helium value is found, In spite of the anisotropic interaction betwe en HF and helium all clusters have almost perfectly spherical helium d ensity profiles and indicate close to free rotor behavior of HF inside the cluster. The cluster size dependence of The redshift can be quali tatively described by an induced dipole model. (C) 1996 American Insti tute of Physics.