Helium aggregates doped with alkali dimer impurities: A finite temperature study of complexes

A path integral Monte Carlo method has been used to study small ( 4 He ) N – Rb 2 ( 3 Σ u + ) complexes at low temperatures. We focus our investigation on the range 1 K ⩽ T ⩽ 2 K for aggregates of different sizes containing between 10 and 40 helium atoms. The extremely weak He–Rb2 interaction, with a shallower potential depth and a longer equilibrium bond length than in the case of the He–He interaction, favors the preference of the alkali molecular dopant to reside on the helium droplet surface. In spite of the slight perturbation introduced by the rubidium dimer, it serves like a glue for the helium host that, at a given temperature, would not exist as a pure aggregate without the presence of the dopant molecule. It is found that the stability of the clusters increases with the number of He atoms surrounding the Rb2 dimer.