Ground-state characteristics of helium–alkali tetramers

Helium–alkali tetramers ${}^4{He}_{3}A$, ${}^3{He}^4{He}_{2}A$ and ${}^3{He}_2^{4}HeA$, where A is one of the alkali atoms ^6,7Li, ²³Na, ^39,40,41K, ^85,87Rb or ¹³³Cs, were studied using diffusion Monte Carlo method. Besides ground-state energies, exact distribution functions were evaluated. Spatial arrangements of atoms in clusters qualify He–A tetramers as transitional systems, which reveal halo-like spatial extension. On the one hand, swapping ⁴He by ³He atom in a He–Li tetramer, one can trace how quasi-halo structure transforms into halo structure. On the other hand, swapping ³He by ⁴He in He–A tetramer reveals behavior previously noticed in helium droplets doped with an alkali atom. Helium atoms start grouping on one side of an alkali atom, which is clearer in He–A clusters with more ⁴He atoms that have stronger self-binding. The effect is the more pronounced the bigger alkali atom is, since it is then harder for the remaining He atoms to surround an alkali. (paper)