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 Li-6,Li-7, Na-23, K-39,K-40,K-41,Rb-85,Rb-87 or Cs-133, 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 He-4 by He-3 atom in a He-Li tetramer, one can trace how quasi-halo structure transforms into halo structure. On the other hand, swapping He-4 by He-3 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 He-4 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.