Relation between shrinkage effect and compression of the rotational spectrum in the Λ7Li hypernucleus

It has been shown experimentally that the 6Li nucleus shrinks by adding a Λ particle while its spectrum is also compressed. We discuss the compatibility of these two effects, contrasting also with a relation between the shrinkage effect and the stability of the spectrum in the Λ9Be hypernucleus. To this end, we employ two-body d-Λ5He and α-Λ5He cluster models for the Λ7Li and Λ9Be hypernuclei, respectively. We first argue that a Gaussian-like interaction between two clusters leads to a stabilization of the spectrum against an addition of a Λ particle, even though the intercluster distance is reduced. In the case of Λ7Li, the spin-orbit interaction between the intercluster motion and the deuteron spin has to be considered also. We show that the shrinkage effect makes the expectation value of the spin-orbit potential larger, lowering the excitation energy for the 6Li(3+) x Λ(1/2+) level.