On the properties of charged and neutral, atomic and molecular helium species in helium nanodroplets: interpreting recent experiments

Properties of ground state He(He-boolean AND(=)(1s(2) (1)s)), He=+(boolean AND He(1s (2)s)), He-2(+) ((boolean AND)(=) He-2(1 sigma(2)(g)1 sigma(2)(g)1 sigma(u) (2)Sigma(+)(u))) and excited (metastable) He*(He-boolean AND(=)(1s2s(3)S)), He-2*((boolean AND)(=)He(2()1 sigma(2)(g)1 sigma(u)2 sigma(g) (3)Sigma(+)(u))), He*(-)(He-boolean AND(=)(1s2s2p(4)p)) and He-2*(-)(He-boolean AND(=)2(1 sigma(2)(g)1 sigma(u)2 sigma(g)1 pi(u) (4)Pi g)) are calculated using the coupled-cluster method and basis sets multiply augmented with diffuse functions. The aim of this work is to capture the essential physics needed to describe the qualitatively different behaviour of the above mentioned helium species dissolved in liquid helium. By studying their interaction with atomic ground state helium it is found that ground state He, He+, He-2(+) and excited (metastable) He*- are well bound within a helium droplet. In comparison excited (metastable) He-*, He-2(*) and He-2(*-) are found to be squeezed out due to the high energetic cost associated with the large volume they require inside a helium droplet. In particular, the molecular species He-2(*) and He-2(*-) consist of a positive core in the form of a He-2(+) which is surrounded by a diffuse electronic cloud accounting for one or two electrons, respectively. The implications of these results for recent experimental studies on helium nanodroplets are discussed, particularly for the negatively charged species He*- and He-2(*-). We find that the latter species experience completely different dynamcis in a helium droplet although they are very similar in various other respects (e.g. diffuse electron clouds, size) in good agreement with experimental observations.