Structure and energetics of He2* bubble-states in superfluid 4He

Abstract

Structure and energetics of solvation of the triplet Rydberg states of the He2* excimer in liquid He4 (LHe) are analyzed using ab initio potentials and density functional methods. The results are used to interpret the known spectroscopy. Having established the reliability of the various semiempirical functionals, interfacial properties of the superfluid on molecular scales are discussed. Due to its spherical electron density, the a(Σu3) state solvates in a spherical bubble of 7 Å radius in which the excimer freely rotates. This explains the observed rotationally resolved b3←a3 and c3←a3 absorption spectra. A deep potential minimum occurs at the equatorial node of the a(Σu3) state, where a radially frozen belt of six He atoms can be sustained at R=2.3 Å, inside an ellipsoidal cavity with major axis of 8 Å and a more diffuse minor axis of 6 Å. Despite the absence of a potential energy barrier, or a many-body interfacial tension preventing the wetting of the belt, the bare c3 state is observed in emission. It is argued that in the superfluid, wetting is prevented by the hindered rotation of the excimer, hence the sensitivity of the c3→a3 emission to pressure induced quenching. The nodal plane in the b(3Πg3) state passes through the molecular axis, as such, rotation cannot provide protection against wetting and subsequent quenching of the b3 state via the He3* manifold; hence the absence of b3→a3 emission despite its large transition dipole. In its global minimum, the d3 excimer sustains a shell of 16 He atoms, localized at the radial node of its Rydberg electron, at R∼2.5 Å. The shell, in turn, is contained in a nearly spherical bubble held at a radius of 13 Å by the extra-nodal electron density. The repulsion between extra-nodal electron density and LHe provides a barrier to filling of the deep nodal well, ensuring the stability of the bare d3 excimer in a large spherical bubble. This explains the free-rotor envelopes of the d3→b3 and d3→c3 emissions, and their negligible spectral shifts relative to the gas phase. The predicted minimum energy structures, the belted c3 state and the crusted d3 state, if formed, should be metastable.