Spectroscopic Detection of the Lithium Helium (LiHe) van der Waals Molecule

Abstract

Van der Waals molecules are extremely weakly-bound, extremely long-range molecules. The most weakly bound van der Waals molecules are those containing helium. These special characteristics have motivated many theoretical studies on the structure of these molecules. To date, the only ground-state helium diatomic molecule that has been directly detected in gas phase is the helium dimer 4He2. Using cryogenic helium buffer-gas cooling and laser ablation, we have produced large densities of lithium atoms in a high-density helium gas, from which LiHe molecules form by three-body recombination Li + He + He LiHe + He. These weakly-bound van der Waals molecules were detected spectroscopically using laser induced fluorescence. We have measured the LiHe transition frequency, linewidth and optical density. The observed LiHe spectrum confirmed the theoretical prediction that only a single rovibrational state of LiHe is populated. This state is split into a hyperfine doublet, which is unchanged from the atomic lithium, 7Li, at the level of accuracy of our measurements. The excited-state structure of 7Li4He was calculated using the discrete variable representation (DVR). These calculations were in quantitative agreement with our measurements. From this agreement, and from the ground-state hyperfine splitting, we identified the molecule as 7Li4He and not 6Li4He, 6Li3He, 7Li3He or lithium bound to a helium cluster.