LiHe is a compound of helium and lithium. The substance is a cold low density gas made of molecules each composed of a helium atom and lithium atom bound by van der Waals force. The preparation of LiHe opens up the possibility to prepare other Helium dimers, and beyond that multi-atom clusters that could be used to investigate Efimov states and Casimir retardation effects.
It was detected in 2013. Previously 7Li4He was predicted to have a binding energy of 0.0039 cm−1 (7.7×10−8eV, 1.2×10−26J or 6 mK), and a bond length of 28 Å. Other van der Waals bound helium molecules were previously known including Ag3He and He2. Detection of LiHe was done via fluorescence. The Lithium atom in the X2Σ state was excited to A2Π. The spectrum showed a pair of lines, each split into two with the hyperfine structure of 7Li. The lines had wavenumbers of 14902.563. 14902.591 14902.740 and 14902.768 cm−1. The two pairs are separated by 0.177 cm−1. This is explained by two different vibrational states of the LiHe molecule: 1/2 and 3/2. The bonding between the atoms is so low that it cannot withstand any rotation or greater vibration without breaking apart. The lowest rotation states would have energies of 40 and 80 mK, greater than the binding energy around 6 mK.
LiHe was formed by laser ablation of lithium metal into a cryogenic helium buffer gas at a temperature between 1 and 5 K. The proportion of LiHe molecules was proportional to the density of He, and declined as the temperature increased.
- Tariq, Naima; Taisan, Nada; Singh, Vijay; Weinstein, Jonathan (8 April 2013). "Spectroscopic Detection of the LiHe Molecule". Physical Review Letters. 110 (15): 153201. doi:10.1103/PhysRevLett.110.153201. PMID 25167262. Retrieved 8 February 2015.
- Friedrich, Bretislav (8 April 2013). "A Fragile Union Between Li and He Atoms". Physics. 6: 42. doi:10.1103/Physics.6.42.