LiHe is a compound of

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic ta ...

and

lithium Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense soli ...

. The substance is a cold low-density gas made of Van der Waals molecules, each composed of a helium atom and lithium atom bound by

van der Waals force In molecular physics, the van der Waals force is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical electronic bond; they are comparatively weak and ...

. 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 state The Efimov effect is an effect in the quantum mechanics of few-body systems predicted by the Russian theoretical physicist V. N. Efimov in 1970. Efimov’s effect is where three identical bosons interact, with the prediction of an infinite serie ...

s and Casimir retardation effects.

Detection

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 In atomic physics, hyperfine structure is defined by small shifts in otherwise degenerate energy levels and the resulting splittings in those energy levels of atoms, molecules, and ions, due to electromagnetic multipole interaction between the n ...

of ⁷Li. The lines had

wavenumber In the physical sciences, the wavenumber (also wave number or repetency) is the '' spatial frequency'' of a wave, measured in cycles per unit distance (ordinary wavenumber) or radians per unit distance (angular wavenumber). It is analogous to te ...

s 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 Laser ablation or photoablation (also called laser blasting) is the process of removing material from a solid (or occasionally liquid) surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser ...

of lithium metal into a

cryogenic In physics, cryogenics is the production and behaviour of materials at very low temperatures. The 13th IIR International Congress of Refrigeration (held in Washington DC in 1971) endorsed a universal definition of “cryogenics” and “cr ...

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.

Properties

LiHe is polar and

paramagnetic Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, ...

. The average separation between the lithium and helium atoms depends on the isotope. For ⁶LiHe the separation is 48.53 Å, but for ⁷LiHe the distance is much smaller at 28.15 Å on average. If the helium atom of LiHe is excited so that the 1s electron is promoted to 2s, it decays by transferring energy to ionise lithium, and the molecule breaks up. This is called

interatomic Coulombic decay Interatomic Coulombic decay (ICD) is a general, fundamental property of atoms and molecules that have neighbors. Interatomic (intermolecular) Coulombic decay is a very efficient interatomic (intermolecular) relaxation process of an electronically e ...

. The energy of the Li⁺ and He decay products is distributed in a curve that oscillates up and down about a dozen times.

References

{{Lithium compounds Helium compounds Lithium compounds Van der Waals molecules

Detection

Properties

See also

References