Spectroscopic Detection of the LiHe Molecule

We use cryogenic helium buffer-gas cooling to form large densities of lithium atoms in a high-density helium gas, from which LiHe molecules form by three-body recombination. These weakly bound van der Waals molecules are detected spectroscopically, and their binding energy is measured from their equilibrium thermodynamic properties.

Figure 1

Fluorescence spectrum of LiHe molecules. The LiHe fluorescence is plotted against the lower axis; the iodine signal used for frequency calibration is shown on the upper axis. The graph is an average of multiple spectra, taken at various times ranging from 16 to 40 ms after the ablation pulse, temperatures from 2.0 to 2.5 K, and a helium density of $7\times {10}^{17}{\mathrm{cm}}^{-3}$.

Figure 2

Schematic of the relevant LiHe energy levels, showing the ground-state hyperfine structure and the excited-state rotational structure and lambda doubling. Levels are labeled $+$ and $-$ according to their parity. The hyperfine structure in the excited state is not shown, as it is not resolved in our spectra. This is expected, as the hyperfine splitting of the excited state of atomic lithium is small compared to the Doppler broadening. The listed transition frequencies are the experimentally measured values, with experimental error of $\pm 0.004{\mathrm{cm}}^{-1}$.

Figure 3

LiHe OD plotted as a function of ${}^7\mathrm{Li}$ OD at a constant helium density of $2.3\times {10}^{17}{\mathrm{cm}}^{-3}$ and a temperature of $2.45\pm 0.2\mathrm{K}$.

Figure 4

The ratio of LiHe OD to ${}^7\mathrm{Li}$ OD as a function of helium density at temperatures from 2.5 to 6 K. Missing from the horizontal error bars is an additional 8% uncertainty from the uncertainty in our helium fill line diameter.

Figure 5

The ratio of LiHe OD to ${}^7\mathrm{Li}$ OD as a function of temperature at constant helium density of about $7\times {10}^{17}{\mathrm{cm}}^{-3}$. The fit is as discussed in the text.