Cold Anisotropically Interacting van der Waals Molecule: TiHe

We have used laser ablation and helium buffer-gas cooling to produce titanium-helium van der Waals molecules at cryogenic temperatures. The molecules were detected through laser-induced fluorescence spectroscopy. Ground-state $\mathrm{Ti}(a{{}^{3}F}_2)\text{−}\mathrm{He}$ binding energies were determined for the ground and first rotationally excited states from studying equilibrium thermodynamic properties, and found to agree well with theoretical calculations based on newly calculated ab initio Ti-He interaction potentials, opening up novel possibilities for studying the formation, dynamics, and nonuniversal chemistry of van der Waals clusters at low temperatures.

Figure 1

Fluorescence spectrum of TiHe molecules taken at a helium density of $6.5\times 10^{17}{\mathrm{cm}}^{-3}$, cell temperature of 1.2 K, and a probe power of 1.20 mW. Not shown is an additional small peak at $25227.5{\mathrm{cm}}^{-1}$. The peaks are labeled as ($N$, $A$), where $N$ is the ground state rotational quantum number, and $A$ is the isotope. Identification as described in the text. The peaks at 25 227.95, 25 227.98, and $25228.02{\mathrm{cm}}^{-1}$ have higher intensities than would we expected from isotopic analysis, we suspect they may be overlapped with unidentified transitions from the $N=1$ state. The frequency center of the scan is determined from a wave meter with an uncertainty of $0.1{\mathrm{cm}}^{-1}$.

Figure 2

The optical depth (OD) of TiHe as a function of the OD of Ti, colored according to the time after the ablation pulse, and fit to a linear dependence $y=bx$. The data was taken at temperatures between 1.97 K and 2.10 K at $n_{\mathrm{He}}=3.4\times 10^{16}{\mathrm{cm}}^{-3}$.

Figure 3

The ratio of the TiHe and ${}^{48}\mathrm{Ti}$ optical depths as a function of temperature at $25228.03{\mathrm{cm}}^{-1}$. The data were taken at different He densities. $E$ and $c$ are obtained from the fit.

Figure 4

$c$ coefficient vs helium density for the ${}^{48}\mathrm{Ti}{}^4\mathrm{He}$ transition at $25228.03{\mathrm{cm}}^{-1}$. The fit is to a line $y=bx$.

Figure 5

Ab initio potentials and bound levels of the TiHe vdW complex. The anisotropy-induced splitting of the rotational levels is exaggerated for clarity.