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Physical Review Letters

Alkali-Metal Hyperfine Shift in the Van der Waals Molecule KAr

Richard R. Freeman, Edward M. Mattison, David E. Pritchard, Daniel Kleppner

Published Aug 12, 1974• 19 citations
Publisher

Abstract

The hyperfine interaction of K in KAr has been measured in several vibrational states of KAr. The results yield a value for the hyperfine shift $\ensuremath{\alpha}$ as a function of internuclear separation $R$ given by $\ensuremath{\alpha}=\ensuremath{-}115(R\ensuremath{-}{R}_{0})$ kHz, where $R$ is in bohrs and ${R}_{0}=12.0$ bohr, valid over the range $10.25 \mathrm{bohr}<R<12.0 \mathrm{bohr}$.

DOI
10.1103/physrevlett.33.397

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The hyperfine interaction of K in KAr has been measured in several vibrational states of KAr. The results yield a value for the hyperfine shift \ensuremathα\ensuremath{\alpha}\ensuremathα as a function of internuclear separation RRR given by \ensuremathα=\ensuremath−115(R\ensuremath−R0)\ensuremath{\alpha}=\ensuremath{-}115(R\ensuremath{-}{R}_{0})\ensuremathα=\ensuremath−115(R\ensuremath−R0​) kHz, where RRR is in bohrs and R0=12.0{R}_{0}=12.0R0​=12.0 bohr, valid over the range 10.25bohr<R<12.0bohr10.25 \mathrm{bohr}<R<12.0 \mathrm{bohr}10.25bohr<R<12.0bohr.
Collisions with chemically inert atoms or molecules change the hyperfine coupling $A\phantom{\rule{0.16em}{0ex}}\mathbf{I}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{S}$ of an alkali-metal atom through the hyperfine-shift interaction $\ensuremath{\delta}A\phantom{\rule{0.16em}{0ex}}\mathbf{I}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{S}$. This interaction is responsible for the pressure shifts of the microwave resonances of alkali-metal atoms in buffer gases, is an important spin interaction in alkali-metal--noble-gas van der Waals molecules, and is anticipated to enable the magnetoassociation of ultracold molecules such as RbSr. An improved estimate is presented for the long-range asymptote of this interaction for Na, K, Rb, and Cs. To test the results, the change in hyperfine coupling due to a static electric field is estimated and reasonable agreement is found.
We report a study, by molecular beam methods, of the alkali–rare gas van der Waals molecule KAr. Paramagnetic potassium–argon molecules were produced in a supersonic molecular beam, and their magnetic hyperfine structure probed by resonance techniques. The principle interaction studied is the spin–rotational coupling of the spin angular momentum h/S of the alkali valence electron and the rotational angular momentum h/N of the molecule. The measured value of the interaction constant γ in the (v=0, N=0) molecular state is 240(10) kHz. Limits have been set on the variation of γ with R, the internuclear separation, over the range R=9 to 13 bohr.
The ggg factor of the $^{2}\ensuremath{\Sigma}$ Van der Waals molecule KAr has been measured relative to the ggg factor of free potassium using molecular-beam magnetic-resonance techniques. A theoretical treatment of the ggg factor of the molecule is presented that predicts the ggg factor to be shifted from the value for the free potassium atom by an amount proportional to the ratio $\frac{\ensuremath{\gamma}}{B}$, where $\ensuremath{\gamma}$ and BBB are the spin-rotational and rotational constants of the molecule, respectively. The experimental results show the molecular ggg factor to be less than the value for free potassium by approximately 25 ppm, in agreement with the prediction.
Theoretical pseudopotentials and dispersion potentials are used to study ground-state hyperfine splitting frequencies of alkali-metal atoms (Li, Na, K, Rb, and Cs) in noble gases (He, Ne, Ar, Kr, and Xe) in all combinations. With a single fitting parameter, calculations based on first-order perturbation theory qualitatively present each temperature dependence of the measured frequency shift. With this parameter and excitation energies of alkali-metal and noble-gas atoms, the hyperfine splitting frequency of alkali-metal atoms is suitable for investigating the properties of noble-gas atoms, such as the s-wave scattering length of electrons, the electric-dipole polarizability, and the van der Waals radius. This study suggests the possibility of improving excitation energies and van der Waals potentials of colliding pairs.
In this paper, we have calculated the values of the C6{\mathrm{C}}_{6}C6​ coefficients and the anisotropy parameters $\ensuremath{\gamma}$ of the leading term of the van der Waals interactions between molecular hydrogen and alkali-metal atoms from Li to Fr. The uncertainties in our results are less than 22%.2