Ab Initio Potentials and Hyperfine Constants for Electronically Excited K-3He

Hyperpolarized nuclear-spin noble-gas atoms are a key component in many applications and areas of physics such as Atomic Magnetometry and use in gyroscopes, Nuclear Magnetic Resonance Imaging and Quantum Information Processing and even have their applications in the search for new physics beyond the standard model. 3He in particular offers a macroscopic ensemble of spins with coherence times on the order of hours at room temperature, in conjunction with its chemical inertness this makes it ideal for various medical imaging applications.

As the nuclear magnetic moment is small, polarization through the application of magnetic fields is thermally limited and ineffective at all but extreme cryogenic temperatures. Therefore, other methods of generating hyperpolarization, beyond the thermal limit, are commonly used. One such method is hyperpolarization through interactions with polarized alkali atoms in their ground-state, in a mechanism known as Spin Exchange Optical Pumping (SEOP). In SEOP, the valence electron of the alkali atoms can easily be polarized through the absorption of circularly polarized resonance light, subsequently polarizing the nuclear spin of the noble gas atoms via hyperfine interaction, reaching polarizations of several tens of percent.

The hyperfine interaction is only significant when the two atoms are in close proximity. At present there are two known mechanisms. The first is binary collisions. Each such collision is short and contributes little to the spin exchange rate, but binary collisions are plentiful. In contrast, the second mechanism is the formation of van der Waals (vdW) molecules, where the interaction time is long, but such molecules form in three body collisions which are rare.

In this work we consider interactions of 3He atoms with optically pumped, electronically excited K atoms. The first step in providing theoretical predictions for both known spin exchange mechanisms is the estimation of several molecular parameters. Namely, the potential energy curves governing the dynamics of K-3He collision complexes, and the hyperfine coupling constant, proportional to the spin density at the position of the 3He nucleus, responsible for the spin exchange interaction. Our research objective is to explore the possibility of enhancing the rate of polarization by accessing the electronic excitations of K atoms.

Ab initio calculations were performed for the ground state and lowest lying electronic excitations of the K-3He complex. The results of these calculations, the potential energy curves and charge and spin densities of the wavefunctions, are discussed and explained. The inadequacy of the semi-classical approach to calculating the spin exchange cross section is demonstrated, and future work is outlined.