Spin-exchange collisions in optical pumping of 129Xe: A multiscale simulation

Abstract

Xenon nuclei may be hyperpolarized through collisions with optically pumped rubidium, with the Xe hyperfine coupling (HFC) between the unpaired Rb electron and xenon nucleus transferring spin polarization from the Rb atom to Xe. We model the polarization transfer to 129 Xe in short binary collisions and long van der Waals (VDW) complexes between gaseous Rb and Xe. The simulations feature molecular dynamics of the Rb-Xe mixture, spin-Hamiltonian parameters extracted from relativistic quantum chemistry, and explicit spin dynamics simulations of the Rb-Xe interaction events on a novel Python code. Oscillation in the VDW bond-length R strongly modulates the Xe HFC, imposing steps in the Xe polarization build-up. The Rb nucleus and the unpaired electron constitute a strongly coupled two-spin system, with practically constant Rb HFC regardless of R. Explicit numerical propagation of the time-dependent Schrödinger equation for a simplified model VDW complex reveals that the spin system undergoes two simultaneous Rabi-like oscillations between the initial, fully spin-polarized state of the Rb atom, and two final states after the Xe spin flip, one in each of the two branches of the hyperfine spectrum of the Rb atom. The simulations indicate that incorporating the Rb HFC explicitly results in an overall reduction of the Xe polarization as compared with the simple model, in which the Rb HFC is neglected. Slow oscillations between the Rb spin states constitute an envelope on which the steps in the Xe polarization are superimposed, and this determines the achievable Xe polarization in long-lived VDW events. A simple empirical polarization transfer model based on the lifetime distribution of the VDW events and the Rabi transition probabilities reveals that, in the simulated conditions, a large part of the polarization transfer takes place due to the transition between separate branches of the hyperfine states of the Rb atom. This is due to the fact that maxima of the transition probability can, for this faster oscillation, be reached within the lifetime of the VDW complexes. We expect that these findings will contribute to the understanding of the spin-exchange optical pumping process and aid in its optimization.