Nathan A. Prisco, Arthur C. Pinon, Lyndon Emsley, Bradley F. Chmelka, “Scaling analyses for hyperpolarization transfer across a spin-diffusion barrier and into bulk solid media,” Physical Chemistry Chemical Physics, 23, 1006–1021 (2021).
The significance of this work is that it demonstrated, for the first time, the use of dimensional scaling analyses to elucidate key rate-limiting processes and phenomena that are central to the use of hyperpolarization to analyze and understand complicated multi-spin systems. The time evolution of such multi-spin systems are complicated to treat quantum mechanically, and this paper showed that classical scaling analyses render them tractable. In particular, by analogy to heat and mass transfer film theory, we developed a general approach for determining hyperpolarization transfer rates between dilute electron spins and a surrounding ensemble of nuclear spins (Fig. 1A). The analyses yielded new quantitative relationships for understanding, predicting, and optimizing the effectiveness of hyperpolarization protocols, such as Dynamic Nuclear Polarization (DNP). In particular, a “polarization-transfer coefficient” was defined and measured and shown to be analogous to heat and mass transfer coefficients. Spin-polarization analogues of dimensionless Biot, Thiele, and Damköhler numbers were developed to describe the accumulation, propagation, and dissipation of net nuclear magnetization produced by hyperpolarization processes, the results of which were in excellent quantitative agreement with experiment with no adjustable parameters.
Depending on experimental conditions, two distinct kinetic regimes were found to be associated with different rate-limiting polarization transfer phenomena (Fig. 1B). Specifically, the quantitative analyses provided evidence for kinetically-limited transfer associated with a so-called “spin-diffusion” barrier. The analyses notably yielded new insights on the transfer of hyperpolarization over mesoscopic (2–100 nm) length scales, including across interfaces, that are much larger than those associated with quantum mechanical models that have typically been the focus of the spectroscopy community. The results and classical analyses yielded general design criteria for optimization of polarization transfer processes involving complicated interfaces and composite media for applications in materials science, physical chemistry, chemical engineering, and spintronics. This paper represents a novel application of chemical engineering principles to physical systems and processes that extended quantitative understanding to longer length and time scales than quantum mechanical analyses alone.