Resonant Mixing Dynamic Nuclear Polarization

Abstract

We propose a new mechanism for dynamic nuclear polarization that is different from the well-known Overhauser effect, solid effect, cross effect and thermal mixing processes. In particular, we discovered that the evolution of the density matrix with the simple Hamiltonian of a coupled electron-nuclear spin pair with weak microwave irradiation yields a nuclear polarization enhancement when irradiating near the electron Larmor frequency. We denote the mechanism as Resonant Mixing (RM). We believe that this mechanism is responsible for the observed dispersive shaped DNP field profile for trityl samples near the electron paramagnetic resonance center. This new effect is due to mixing of states by the microwave field together with the electron-nuclear coupling, and involves the same interactions as the SE. However, the SE is optimal when the microwave field is off-resonance, whereas RM is optimal when the microwave field is on-resonance.

Dynamic Nuclear Polarization with Conductive Polymers

“…5 and of trityl in Fig. 6B was observed by other authors in a large range of experimental conditions; by the Han g roup and the Griffin group using the radical trityl from low to high fields, from 20 to 100 K, and in both static mode and under magic angle spinning (MAS); [29,30] and by the Prisner group for BDPA in lipid bilayers above room temperature. [31] The Han group reported that the effect was stronger at higher radical concentrations.…”

“…[29,32] On the other hand, the Griffin group recently proposed a 1 electron-1 nucleus mechanism that they named "resonant mixing" (RM), based on numerical simulations and on an analytical model. [30] The absorptive pattern which is negative in Fig. 5 (for high radical concentrations) and positive in Fig.…”

“…TM and OE are mediated by two fundamentally different processes, namely, electron-electron-nucleus triple spin flips [22] and stochastic modulations of the hyperfine interaction, [9] respectively, while RM requires none of them. [30] As the radical concentration increases, electronelectron interactions (in particular dipolar interactions) increase making TM more probable. On the other hand, the efficiency of OE could also be affected by radical concentration, as the conductivity of PANI increases with radical concentration.…”

Non-perturbative treatment of the solid effect of dynamic nuclear polarization

“…Such numerical simulations are currently often employed to explore the efficiency of the solid effect for various experimental parameters. However, even in the relatively simple case of a four-level system, observing a certain effect in the simulations does not automatically provide understanding about the mechanism of this effect, as demonstrated recently by , who strive to explain the origin of a dispersive DNP component seen both in experiments and in their numerical simulations. Clearly, developing intuition about the spin dynamics that is relevant for a given phenomenon is invaluable.…”