Evidences of spin-temperature in Dynamic Nuclear Polarization: an exact computation of the EPR spectrum

TL;DR

This paper develops an exact numerical method to compute the EPR spectrum in DNP experiments, confirming the spin-temperature approach's validity and explaining spectral diffusion effects at low temperatures.

Contribution

It introduces a novel numerical technique for exact EPR spectrum computation considering dipolar interactions in DNP, supporting the spin-temperature model.

Findings

Reproduces broad depolarization observed in experiments

Validates the spin-temperature approach in DNP conditions

Explains non-monotonic EPR spectrum behavior with frequency

Abstract

In dynamic nuclear polarization (DNP) experiments, the compound is driven out-of-equilibrium by microwave (MW) irradiation of the radical electron spins. Their stationary state has been recently probed via electron double resonance (ELDOR) techniques showing, at low temperature, a broad depolarization of the electron paramagnetic resonance (EPR) spectrum under microwave irradiation. In this theoretical manuscript, we develop a numerical method to compute exactly the EPR spectrum in presence of dipolar interactions. Our results reproduce the observed broad depolarisation and provide a microscopic justification for spectral diffusion mechanism. We show the validity of the spin-temperature approach for typical radical concentration used in dissolution DNP protocols. In particular once the interactions are properly taken into account, the spin-temperature is consistent with the non-monotonic behavior of the EPR spectrum with a wide minimum around the irradiated frequency.