Multidimensional low-power pulse EPR under DNP conditions

Several different processes and potentially even multiple different mechanisms are involved in the dynamic nuclear polarization (DNP) of bulk nuclei in a solid matrix doped with paramagnetic centers. This has to date prevented the quantification of DNP on the basis of a mechanistic understanding for paramagnetic agents with an electron paramagnetic resonance (EPR) spectrum broader than the Larmor frequency of the nuclei that are polarized. To compare theoretical models with experiments, it is necessary to gather experimental data to quantify all the involved processes. On the basis of an EPR setup using the same unstabilized microwave source as is used for DNP, we present multidimensional correlation EPR experiments to study the electron spin dynamics under DNP conditions. Low-power pulse EPR methods are used to measure the transient saturation and saturation-recovery on a timescale of tens to hundreds of milliseconds. Furthermore, the absence of a microwave resonator or cavity enables us to perform pump-probe experiments with switching of the microwave frequency. The correlation patterns obtained with these electron-electron double-resonance experiments can be used to analyze spectral diffusion in the EPR spectrum of the investigated radicals. In addition to studying the dynamical properties of the electron spins, it is shown for TEMPO in a glassy matrix that these experiments can be used to directly measure DNP of strongly hyperfine-coupled nuclei, in this case the nitroxyl-N-14.