Effects of Microwave Gating on Nuclear Spin Echoes in Dynamic Nuclear Polarization

Abstract

Dipolar or quadrupolar echoes allow one to observe undistorted powder patterns, in contrast to simple Fourier transformations of free induction decays (FIDs). In this work, the build-up of proton polarization due to dynamic nuclear polarization (DNP) is monitored by observing echoes rather than FIDs. When the microwave irradiation is interrupted during the build-up of DNP, the electrons relax back to their Boltzmann distribution at high fields (B0 = 6.7 T) and low temperatures 1.2 < Tsample < 4.0 K, so that dipolar flip-flop-flip terms involving two electrons and one proton become largely ineffective as a mechanism of proton decoherence. This leads to a prolongation of the nuclear coherence lifetime T2'( 1 H). The increase in T2'( 1 H) leads to transient surges of the amplitudes of spin echoes. Conversely, transient slumps of spin echoes are observed when the microwave irradiation is switched back on, due to a shortening of nuclear coherence lifetimes.

Relaxation enhancement by microwave irradiation may limit dynamic nuclear polarization

“…Microwave (MW) irradiation in DNP generates not only hyperpolarization of the nuclei but can cause a relaxation enhancement of the nuclei by more than a factor of ten at temperatures of a few Kelvin compared to the decay of nuclear polarization without MW irradiation. This is similar to the experimentally observed shortened relaxation times T 1r 18 and T 0 2 19 under MW irradiation at liquid-helium temperatures. The relaxation enhancement was measured on 1 H in samples consisting of a glassy matrix containing TEMPO radicals.…”

“…2 ) 19 at liquid helium temperatures and 6.7 T field strength. In this work, the increased nuclear relaxation-rate constants under microwave irradiation during the build up of hyperpolarization is found to limit the achievable maximum polarization compared to the prediction when using the electron equilibrium polarization, nuclear relaxation-rate constants (without MW irradiation) and polarization injection rates.…”

Frozen water NMR lineshape analysis enables absolute polarization quantification

“…We note that this measurement was performed under microwave irradiation of the sample; gating the microwave source during NMR signal acquisition was recently demonstrated to prolong spin decoherence times and result in increased echo intensities. 51 This is considerably longer than the dead time employed in pulse-acquire experiments, but on the order of the echo time 2 t echo used in echo-based experiments and as a result T 2 relaxation reduces the intensity and also could potentially influence the lineshape of echo-based spectra, which could be related to some discrepancy between the experimental and simulated spectra shown in Fig. 5, but is unlikely to influence those of the pulse-acquire measurements;…”

The role of electron polarization on nuclear spin diffusion

“…As depicted by the hollow circles in Fig. 3, our model predicts that spin diffusion at the interface is ≈ 7 times faster when the electron polarization is P ≈ 50% (that is, under µw irradiation [32,33]) compared to that at Boltzmann equilibrium P = 99.93%. As Fig.…”

“…3. The µw irradiation is gated and a delay t g allows the electron spins to return to Boltzmann equilibrium [32,33].…”