On the role of electron–nucleus contact and microwave saturation in thermal mixing DNP

Abstract

We have explored the manifold physical scenario emerging from a model of Dynamic Nuclear Polarization (DNP) via thermal mixing under the hypothesis of highly effective electron-electron interaction. When the electron and nuclear reservoirs are also assumed to be in strong thermal contact and the microwave irradiation saturates the target electron transition, the enhancement of the nuclear polarization is expected to be considerably high even if the irradiation frequency is set far away from the centre of the ESR line (as already predicted by Borghini) and the typical polarization time is reduced on moving towards the boundaries of said line. More reasonable behaviours are obtained by reducing the level of microwave saturation or the contact between electrons and nuclei in presence of nuclear leakage. In both cases the function describing the dependency of the steady state nuclear polarization on the frequency of irradiation becomes sharper at the edges and the build up rate decreases on moving off-resonance. If qualitatively similar in terms of the effects produced on nuclear polarization, the degree of microwave saturation and of electron-nucleus contact has a totally different impact on electron polarization, which is of course strongly correlated to the effectiveness of saturation and almost insensitive, at the steady state, to the magnitude of the interactions between the two spin reservoirs. The likelihood of the different scenario is discussed in the light of the experimental data currently available in literature, to point out which aspects are suitably accounted and which are not by the declinations of thermal mixing DNP considered here.

Relevance of electron spin dissipative processes to dynamic nuclear polarization via thermal mixing

“…A similar improvement is achievable also by assuming a combination of bad electron-nucleus contact and finite nuclear leakage [13] or by imposing a partial saturation of the irradiated packet [11,13]. Here however it is obtained simply by removing the non-physical assumption T CSD = 0, which is normally accepted in all TM models in order to simplify their computation.…”

“…While a speed up of the DNP process with the increase in the radical concentration has to be expected, since there are more polarization transfer centers, the significant decrease of the steady state polarization has not been accounted for by any previous theoretical description of TM-DNP. In fact both the Borghini [22] and the finite contact model introduced in [12] and [13] predict no effect of the radical concentration as long as T 1n is negligible whereas, in presence of leakage, a higher number of polarizing centres would push the system towards higher steady state polarizations. Gadolinium doping on the other hand leaves substantially unaffected the nuclear polarization time, while shortening the typical electron spin lattice relaxation time T 1e .…”

“…First of all, it has to be realized that the Borghini model and the model introduced in [12,13] rely on the assumption that the energy exchange between the electron system and the lattice occurs only via the Zeeman transitions depicted in FIG. 3, panel C, whereas all transitions involving more than one electron spin are always energy conserving.…”

“…Improvements to the original theoretical picture have been proposed in [11] and [12,13]. In these latter papers, in particular, a novel approach based on rate equations has been introduced, that overcomes several limitations of the traditional approaches and provides the dynamics of the spin polarization of spin ensembles by calculating the evolution induced by any single or multiple spin transitions.…”

The role of the glassy dynamics and thermal mixing in the dynamic nuclear polarization and relaxation mechanisms of pyruvic acid

“…The effect of relevant parameters, including the radical concentration [14][15][16], the concentration of gadolinium complexes [14,17], the nuclear concentration [18,19], the amount of matrix deuteration [20], the effect of microwave saturation and the field strength [15,16,21] on the DNP performances of this molecule have been experimentally studied. Remarkably very recently, the relevance of these physical quantities on DNP kinetics has also been considered in the development of novel models describing TM throughout a rate equation approach [19,22,23]. Nevertheless, the role of the properties of the glassy matrix formed by the polarized molecules and radicals has not been investigated to a deep level.…”