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

Marta Filibian 1 Sonia Colombo Serra 2 Marco Moscardini 1 Alberto Rosso 3 Fabio Tedoldi 2 Pietro Carretta 1

Physical Chemistry Chemical Physics, Royal Society of Chemistry, 2014, 16, pp.27025

The temperature dependence of $^1$H and $^{13}$C nuclear spin-lattice relaxation rate $1/T_1$ has been studied between 4.2 K and 1.6 K in pure pyruvic acid and in pyruvic acid containing trityl radicals at a concentration of 15 mM. The temperature dependence of $1/T_1$ is found to follow a quadratic power law for both nuclei in the two samples. Remarkably that is the same temperature dependence found also for the electron spin-lattice relaxation rate in the sample containing radicals. Dynamical nuclear polarization experiments on this latter sample show that below 4 K $^{13}$C build up rate and $1/T_1$ scale with $1/T_{1e}$ and their values are quantitatively consistent with the presence of a thermal mixing regime between the nuclear and electron spin reservoirs. These results are explained by considering the effect of the pyruvic acid glassy dynamics on the relaxation rates and by assuming that below 4 K dynamical nuclear polarization is driven by a very good thermal contact between the nuclear and electron spin reservoirs.

  • 1. CNISM - Dipartimento di Fisica and Unità
  • 2. Centro Ricerche Bracco
  • 3. LPTMS - Laboratoire de Physique Théorique et Modèles Statistiques