Thermal Mixing between Spin Systems in Solids

Abstract

The present work is a study of thermal mixing in zero field between protons and bromine nuclei in paradibromobenzene, induced by rf irradiation at a frequency close to the bromine quadrupole resonance frequency. Experiments have been conducted at 77 and 4.2°K. This mixing leads to the cooling of the dipole-dipole interactions to a temperature which depends on the relative couplings to the lattice of the different interactions—dipolar and quadrupolar—experienced by the spin system. The ordering of the dipolar interactions is transferred to Zeeman interactions by adiabatic magnetization, which results in an enhanced proton polarization with respect to its thermal-equilibrium value. A detailed theory of the thermal mixing, together with experimental results obtained with paradichlorobenzene, has been published in a preceding article. The present work pays special attention to the dynamics of the evolution of the spin system during the thermal contact. This evolution proceeds from two distinct phenomena: The first is the establishment of a spin temperature in the rotating frame by rf-induced thermal mixing between dipole-dipole and quadrupole interactions. The second is a variation of this temperature at a rate and toward a steady-state limit determined by the respective spin-lattice relaxation times of the dipole-dipole and quadrupole interactions. The experiments performed at 4.2°K with paradibromobenzene verify this two-step evolution. They also demonstrate that even very low concentrations of paramagnetic impurities provide the dominant relaxation mechanism for the dipole-dipole interactions, resulting in a drastic decrease of the dynamic polarizations obtainable by this method.