Dynamic Polarization of Protons in Copper Tutton Salts (Dilute Cu${\mathrm{K}}_4$S${\mathrm{O}}_4$·6${\mathrm{H}}_2$O) and Their Spin-Lattice Relaxation Times

The protons from waters of hydration of copper and zinc salts are polarized by saturating the electron-spin transitions (forbidden) of copper ions at liquid-helium temperatures. The enhancement of the proton resonance was observed to be due to dipole-dipole interaction with the electron spins (so-called solid-state effect). The rates of decay and growth of these enhanced proton resonance signals were measured as functions of the temperature and the intensity of the microwave field applied to saturate the electron-spin transition. From the measured proton relaxation time (in the range of milliseconds) and its temperature dependence, the electron-spin relaxation times (in the range of microseconds) and their temperature dependence were deduced. It was found that the ${\mathrm{Cu}}^{++}$ ions in the 1:50 dilute Cu:Zn sample relax through spin-orbit interaction (Raman process), and in the 1:5 Cu:Zn sample through both spin-orbit (Raman) and exchange interactions. The observed small values of the relaxation times for electrons indicate that the protons relax preferentially through those electron spins which have no phonon bottleneck.