Spin‐lattice relaxation of spin‐½ nuclei in solids containing diluted paramagnetic impurity centers. II. Dynamic polarization of nuclear spin system by thermal mixing and solid‐state effect

Abstract

AbstractDynamic nuclear polarization of nuclear spins via the solid‐state and thermal mixing effects is discussed. Continuous‐wave S‐ and X‐band microwave radiation have been employed to measure 13C signal enhancements and polarization times for 13C nuclei in a natural type Ib diamond as a function of magnetic field. It was found that thermal mixing plays an important role in the 13C signal enhancement because the central electron spin resonance (ESR) line width HL ≃ H0γC/γe, resulting in flip‐flip and flip‐flop forbidden transitions taking place simultaneously. On the other hand, the 13C spin‐lattice relaxation rate is determined to a large extent by the solid‐state effect (forbidden transitions). 13C polarization rates have also been measured for a suite of natural diamonds. It is shown that the polarization rate is proportional to the paramagnetic impurity concentration, in agreement with the theory. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson 19A: 36–43, 2003.