DYNAMIC NUCLEAR-POLARIZATION OF DIAMOND - I - SOLID-STATE AND THERMALMIXING EFFECTS

The dynamic nuclear polarization of C-13 nuclei in a suite of seven na tural type Ia and Ib diamonds, using continuous wave S- and X-band mic rowave radiation, is described. The C-13 signal enhancement and polari zation time have been measured for one of the type Ib diamonds as a fu nction of magnetic field in the vicinity of the resonance field. The t otal paramagnetic impurity concentration (P1 and other centers) in thi s diamond is 2 x 10(18) cm(-3) (23 atomic parts per million), while th e concentration of pi centers is 9.3 ppm. Since the central electron s pin resonance (ESR) linewidth H-L is comparable with H(0)gamma(C)/gamm a(e), flip-flip and hip-hop forbidden transitions take place simultane ously. Consequently thermal mixing plays an important role in the C-13 Signal enhancement. However, the C-13 spin-lattice relaxation rate is determined to a large extent by the solid state effect (forbidden tra nsitions). The C-13 polarization rates have been measured for the suit e of diamonds by executing dynamic nuclear polarization (DNP) experime nts on both hyperfine and central ESR lines. It is shown that the pola rization rate is proportional to the paramagnetic impurity concentrati on of the sample, in agreement with the existing theory. It has been f ound that in type Ib diamonds with relatively low nitrogen impurity co ncentrations the dynamic nuclear polarization of a single hyperfine li ne yields an equilibrium C-13 polarization that is one-quarter of that obtained in the case of dynamic nuclear polarization of the central l ine. In samples containing P1 and P2 centers (type Ia), or in type Ib samples with relatively high concentrations of P1 centers, the same eq uilibrium C-13 polarization is obtained for the DNP of hyperfine and c entral transitions. This phenomenon is explained in terms of a model i n which thermal contact is established between the electron Zeeman res ervoir and the nuclear spin reservoirs via the spin-spin interaction r eservoir if H-L greater than or equal to H(0)gamma(C)/gamma(e). (C) 19 98 American Institute of Physics. [S0021-9606(98)70534-3].