Electron-nuclear Spin System Research Articles

An EPR–ENDOR study of phosphorus in phenacite

An EPR and ENDOR study on a phosphorus impurity in natural single crystals of phenacite (Be2SiO4) is reported. The EPR studies on this strongly coupled electron–nuclear spin system include the forbidden or ΔMF=0 transitions. The nuclear g factor of the impurity ion which was calculated from an analysis of the forbidden transitions shows the impurity to be 31P. The principal values and directions of the electronic g tensor and the hyperfine coupling tensor were determined through a least-squares fit to the data employing direct diagonalization of the Hamiltonian matrix. ENDOR studies show a superhyperfine (shf) coupling of this center to several near neighbor 9Be nuclei. The shf coupling parameters for three 9Be nuclei were determined and are presented along with a detailed model of the center and a discussion of the crystallographic positions of the Be nuclei. Our analyses indicate a P5+ ion substitutes for a Si ion at a normal site and is converted to P4+ by x rays.

Consideration of the dipole-dipole reservoir in nonstationary double resonance

Manifestations of the dipole-dipole reservoir (DDR) of electron spins in nonstationary NNDOR and ENDOR are investigated, taking as an example the electron-nuclear spin system Cr 3+ − 53Cr− 27 Al in ruby. It is demonstrated, that a consistent consideration of the hyperfine structure of the EPR spectrum of the Cr 3+ ions explains the behaviour of the NMR signal of the 27Al nuclei, observed experimentally during saturation of NMR of the 53Cr nuclei. The ENDOR signals are calculated with allowance for the shift of the DDR temperature, and it is pointed out that the absorption signal of ENDOR can change its sign depending on nuclear detuning.

Effect of amplitude modulation of the high-frequency field saturating the esr on the nmr signals in a coupled electron-nuclear spin system

where F = v d + jud; v d and u d a re the absorption and dispers ion signals of the NMR, respect ively , during the Overhauser effect; a = a + jAwn; ~ = 1 / T t n and a = 1/T2n a re the rec iproca ls of the longitudinal and t r ansve r se nuclear relaxat ion t imes, respectively+ Aw n = YnH0 Wn; Yn is the nuclear gyrorat io ; H 0 is the constant external magnet ic field (parallel to the z axis); Win = YnHln; co n and H m are the frequency and amplitude of the magnetic field 2Hln cos COn t (in the xy plane) which induces the nuclear t ransi t ions; and M d is the longitudinal nuclear magnetizat ion to which the z component of the nuclear magnetic moment relaxes.

Theory of nuclear relaxation in concentrated paramagnetic solutions

Relaxation equations are derived for an electron-nuclear spin system, with account of the exchange interaction between electronic spins. It is shown that the relaxation equations can be derived by standard perturbation theory, with the help of the random-local-field method, based on a certain stochastic model of the exchange coupling. The perturbation theory is carried out to second order only for the interactions which are directly responsible for the relaxation, and the exchange interaction can be arbitrarily strong. The relaxation times are expressed in terms of the spectral densities of the correlation functions, which describe not only the thermal broadening of the electron-nuclear interactions, but also the exchange interaction. An explicit expression is obtained for these correlation functions.