New determination of the nuclear-quadrupole moment of the NMR probe 99Tc ground-state

L. A. ERRICO, G. N. DARRIBA Y M. RENTERÍA.

La Plata, Bs As, Argentina

Workshop; 35th anniversary of Hyperfine Interactions at La Plata International Workshop; 2005

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The experimental study of hyperfine interactions is often used as a powerful tool to characterize different atomic sites in a given material and to obtain information about electronic, magnetic and structural properties of solids [1]. In the case of pure electric-quadrupole interactions, the measured quantities are the quadrupole coupling constant nQ = eQV33/h and the asymmetry parameter h = (V11-V22)/V33, where Vii (i = 1, 2, 3) are the components of the diagonalized electric-field gradient (EFG) tensor with the convention |V33|>|V22|>|V11|. The EFG is measured via its interaction with the nuclear-quadrupole moment Q of a suitable probe-atom (generally an impurity in the system under study) by different techniques, such as Mössbauer Spectroscopy, Nuclear Magnetic and Nuclear-Quadrupole Resonance (NMR and NQR) or Perturbed-Angular Correlations (PAC). Since the EFG tensor is directly related to the asphericity of the electronic density in the vicinity of the probe nucleus, the determination of the quadrupole coupling constant nQ allows the investigation of electronic and structural properties in the solid, provided Q is known. Although Q is a purely nuclear quantitity, the quadrupole moments for some isotopes are only known with limited accuracy and their determination is still an active field of research. One of the possible methods to determine Q is to use information of experimental quadrupole coupling constants and accurate theoretical EFG calculations. In this work we present a new determination of the nuclear-quadrupole moment  of the ground state (I = 9/2+) of 99Tc. This nuclear state is used as nuclear probe in NMR spectroscopy. This determination was obtained by comparing experimentally determined nuclear-quadrupole frequencies with the EFG calculated at Tc sites in metallic Tc using the ab initio “Full-Potential Linearized-Augmented-Plane-Waves” method [2]  in the framework of the Density Functional Theory. The obtained result for the quadrupole moment, |Q(99Tc, 9/2+)| = 0.141 b, has an indetermination ten times smaller than those previously reported in the literature [Q(99Tc, 9/2+) = +0,3434 b [3]; Q(99Tc, 9/2+) = +0,2810 b [4]]. These determinations are compared with predictions of the nuclear shell model. The sign of the 99Tc quadrupole moment is also discussed.