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Conference: Bucharest University Faculty of Physics 2001 Meeting

Section: Atomic and Molecular Physics; Astrophysics

Title:

High Field-High Frequency Electron Paramagnetic Resonance of Pb3+ in Calcite

Authors:

M. Martinelli, C. A. Massa , L.A. Pardi, V. Bercu1, and F.F. Popescu1

Affiliation:

Instituto di Fisica Atomica e Molecolare CNR, via Alfieri 1, 56010, Ghezzano, Pisa

1 Department of Physics, University of Bucharest, Magurele, RO-76900, Bucharest, Romania

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Keywords:

Abstract:

The paramagnetic center under focus is Pb3+ in calcite. Due to the fact that this ion has a (6s)1 its EPR spectra are approximately isotropic, and characterized by a large hyperfine interaction. In this case the hyperfine constant represents the zero field splitting. The hyperfine interaction for 207Pb3+ is about 40 GHz. This zero field splitting is larger than the frequencies of the usual microwave bands (9 - 35 GHz). Thanks to the possibility of High Field-High Frequency Electron Paramagnetic Resonance [1] (HF2EPR) we could study some transitions that in the usual microwave bands can not be detected. Due to the favorable condition of containing only isotopes with zero nuclear spin, calcite is a special case of diamagnetic crystal. So, the EPR lines are not inhomogeneously broadened by the super-hyperfine interaction. In addition, the high isotropic character of the EPR spectra of ions with (ns)1 configuration leads to negligible mosaic structure effects in such spectra. That is why, the EPR lines of the paramagnetic ions hosted in calcite are at higher temperature homogeneously broadened [2]. The linewidths at relatively high temperatures are strongly dependent on the spin-lattice relaxation rates, and on the magnetic field intensity [2]. By using the specific advantages of HF2EPR, it becomes possible to study on one hand, the dependencies on the linewidths on the magnetic field intensity, and on the other hand to obtain a parallel detection (where the magnetic component of the FIR field is parallel to the static magnetic field). The measurements have been made at 95, GHz, 190 GHz, 240 GHz, and 285 Ghz. Our measurement confirms the recent theory of the Raman relaxation processes. In addition, unlike the usual microwave bands experiments, a direct spin-lattice relaxation process via modulation of the hyperfine interaction is revealed even at high temperatures. This fact is not surprising because at higher frequencies, the phonon spectral density of the lattice increases strongly. [1] J. H. Freed, Annu. Rev. Phys. Chem., 51, 655 (2000) [2] F. F. Popescu, and V.V. Grecu, J. Phys. C, 15, 1547 (1982) [3] F. F. Popescu, Phys. Stat. Sol. (b), 214, 113 (1999)