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Conference: Bucharest University Faculty of Physics 2005 Meeting

Section: Atomic and Molecular Physics; Astrophysics

Title:

High-Field Effects in HF-EPR spectra: delocalized spin projection in paramagnetic molecular complexes and azimutal angular dependence of axial spectra

Authors:

F.F. Popescu1, M. Martinelli2, C.A. Massa2, L.A. Pardi2, and V. Bercu1-3

Affiliation:

1 Department of Physics, University of Bucharest, Magurele, Ro-76900, Bucharest, Romania;

2 HF2EPR - Istituto per i Processi Chimico-Fisici CNR - via G. Moruzzi, 1 – 56124 Pisa

3Dipartimento di Fisica “Enrico Fermi”, Università di Pisa, Largo Bruno Pontecorvo 3, I-56127 Pisa

E-mail

ffpopescu@k.ro

Keywords:

Abstract:

High-Field Electron Paramagnetic Resonance (HF-EPR) spectroscopy was used to investigate the unusual temperature and frequency dependence of the powder spectrum of the Gd(HBPz3)2 tropolonate complex (GdTrp)[1]. It was found that, compared to the calculated powder spectra, the intensity of the lines to Gd3+ and Ce3+ HF-EPR powder spectra corresponding to the magnetic fielf along the binary axis was increasingly depleted by increasing the frequency and decreasing the temperature. A new type of H/T effect [2] is evidenced. This effect is interpreted in terms of the formation of spin projection states delocalized and quasidelocalized along linear chains of Gd3+ ions in high magnetic fields, due to the competition between the weak dipole and exchange spin-spin interaction and to the particular structure of the molecular complex. The number of ions in the chain depends strongly on the orientation of the magnetic field and on the relaxation processes[3]. Another high field effect was obseved in Ce3+ HF-EPR powder spectra corresponding to a ground multiplet with J>1/2 and a large orbital contribution[4]. Consequently these paramagnetic ions exhibits a ground doublet and other closed excited doublets. In many cases, this zero-field splitting is small to be measured by optical spectroscopy, while the susceptibility measurements are not suitable to locate them with a reasonable precision. The conventional microwave EPR spectroscopy usually detects the transitions corresponding to the ground doublet of the J manifold. As a result, depending on symmetry, are available only one, two, or maximum three values of the effective g. These values on one hand are magnetic field independent, and on the other hand depend on small reductions of the gJ Zeeman interaction factor which can exhibits also a weak anisotropy. It is difficult to determine these adjustable factors and the relevant crystal-field parameters from the observed effective g-values and the normalization relationship because of the uncertainty. However, if the Zeeman interaction is not very small in comparison with the zero-field splitting of the ground manifold, the above effective g values may exhibit important magnetic field dependences. Consequently, a multifrequency HF-EPR spectroscopy could be very suitable to determine the zero-field splitting and the adjustable factors of the Zeeman interaction. In addition, even in axial symmetries, the effective g factor may exhibit not only a polar angular dependence, but also an azimuthal angular dependence. The azimuthal angular dependence is in the first approximation proportional to for a fourfold axis, for a sixfold axis, etc. The azimuthal angular dependence splits the perpendicular type transition of the EPR powder spectrum. This splitting may be used to find with a good precision the zero-field splitting of the ground manyfold, even direct transitions between the ground doublet and the excited doublets are not detectable. Consequently, the splitting of the perpendicular transition in a powder HF-EPR spectra exhibits an important advantage, since angular dependences on single-crystals at high-fields and low temperatures are very difficult to be obtained. References: [1] A. Caneschi, A. Dei, D. Gatteschi, C.A. Massa, L.A. Pardi, S. Poussereau, and L. Sorace, Chem:Phys:Lett: 371, 694 (2003). [2] F.F. Popescu, M. Martinelli, C.A. Massa, L.A. Pardi, and V. Bercu, sent to Magnetic Resonance in Chemistry . [3] M. Martinelli, C.A. Massa, L.A. Pardi, V. Bercu, and F.F. Popescu, Phys:Rev: B, 67, 014225, 1-11,(2003) [4] F.F. Popescu, M. Martinelli, C.A. Massa, L.A. Pardi, and V. Bercu, sent to J: Phys: Condens: Matter.