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Conference: Bucharest University Faculty of Physics 2003 Meeting

Section: Atomic and Molecular Physics; Astrophysics

Title:

INVESTIGATION OF THE RELAXATION MECHANISMS BY LINESHAPE ANALYSIS OF THE MULTIFREQUENCY HF2EPR POWDER SPECTRA

Authors:

M. Martinelli,1 C.A. Massa,1 L.A. Pardi,1 V. Bercu,2 and F.F. Popescu2

Affiliation:

1 Istituto per i Processi Chimico-Fisici, CNR, Pisa, Italy

2Department of Physics, University of Bucharest, Magurele, Romania

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Keywords:

Abstract:

There are several mechanisms that broaden the width of an EPR lineshape. Some of these mechanisms lead to a Gaussian broadening. Relaxation mechanism governed by spin-spin interaction, spin-lattice interaction and spin exchange interaction give rise to Lorentzian lineshapes. As a consequence of the angular dependence of the line positions, the lineshapes of the powder spectra become asymmetric inhomo-geneously broadened. All the above mechanisms explain why the powder EPR spectra exhibit usually very broad resonance lines. In a previous paper [1], we have described a procedure of investigation of the relaxation processes in a multilevel spin-system by linewidth analysis of the multifrequency high-field, high-frequency EPR (HF2EPR) spectra. In order to extract the spin-relaxation rates from magnetic resonance lines it is necessary to separate the Lorentzian and Gaussian contributions to the lineshapes. De-convolution procedures needed to analyze lineshapes usually involve Voight profiles. In practice, spectroscopic lineshapes extend over a limited range and involve experimental noise, baseline drift, and wing dissymetry. Consequently, the deconvolution procedure involving Voigt profiles is usually very difficult. In the case of single-crystal EPR spectra, it is possible to separate the Lorentzian contribution from the Gaussian contribution simply by using the Stoneham formulae [1,2]. If the EPR spectra are strongly anysotropic, the spin-relaxation rates could have important angular dependencies. For multilevel spin systems, the angular dependence analyze of the line-shape of the line positions that are spread in a large magnetic field range is very difficult because of the field remanence. The aim of the present communication is to show that in many cases it is possible to investigate the spin-relaxation in a multilevel system by multifrequency HF2EPR powder spectra. A program written by Weihe was used to generate powder pattern EPR spectra [3]. It allows direct assignment of the observed lineshape of the EPR transitions by considering a Lorentzian lineshape of the individual lines of the crystallites, and a Gaussian distribution of the spin Hamiltonian parameters. The vali-dity of the method was tested on the EPR powder spectra of Pb3+ in calcite, whose lineshape of the single crystal transitions are well known[1]. The possibility to analyze the spin relaxation and the Gaussian spread of the EPR parameters caused by lattice imperfections of the crystallites was extended to Gd3+ in a tropolonate complex by temperature dependence lineshape investigation of the powder spectra performed at 95, 190, and 240 Ghz. [1] M. Martinelli, C.A. Massa, L.A. Pardi, V. Bercu, and F.F. Popescu, Phys. Rev. B 67, 014425 (2003) [2] A.M. Stoneham, J. Phys. D 5, 670 (1972) [3] C.J.H. Jacobse, E. Pedersen, J. Villadsen, H. Weihe, Inorg. Chem. 32, 1216 (1993)