engleski

EPR study of relaxation rate of glassy modes in hydrogen bonded solid

EPR study of relaxation rate of glassy modes in hydrogen bonded solid B. Rakvin Ruđer Bošković Institute, Bijenička 54, Zagreb, Croatia e-mail: rakvin@irb.hr Keywords: EPR, electron spin lattice relaxation, glassy state, spin probes Abstract It is well known that amorphous materials show some universal thermal, elastic, and dielectric properties at low temperatures, which differ from the crystalline materials. These properties are mostly based on the presence of low-frequency modes in amorphous material in addition to acoustic phonons. The extra modes, denoted as glassy or disorder mode, are commonly divided into two types: the modes corresponding to the double-well systems that exist in glasses and the so-called “boson peak” (BP) modes. The BP modes are quasi-harmonic modes producing the excess of vibrational density of states over that predicted by the Debye model. Electron spin-lattice relaxation (SLR) rates of TEMPO radical in ethanol and stable radicals induced by ϒ-irradiation of trehalose were measured in the crystalline and glassy states of host matrices which were considered as model systems of hydrogen bonded solid. The obtained results suggest the existence of extra modes in glassy state, which produce the excess SLR rate by modulating the electron-nuclear dipolar (END) interaction between the electron spin of radical and the matrix protons. In order to elucidate the effective SLR mechanisms of these glassy modes, the SLR rate was calculated within the soft-potential model (SPM), assuming the END interaction between the electron spin and the matrix protons[1]. Evaluations of SLR rates for glassy ethanol indicate two main mechanisms of glassy modes: thermally activated relaxation of double-well system and phonon-induced relaxation of quasi-harmonic local modes. On the other side the SLR rate of radicals in trehalose glass can be understood as being affected by the Raman process with BP modes. The correlation of SLR rates induced by these mechanisms with the experimental excess SLR rate will be discussed in detail in the presentation. References [1] D. Merunka, M. Kveder, M. Jokić and B. Rakvin, J. Magn. Reson. 228 (2013) 50-58.

EPR ; electron spin lattice relaxation ; glassy state ; spin probes

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