engleski

The effect of matrix disorder on electron spin relaxation: EPR study of solid ethanol

Amorphous solids in comparison with crystals exhibit anomalous dynamics at low temperatures, an effect addressed by different experimental techniques [1-3] and theoretical models [4-6]. However, the microscopic nature of processes governing molecular dynamics at temperatures well below the glass transition is still in the focus of the research interest [7]. In this context the application of pulsed electron paramagnetic resonance (EPR) spectroscopy offers the advantage of studying local properties in the vicinity of paramagnetic center. Since solid ethanol can be prepared in phases characterized by different types of disorder [8], it has been used as a very convenient model system for studying low-molecular weight solids by EPR upon incorporation of paramagnetic nitroxyl radicals [9]. In this study ethanol host was doped with TEMPO and electron spin-lattice relaxation, measured in the temperature interval between 5 and 80 K, was compared for crystalline and glassy state of solid ethanol. The experimental data were evaluated assuming three physical processes governing spin relaxation: two-phonon Raman process, low-frequency pseudolocal vibrations and localized quantum-mechanical two-level tunneling excitations/boson peak (TLS/BP). In order to delineate the impact of matrix protons on electron spin relaxation the comparison was made between the data acquired with protonated and deuterated host. The largest difference between spin-lattice relaxation rates was detected at lowest temperatures studied. The fastest exchange of energy between the spin system and the lattice was observed for TEMPO in protonated ethanol glass. The phenomenon can be ascribed to TLS/BP excitations which in disordered solids, as compared with crystalline ones, originate from the complex, multiple shallow minima energy landscape and an excess of the vibrational density of states over that predicted by the Debye model. This reasoning is supported by the finding that isotope substitution of matrix protons for deuterons increased spin-lattice relaxation of TEMPO in ethanol glass. In the case of crystalline ethanol no effect of matrix deuteration was observed. This finding can be explained in terms of a more ordered local structure in which the density of TLS centers is decreased and therefore are not involved in coupling of the spin system with the lattice. References [1] C. Talón, M. A. Ramos, S. Vieira, G. J. Cuello, F. J. Bermejo, A. Criado, M. L. Senent, S. M. Bennington, H. E. Fischer, and H. Schober, Phys. Rev. B 58, 745 (1998). [2] C. Cabrillo, F. J. Bermejo, and S. F. J. Cox, Phys. Rev. B 67, 184201 (2003). [3] M. A. Ramos, S. Vieira, F. J. Bermejo, J. Dawidowski, H. E. Fischer, H. Schober, M. A. González, C. K. Loong, and D. L. Price, Phys. Rev. Lett. 78, 82 (1997). [4] M. K. Bowman, and L. Kevan, J. Phys. Chem. 81, 456 (1977). [5] Misra [6] D. A. Parshin, H. R. Schober, and V. L. Gurevich, Phys. Rev. B 76, 064206 (2007). [7] A. V. Naumov, Yu. G. Vainer, and L. Kador, Phys. Rev. Lett. 98, (2007) 145501. [8] M. A. Ramos, V. Rodríguez-Mora, and R. J. Jiménez-Riobóo, J. Phys.: Condens. Matter 19, 205135 (2007). [9] M. Kveder, D. Merunka, M. Jokić, J. Makarević, and B. Rakvin, Phys. Rev. B 80, 052201 (2009).

glass; epr; relaxation

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