Abstract

WILLIAMS, R.T.  and  SONG, K.S.. Stochastic resonance interpretation of temperature-dependent F-center formation in NaCl. Rev. mex. fis. [online]. 2008, vol.54, suppl.2, pp.1-9. ISSN 0035-001X.

Experiments by Tanimura and Hess have revealed that the formation of F centers after exciton creation in NaCl appears complete within 6 ps, a time too short to be consistent with thermally-activated conversion from self-trapped excitons (STE) in equilibrium. Yet the yield of prompt F centers is temperature-dependent. Molecular dynamics simulations of the relaxation from self-trapped excitons to F centers in NaCl have been performed as a function of modeled temperature, and are found to duplicate the main features of the seemingly paradoxical experimental observation. Additional insight is gained from the MD simulations by being able to observe excitation of a long-lived local vibrational mode on the compacted anion row produced by the off-center STE. The MD results indicate that the defect formation rate increases with temperature from 10 K up to about 100 K, and decreases above about 200 K. This thermal "resonance" in the yield of prompt defect formation, along with the presence of a vibrational soliton forcing motion along the reaction coordinate, are interpreted as an example of stochastic resonance in defect formation. The same explanation may explain the dynamic interstitial phenomenon, i.e. observation that a freshly created H center has a lower thermal activation energy for transport than an equilibrated H center.

Keywords : F-center formation; NaCl; stochastic resonance; molecular dynamics; self-trapped exciton.

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