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.]]>
6 ps, un tiempo muy corto e inconsistente con la conversión térmicamente activa de excitones auto atrapados (STE) en equilibrio. Por otro lado es un hecho conocido que la subita formación de centros F es dependiente de la temperatura. Simulaciones de dinámica molecular (DM) del proceso de relajación de excitones auto atrapados en centros F llevados a cabo en NaCl en función de la temperatura contradicen las observaciones experimentales. La simulación mediante DM permite obtener un conocimiento más profundo debido a que se pueden examinar los modos vibracionales locales de vida media extensa en el arreglo compacto de iones producidos por STE desplazados del centro. Los resultados de DM indican que la rapidez de formación de defectos aumenta conforme aumenta la temperatura en el rango de 10 - 100 K, y disminuye por arriba de los 200 K. Esta "resonancia" termica en la rapidez de formación repentina de defectos, conjuntamente con movimiento forzado de un soliton vibracional a lo largo de la coordenada reactiva, se interpreta como una resonancia estocástica en la formación de defectos. Una situación similar puede explicar el fenómeno de dinámica intersticial, es decir, la observación de que los centros H recien creados fuera de equilibrio exhiban una energía de activación térmica menor a la de un centro H en equilibrio.]]>
Stochastic resonance interpretation of temperature–dependent F–center formation in NaCl
R.T. Williamsª and K.S. Songb
ª Department of Physics, Wake Forest University, Winston–Salem, NC 27109 USA, e–mail: williams@wfu.edu
b Department of Physics, University of Ottawa (ret.), 27Morenz Terrace, Ottawa, ONK2K3H1, Canada, e–mail: akssong@sympatico.ca
Recibido el 12 de octubre de 2007 Aceptado el 9 de agosto de 2008
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Abstract
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.
Resultados experimentales de Tanimura y Hess han revelado que en NaCl la formación de centros F después de creación excitónica ocurre completamente en 6 ps, un tiempo muy corto e inconsistente con la conversión térmicamente activa de excitones auto atrapados (STE) en equilibrio. Por otro lado es un hecho conocido que la subita formación de centros F es dependiente de la temperatura. Simulaciones de dinámica molecular (DM) del proceso de relajación de excitones auto atrapados en centros F llevados a cabo en NaCl en función de la temperatura contradicen las observaciones experimentales. La simulación mediante DM permite obtener un conocimiento más profundo debido a que se pueden examinar los modos vibracionales locales de vida media extensa en el arreglo compacto de iones producidos por STE desplazados del centro. Los resultados de DM indican que la rapidez de formación de defectos aumenta conforme aumenta la temperatura en el rango de 10 – 100 K, y disminuye por arriba de los 200 K. Esta "resonancia" termica en la rapidez de formación repentina de defectos, conjuntamente con movimiento forzado de un soliton vibracional a lo largo de la coordenada reactiva, se interpreta como una resonancia estocástica en la formación de defectos. Una situación similar puede explicar el fenómeno de dinámica intersticial, es decir, la observación de que los centros H recien creados fuera de equilibrio exhiban una energía de activación térmica menor a la de un centro H en equilibrio.
Descriptores: Formación de centros F; resonancia estocástica; NaCl; dinámica molecular; excitones auto atrapados.
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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.