Electronic Excitations in Initiation of Chemistry in Molecular Solids
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Electronic Excitations in Initiation of Chemistry in Molecular Solids Maija M. Kuklja Department of Electrical and Computer Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA ABSTRACT An ab initio study is performed for the initiation of chemistry in high explosive crystals from a solid-state physics viewpoint. Specifically, we are looking for the relationship between the defect-induced deformation of the electronic structure of solids, electronic excitations, and chemical reactions under shock conditions. Band structure calculations by means of the HartreeFock method with correlation corrections were done to model an effect of a strong compression induced by a shock/impact wave on the crystals with and without edge dislocations. Based on the results obtained, an excitonic mechanism of the earliest stages for initiation of high explosive solids is discussed with application to cyclotrimethylene trinitramine (also known as RDX) crystal. Experimental verification of the validity of the proposed model is reported for RDX and heavy metal azides. Thus, the key role of electronic excitations facilitated by edge dislocations in explosive solids is established and analyzed. Practical applications of the suggested mechanisms are discussed.
INTRODUCTION Multiscale modeling of materials that encompass a range of length and time scales is crucial to advances in the understanding of complex phenomena in materials. In this article we discuss a possibility of a solid state chain reaction involving electronic excited states. We attempt to link a theoretically predicted model for the excitonic mechanism of initiation in high explosive (HE) solids and experimentally discovered new optical and electronic properties of HE solids. We also show how macro-behavior of the solids such as electrical conductivity, optical absorption and luminescence, observed for seconds and minutes, can be explained and consistently interpreted using theoretically developed quantum chemical models for lattice defects and an atomic/electronic rearrangement taking place in pico- to femto- second range time scales. We believe that accurate models and simulations connecting the microscopic properties of atoms and molecules to the macroscopic behavior of materials will open very new perspectives in an initiation of detonation theory at large. A wealth of data on the mechanisms of chemical chain reactions in gases and liquids has been accumulated1. A chain reaction occurs due to real migration of active particles (free atoms or radicals), their collisions and interaction with virgin molecules. The nature of chain reactions in solids is more complex. There is a paucity of experimental data demonstrating the chain-reaction nature of the processes in solids. Serious difficulties exist for understanding the nature of active particles providing a solid state chain reaction. Apparently, real migration of reagents such as atoms or radicals over the crystal lattice may occur only as a result of sufficiently slow diffusion process
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