Multiple Roles of Highly Vibrationally Excited Molecules in the Reaction Zones of Detonation Waves

Abstract

Recent experimental and theoretical advances in the understanding of high-pressure, high-temperature chemical kinetics are used to extend the nonequilibrium Zeldovich−von Neumann−Doring (NEZND) theory of self-sustaining detonation in liquid and solid explosives. The attainment of vibrational equilibrium behind the leading shock front by multiphonon up-pumping and internal vibrational energy redistribution establishes a high-temperature, high-density transition state or series of transition states through which the chemical decomposition proceeds. The reaction rate constants for the initial unimolecular decomposition steps are accurately calculated using high-temperature, high-density transition-state theory. These early reactions are endothermic or weakly exothermic, but they channel most of the available energy into excited vibrational states of intermediate product species. The intermediate products transfer some of their vibrational energy back into the transition states, accelerating the overall reaction rates. As the decomposition progresses, the highly vibrationally excited diatomic and triatomic molecules formed in very exothermic chain reactions are rapidly vibrationally equilibrated by “supercollisions”, which transfer large amounts of vibrational energy between these molecules. Along with vibrational−rotational and vibrational−translational energy transfer, these excited vibrational modes relax to thermal equilibrium by amplifying pressure wavelets of certain frequencies. These wavelets then propagate to the leading shock front and reinforce it. This is the physical mechanism by which the leading shock front is sustained by the chemical energy release.

Detonation Reaction Zones in Condensed Explosives

“…The Non-Equilibrium Zeldovich -von NeumannDoring (NEZND) theory was developed to identify the non-equilibrium chemical processes that precede and follow exothermic chemical energy release within the reaction zones of self-sustaining detonation waves in gaseous, liquid and solid explosives (1)(2)(3)(4)(5)(6)(7)(8)(9)(10). Prior to the development of the NEZND model, the chemical energy released was merely treated as a heat of reaction in the conservation of energy equation in the ChapmanJouguet (C-J) (11,12), Zeldovich -von NeumannDoring (ZND) (13)(14)(15), and curved detonation wave front theories (16).…”

Chemical reaction and equilibration mechanisms in detonation waves

“…Once the exotherrnic chain reaction process is initiated, highly vibrationallyexcited productsform and interact with the umeackd molecules and each other to greatly increase the rates of decomposition. Large quantities of vibrational energy have been shown to be rapidly transferred by "superCollisions" involving highly vibrationally excited molecules (5).…”

Effect of electric fields on the reaction rates in shock initiating and detonating solid explosives

“…This excitation has been shown experimentally [12] and theoretically [13] to be followed within picoseconds by a nonradiative transition back to the ground electronic state accompanied by excitation of that state's vibrational levels. The highly vibrational excited molecules undergo more frequent reactive collisions than those in lower vibrational states [14], thus increasing the chemical energy release rate and reducing the overall reaction zone length of the detonation wave [14] or lower pressure chemical processes [15]. A simplified reaction sequence is:…”