Shock-induced molecular excitation in solids

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Abstract
Initiation of condensed explosives is studied on a molecular level with a quantum-mechanical calculation of transition rates for shock-induced transitions between the low-lying internal molecular normal-mode states in a molecular solid. It is assumed that the shock produces a distribution of acoustic phonons which become thermalized before any significant number of internal-mode phonons is created. The calculation uses the Born-Oppenheimer approximation in which the internal modes constitute the fast subsystem and the acoustic modes the slow subsystem. A sample calculation is done for nitromethane. Generally speaking, the lowest-frequency internal modes have the fastest shock-induced transition (highest rates), with the transition from the ground to first excited state being the slowest. The transition rates increase by 6 to 10 orders of magnitude from the values under normal conditions when nitromethane is subjected to shocks of 50 to 300 kbar. The transition lifetimes are compared with, and show some correlation with, the pressure-time critical-shock initiation data obtained by de Longueville, Fauquignon, and Moulard.