Shocked molecular solids: Vibrational up pumping, defect hot spot formation, and the onset of chemistry

Abstract

A model and detailed calculations are presented to describe the flow of energy in a shocked solid consisting of large organic molecules. The shock excites the bulk phonons, which rapidly achieve a state of phonon equilibrium characterized by a phonon quasitemperature. The excess energy subsequently flows into the molecular vibrations, which are characterized by a vibrational quasitemperature. The multiphonon up pumping process occurs because of anharmonic coupling terms in the solid state potential surface. Of central importance are the lowest energy molecular vibrations, or ‘‘doorway’’ modes, through which mechanical energy enters and leaves the molecules. Using realistic experimental parameters, it is found that the quasitemperature increase of the internal molecular vibrations and equilibration between the phonons and vibrations is achieved on the time scale of a few tens of picoseconds. A new mechanism is presented for the generation of ‘‘hot spots’’ at defects. Defects are postulated to have somewhat greater anharmonic coupling, causing the vibrational temperature in defects to briefly overshoot the bulk. The influence of the higher defect vibrational temperature on chemical reactivity is calculated. It is shown that even small increases in defect anharmonic coupling have profound effects on the probability of shock induced chemistry. The anharmonic defect model predicts a size effect. The defect enhanced chemical reaction probability is reduced as the particle size is reduced.