No abstract

Molecular mechanical energy transfer in energetic materials is investigated because of the likely possibility of a relationship between energy transfer rates and impact sensitivities. Energy transfer in the liquid high explosive nitromethane (NM) is studied by picosecond infrared pumping of C-H stretching vibrations (-3000 cm-') and picosecond incoherent anti-Stokes Raman probing of six lower energy Raman-active vibrations in the 1400-480 cm-I range. Vibrational cooling of C-H excited NM is shown to require at least 200 ps.During vibrational cooling, substantial transient overheating is observed in the higher energy vibrations in the 1400-900 cm-' range. Overheating refers to instantaneous vibrational quasitemperatures which are temporarily greater than the final temperature of the bulk liquid. The overheating and the increasing delay in the rise of excitation in certain vibrations is used to infer that ladder (cascade) type vibrational cooling processes are important in ambient temperature NM. Molecular thermometry is used to estimate the absolute efficiencies of energy transfer between some of the pumped and probed vibrations. This detailed study of energy transfer in a high explosive presents a more complete picture than the relatively simplified theoretical models for energetic material initiation presently in use.

Time resolved vibrational spectroscopy with picosecond tunable mid-infrared pulses is used to measure the rates and investigate the detailed mechanisms of multiphonon up-pumping and vibrational cooling in a condensed high explosive, nitromethane. Both processes occur on the ˜100 ps time scale under ambient conditions. The mechanisms involve sequential climbing or descending the ladder of molecular vibrations. Efficient intermolecular vibrational energy transfer from various molecules to the symmetric stretching excitation of NO2 is observed. The implications of these measurements for understanding shock initiation to detonation and the sensitivities of energetic materials to shock initiation are discussed briefly.