Ultrafast Energy Transfer in High Explosives: Vibrational Cooling

摘要

Molecular mechanical energy transfer in energetic materials is investigatedbecause 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 (-3000cm-1) and picosecond incoherent anti-Stokes Raman probing of six lower energy Raman-active vibrations in the 1400-480 cm-1 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-1 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.