ULTRAFAST ENERGY-TRANSFER IN HIGH EXPLOSIVES - VIBRATIONAL COOLING

Molecular mechanical energy transfer in energetic materials is investi gated because of the likely possibility of a relationship between ener gy transfer rates and impact sensitivities. Energy transfer in the liq uid high explosive nitromethane (NM) is studied by picosecond infrared pumping of C-H stretching vibrations (similar to 3000 cm(-1)) and pic osecond incoherent anti-Stokes Raman probing of six lower energy Raman -active vibrations in the 1400-480 cm(-1) range. Vibrational cooling o f C-H excited NM is shown to require at least 200 ps. During vibration al cooling, substantial transient overheating is observed in the highe r energy vibrations in the 1400-900 cm(-1) range. Overheating refers t o instantaneous vibrational quasitemperatures which are temporarily gr eater than the final temperature of the bulk liquid. The overheating a nd the increasing delay in the rise of excitation in certain vibration s is used to infer that ladder (cascade) type vibrational cooling proc esses are important in ambient temperature NM. Molecular thermometry i s used to estimate the absolute efficiencies of energy transfer betwee n some of the pumped and probed vibrations. This detailed study of ene rgy transfer in a high explosive presents a more complete picture than the relatively simplified theoretical models for energetic material i nitiation presently in use.