| Molecular mechanical energy transfer in energetical materials is investigated because of the likely possibility of a relationship between energy transfer rates and impact sensitivities. Picosecond coherent Raman scattering, and anti-Stokes Raman spectroscopy following an ultrafast temperature and pressure jump, are used to study vibrational energy relaxation and multiphonon up-pumping in a high explosive nitromethane (NM). The relationships between these energy transfer processes and shock wave-induced initiation to detonation are discussed. The principal mechanism of vibrational cooling in solid NM below 150 K is shown to be a vibrational ladder relaxation process giving rise to a vibrational cascade occurring on the {dollar}>{dollar}100-ps time scale. Ambient temperature up-pumping measurements show the 657- and 918- cm{dollar}sp{lcub}-1{rcub}{dollar} vibrations are populated sequentially, and therefore vibrational ladder climbing is involved. The overall time scale for up-pumping is {dollar}approx{dollar}100 ps, which is consistent with what would be predicted from low-temperature CARS measurements, provided the ladder mechanism remained dominant at all temperatures. These measurements yield an estimate for the width of the up-pumping region behind weak shock waves characteristics of initiation process of {dollar}lsb{lcub}rm up{rcub}approx2times10sp{lcub}-7{rcub}{dollar} m. Vibrational cooling in the liquid high explosive nitromethane (NM) is studied by picosecond infrared pumping of C-H stretching vibrations ({dollar}approx{dollar}3000 cm{dollar}sp{lcub}-1{rcub}{dollar}) and picosecond incoherent anti-Stokes Raman probing of six lower energy Raman-active vibrations in the 480-1400 cm{dollar}sp{lcub}-1{rcub}{dollar} 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 high energy vibrations in the 900-1400 cm{dollar}sp{lcub}-1{rcub}{dollar} range. Overheating refers to instantaneous vibrational quasitemperatures which are temporally 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 energetical material initiation presently in use. |
