Direct measurement of ultrafast multiphonon up-pumping in high explosives

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 >100-ps time scale. Ambient temperature up-pumping measurements show the 657- and 918-cm^-1 vibrations are populated sequentially, and therefore vibrational ladder climbing is involved. The overall time scale for up-pumping is ≈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 characteristic of initiation processes of l_up ≈ 2 × 10^-7 m.