Ultrafast Energy Transfer in High Explosives: Vibrational Cooling

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.

Ultrafast infrared-Raman studies of vibrational energy redistribution in polyatomic liquids

“…Several years ago we studied VER and VC in nitromethane (NM) using an Nd : YLF laser system with 30 ps time resolution. 23 VER and VC are particularly interesting in NM because it is a model system for understanding high explosives, and VER has been shown to play important roles in shock-wave initiation to detonation. 46 At early times, we saw the C-H stretch (¾3000 cm 1 ) pumped by the laser, which decayed instantaneously (compared with 30 ps).…”

Evanescent high pressure during hypersonic cluster-surface impact characterized by the virial theorem

“…Earlier experimental and computational work has also used molecular clusters or reactants that are solvated in a chemically inert medium ͓e.g., diatomic molecules embedded inside rare-gas clusters or, say, in CO 2 ͑Ref. 18 Even for systems in equilibrium computing the pressure by molecular dynamics requires the application of a constraint. As the leading edge atoms of the cluster reach and then recede from the surface, the rest of the cluster is still moving forward.…”

Echtzeit‐Beobachtung photoinduzierter Chemie und molekularen Energietransfers in Lösung

“…Von besonderer Bedeutung sind daher Experimente, in denen Schwingungsenergie in wenigen Freiheitsgraden deponiert wird und anschließend eine Umverteilung dieser Energie in niederfrequente intramolekulare Moden sowie in die Lösungsmittelumgebung stattfindet 20–23. 25, 26, 28–36, 61–70 Mehrere zeitaufgelöste Methoden, darunter die IR‐Absorptionsspektroskopie,20, 24, 62, 71–73 die Anti‐Stokes‐Raman‐Spektroskopie28–30, 35, 65, 74–77 und die UV‐Absorptionsspektroskopie,25–27, 56, 61 wurden erfolgreich angewendet, um die IVR‐ und VET‐Zeitkonstanten von organischen Molekülen in Lösung zu bestimmen. Die Literatur enthält ausführliche Studien zu IVR und VET von halogenierten Kohlenwasserstoffen,20, 21, 24, 27, 34, 62, 63, 78 Benzol,65 Nitromethan,30 Acetonitril,28 Alkoholen35, 36, 66, 67 und Wasser,78, 79 in denen solche (zustandsselektiven) Pump‐Probe‐Techniken intensiv zum Einsatz kamen.…”