Multiphonon up-pumping

Chen S, Tolbert W A and DIott D D 1994 Direot measurement of ultrafast multiphonon up pumping in high explosives J. Phys. Chem. 98 7759-66... 

Wen X, Tolbert W A and DIott D D 1992 Multiphonon up pumping and moleoular hot spots in superheated polymers studied by ultrafast optioal oalorimetry Chem. Phys. Lett. 192 315-20... 

Indirect transfer occurs by a two-part mechanism, as shown in Fig. 18. First a vibrational excitation decays by generating phonons. The phonons then produce vibrational excitation on other molecules by multiphonon up-pumping. Indirect transfer will not occur unless the density of vibrational excitations is large enough to produce a real increase in the bath temperature. 

The observation that the reaction requires an induction time of tens of picoseconds can be used to differentiate between proposed mechanisms of how shock wave energy localizes to cause chemical reaction. This induction time is expected for mechanisms that involve vibrational energy transfer, such as multiphonon up-pumping [107], where the shock wave excites low frequency phonons that multiply annihilate to excite the higher frequency modes involved in dissociation. It is also consistent with electronic excitation relaxing into highly excited vibrational states before dissociation, and experiments are underway to search for electronic excitations. On the other hand, prompt mechanisms, such as direct high frequency vibrational excitation by the shock wave, or direct electronic excitation and prompt excited state dissociation, should occur on sub-picosecond time scales, in contrast to the data presented here. 

Picosecond laser pulses in the UV range do not result in better ablation behavior than nanosecond laser pulses. This is different for doped polymers. Experiments with doped PMMA (an IR-absorber, i.e., IR-165 for ablation with near-IR laser and diazomeldrum s acid (DMA) for ablation with UV lasers) with nanosecond and picosecond laser irradiation in the UV (266 nm) and near-IR (1064 nm) range have shown that, in the IR, neat features could be produced with picosecond laser irradiation, while nanosecond irradiation only results in rough surface features [105]. This corresponds well with the different behavior of the two absorbers. With IR-165 the polymer is matrix is heated by a fast vibrational relaxation and multiphonon up-pumping [106]. This leads to a higher temperature jump for the picosecond irradiation, which causes ablation, while for nanosecond pulses only lower temperatures are reached. 

Fig. 12. Experimental measurement of multiphonon up-pumping in nitromethane (NM), reproduced from ref. [127]. Phonons are generated using a picosecond pulse to excite a dye molecular heater. Anti-Stokes Raman spectroscopy is used to monitor population changes that can be converted to vibrational quasitemperatures. The 657 cm 1 doorway vibration is pumped faster than the instrument resolution of 25 ps. The 918 cm C-N stretch, which must be activated to break a C-N bond, is excited 25 ps later.

Interesting is the Multidimensional Reactive Flow model developed by Tarver et al. [5,103,104) it is based on the Non-Equilibrium Zeldovich-von Neuman-Dhring theory. This model starts from the primary chemical changes occurring in the adiabaticaUy compressed thin layer of molecules of the given EM and multiphonon up-pumping due to shock, but in the mathematical description it works with experimental data of thermal explosion of EM [5,103,104) it considers the induction period of initiation of detonation. However, the induction period of the EM decomposition in front of the detonation wave makes the front kinetically unstable and pulsating [101 ]. 

One of the best-developed models of sensitivity (initiation) of EMs is Dlott s Model of the Multiphonon Up-Pumping, which starts from the above-... 

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