Direct Measurement of Ultrafast Multiphonon Up-Pumping in High Explosives

Chen, Sheah; Tolbert, William A.; Dlott, Dana D.

doi:10.1021/j100083a004

Cited by 54 publications

(53 citation statements)

References 17 publications

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“…To fix the parameter ␣, we use the experimental time constant for multiphonon upconversion taken at ambient conditions. For nitromethane, we can use the direct uppumping study of Chen and co-workers, 14 which reveals a time constant of approximately 40 ps for the 657 cm −1 vibration. The other material parameters used for nitromethane are given in Table I. For the larger molecular systems considered here, no such direct information exists and thus we estimate in the following way.…”

Section: Hugoniot Dependencementioning

confidence: 99%

“…Dlott and Fayer 4 and Tokmakoff et al 7 later proposed a model for three-phonon processes in molecular solids based on their extensive experimental work. [13][14][15] They proposed that energy transfer was rate limited by scattering into a small number of low-lying internal modes, and that final thermal equilibrium in a system such as napthalene would be complete in approximately 100 ps independent of shock pressure.…”

Section: Introductionmentioning

confidence: 99%

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Vibrational energy transfer in shocked molecular crystals

Hooper

2010

The Journal of Chemical Physics

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We consider the process of establishing thermal equilibrium behind an ideal shock front in molecular crystals and its possible role in initiating chemical reaction at high shock pressures. A new theory of equilibration via multiphonon energy transfer is developed to treat the scattering of shock-induced phonons into internal molecular vibrations. Simple analytic forms are derived for the change in this energy transfer at different Hugoniot end states following shock compression. The total time required for thermal equilibration is found to be an order of magnitude or faster than proposed in previous work; in materials representative of explosive molecular crystals, equilibration is predicted to occur within a few picoseconds following the passage of an ideal shock wave. Recent molecular dynamics calculations are consistent with these time scales. The possibility of defect-induced temperature localization due purely to nonequilibrium phonon processes is studied by means of a simple model of the strain field around an inhomogeneity. The specific case of immobile straight dislocations is studied, and a region of enhanced energy transfer on the order of 5 nm is found. Due to the rapid establishment of thermal equilibrium, these regions are unrelated to the shock sensitivity of a material but may allow temperature localization at high shock pressures. Results also suggest that if any decomposition due to molecular collisions is occurring within the shock front itself, these collisions are not enhanced by any nonequilibrium thermal state.

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Section: Hugoniot Dependencementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Vibrational energy transfer in shocked molecular crystals

Hooper

2010

The Journal of Chemical Physics

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“…(). The procedure of normalizing to a reference spectrum at known temperature has also been employed for picosecond spontaneous Raman temperature measurements …”

Section: Resultsmentioning

confidence: 99%

Temperature measurements in condensed phases using non‐resonant femtosecond stimulated Raman scattering

Dang

Bolme

Moore

et al. 2012

J Raman Spectroscopy

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We have previously demonstrated the capability of femtosecond stimulated Raman scattering (FSRS) data to measure the temperature (T) of condensed matter at the molecular vibrational level. [Phys. Rev. Lett. 2011, 107, 43001] In this paper, we expand the theory for the FSRS temperature dependence by considering the effects of an isolated change of T as well as a coupled change of T and chemical concentration. We point out that the origin of the temperature sensitivity of the Stokes to anti‐Stokes ratio of FSRS lies in the exponential nonlinearity of the gain and loss. We establish that FSRS of two Raman modes can be used to simultaneously determine the vibrational temperature and the change in concentration of the species contributing to those two modes. Single‐shot experimental results using FSRS are presented to demonstrate over four orders of magnitude higher efficiency than spontaneous Raman in small volume samples with picosecond resolution. Copyright © 2012 John Wiley & Sons, Ltd.

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“…This transformation, termed multiphonon up-pumping [2], is a dissipative process, which does not exist in atomic solids, that can broaden out the shock front [2,5,6]. If the shockfront rise time t r in the presence of up-pumping is faster than the characteristic time scale for thermal equilibration among internal molecular vibrations, typically a few tens of ps [2,3,7,8], the internal vibrations of the molecules can be pumped into highly nonequilibrium states. Several authors have discussed how such nonequilibrium populations might affect chemical reactivity and possibly affect the sensitivity of energetic materials [2][3][4], but until the present work there existed no direct evidence that shockfront rise times in molecular materials were fast enough to produce nonequilibrium vibrational excitations.…”

Section: Ultrafast Raman Spectroscopy Of Shock Fronts In Molecular Somentioning

confidence: 99%

Ultrafast Raman Spectroscopy of Shock Fronts in Molecular Solids

et al. 1997

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The passage of a 4 GPa shock front through an embedded optical nanogauge, a thin ͑ϳ700 nm͒ layer of polycrystalline molecular material (anthracene), is monitored in real time by picosecond coherent Raman scattering. Analysis of high resolution Raman spectra shows the shock rise time is less than 25 ps, and the front is less than 100 molecules wide. The rise time is faster than relaxation of nonequilibrium populations of molecular vibrations, which shows a shock front in a molecular material can leave highly nonequilibrium vibrational states in its wake. The implications for shock initiation of energetic materials, typically polycrystalline molecular solids, are briefly discussed.[S0031-9007(97)03191-8] PACS numbers: 62.50. + p, 31.70.Ks, 42.65.Dr, 78.47. + p We present a new spectroscopic method used to measure ultrafast shock-front rise times in molecular materials. The rise time of a shock front (4.2 GPa) in a polycrystalline layer of anthracene ͑C 14 H 10 ͒ is shown to be #25 ps. Our motivation is the development of a molecular level picture of energetic material initiation [1] by relatively low pressure (say 1-5 GPa) shocks. Most high performance energetic materials are formulations of polyatomic molecular solids. In contrast to simpler atomic solids such as metals, discussed briefly below, the basic unit of these energetic materials is a large molecule with a complicated vibrational structure. Interactions between a shock front and the internal molecular vibrations can transform some of the directed energy of the shock into internal energy in the form of molecular vibrational excitation [2][3][4]. This transformation, termed multiphonon up-pumping [2], is a dissipative process, which does not exist in atomic solids, that can broaden out the shock front [2,5,6]. If the shockfront rise time t r in the presence of up-pumping is faster than the characteristic time scale for thermal equilibration among internal molecular vibrations, typically a few tens of ps [2,3,7,8], the internal vibrations of the molecules can be pumped into highly nonequilibrium states. Several authors have discussed how such nonequilibrium populations might affect chemical reactivity and possibly affect the sensitivity of energetic materials [2-4], but until the present work there existed no direct evidence that shockfront rise times in molecular materials were fast enough to produce nonequilibrium vibrational excitations.For shocks in the 1-5 GPa range, conventional impact measurements ordinarily see steady-state shock fronts with rise times in the 10 28 10 26 s range [5]. These rise times were attributed to viscosity and other dissipative processes, and rise time decreases as shock pressure increases [5]. Recently, ultrashort laser shock generation and probing techniques have been used to investigate shocks which have propagated a very short distance and are not yet in steady state, in opaque atomic solids such as aluminum [9], gold [10], or silicon [11]. Several groups have observed fast rise times (a few tens of ps) for these extreme...

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Direct Measurement of Ultrafast Multiphonon Up-Pumping in High Explosives

Cited by 54 publications

References 17 publications

Vibrational energy transfer in shocked molecular crystals

Vibrational energy transfer in shocked molecular crystals

Temperature measurements in condensed phases using non‐resonant femtosecond stimulated Raman scattering

Ultrafast Raman Spectroscopy of Shock Fronts in Molecular Solids

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