114
4 Vibrational Up-Pumping in Some Molecular Energetic Materials
include a physical underlying mechanism to link a mechanical impact and the subsequent conversion of energy into a chemical reaction. Thus, the reasons for a material
displaying a particular impact sensitivity remain elusive.
The concept of vibrational up-pumping has been demonstrated experimentally and
theoretically for a range of molecular compounds [15–17]. This has led to scattered
analyses of energy transfer rates in molecular EMs based on inelastic neutron scattering spectra [18, 19], or bench-top Raman spectra [20, 21]. While these early works
did suggest strong correlation between sensitivity and energy transfer rates, the data
on which they were based was limited, and the models inconsistent. Most notably,
the mechanism of up-pumping (overtone [20] or combination [18, 19] pathways)
and the target region into which up-pumped energy is considered, differ considerably between these models. Up-pumping phenomena have demonstrated capable of
localising energy [17, 22], i.e. producing ‘hot-spots’, and therefore offer a fundamental approach to understanding the impact and shock-induced chemistry of EMs
[17].
The theoretical and experimental techniques required to enhance the fundamental
understanding of up-pumping models in solids have only recently become available.
A very recent study successfully attempted to correlate zone-centre overtone structure
with the impact sensitivity of a selection of organic EMs [23]. Having constructed
a more complete picture of these processes from first principles in Chap. 3, it is
therefore of interest to further develop and apply the model to a more challenging
series of organic systems.
4.2 Aims
Following on from the success of the up-pumping model to predict the relative
ordering of impact sensitivity for the azide materials, this chapter aims to extend the
model to a series of molecular EMs. In doing so, the work in this chapter seeks to:
• Consider the ‘band gap’ criterion for a series of organic EMs
• Obtain the full vibrational spectrum for a range of organic EMs
• Further develop the vibrational up-pumping mode of Chap. 3 to more complex,
large molecule materials.
• Compare and unify overtone- and combination-based up-pumping approaches.
4.3 Model Systems
Typical organic EMs contain a large number of atoms and, common to molecular
materials, crystallise in large, low symmetry unit cells. Molecular materials were
selected to ensure calculations were computationally tractable, and to ensure that
they spanned a broad range of impact sensitivities and structure types. Many of these
materials have been studied extensively, although a breadth of impact sensitivities
4 Vibrational Up-Pumping in Some Molecular Energetic Materials
include a physical underlying mechanism to link a mechanical impact and the subsequent conversion of energy into a chemical reaction. Thus, the reasons for a material
displaying a particular impact sensitivity remain elusive.
The concept of vibrational up-pumping has been demonstrated experimentally and
theoretically for a range of molecular compounds [15–17]. This has led to scattered
analyses of energy transfer rates in molecular EMs based on inelastic neutron scattering spectra [18, 19], or bench-top Raman spectra [20, 21]. While these early works
did suggest strong correlation between sensitivity and energy transfer rates, the data
on which they were based was limited, and the models inconsistent. Most notably,
the mechanism of up-pumping (overtone [20] or combination [18, 19] pathways)
and the target region into which up-pumped energy is considered, differ considerably between these models. Up-pumping phenomena have demonstrated capable of
localising energy [17, 22], i.e. producing ‘hot-spots’, and therefore offer a fundamental approach to understanding the impact and shock-induced chemistry of EMs
[17].
The theoretical and experimental techniques required to enhance the fundamental
understanding of up-pumping models in solids have only recently become available.
A very recent study successfully attempted to correlate zone-centre overtone structure
with the impact sensitivity of a selection of organic EMs [23]. Having constructed
a more complete picture of these processes from first principles in Chap. 3, it is
therefore of interest to further develop and apply the model to a more challenging
series of organic systems.
4.2 Aims
Following on from the success of the up-pumping model to predict the relative
ordering of impact sensitivity for the azide materials, this chapter aims to extend the
model to a series of molecular EMs. In doing so, the work in this chapter seeks to:
• Consider the ‘band gap’ criterion for a series of organic EMs
• Obtain the full vibrational spectrum for a range of organic EMs
• Further develop the vibrational up-pumping mode of Chap. 3 to more complex,
large molecule materials.
• Compare and unify overtone- and combination-based up-pumping approaches.
4.3 Model Systems
Typical organic EMs contain a large number of atoms and, common to molecular
materials, crystallise in large, low symmetry unit cells. Molecular materials were
selected to ensure calculations were computationally tractable, and to ensure that
they spanned a broad range of impact sensitivities and structure types. Many of these
materials have been studied extensively, although a breadth of impact sensitivities