Insight into the Chemistry of PETN Under Shock Compression Through Ultrafast Broadband Mid-Infrared Absorption Spectroscopy

Abstract

Thin films of pentaerythritol tetranitrate (PETN) were shock compressed using the laser driven shock apparatus at Los Alamos National Laboratory (LANL). Two spectroscopic probes were available to this apparatus: visible white light transient absorption spectroscopy (VIS) from 400 to 700 nm and midinfrared transient absorption spectroscopy (MIR) from 1150 to 3800 cm −1 . Important PETN vibrational modes are the symmetric and antisymmetric NO 2 stretches at 1280 and 1650 cm −1 , respectively, as well as CH stretches at ∼2900 cm −1 . Shock strength was varied from approximately 3 to 55 GPa to span from the chemically unreactive regime to the regime in which fast chemical reaction took place on the 250 ps time scale of the measurements. VIS and MIR results suggest irreversible chemistry was induced in PETN at pressures above 30 GPa. At lower shock pressures, the spectroscopy showed minimal changes attributable to pressure induced effects. Under the higher-pressure reactive conditions, the frequency region at the antisymmetric NO 2 stretch mode had a significantly increased absorption while the region around the symmetric NO 2 stretch did not. No observable increased absorption occurred in the higher frequency regions where CH-, NH-, and OH-bond absorptions would be observed. A broad absorption appeared on the shoulder at the red-edge of the CO 2 vibrational band around 2200 cm −1 . In addition to the experiments, reactive molecular dynamics were carried out under equivalent shock conditions to correlate the evolution of the infrared spectrum to molecular processes. The simulations show results consistent to experiments up to 30 GPa but suggest that NO and NO 2 related features provided the strongest contributions to the shocked infrared changes. Proposed mechanisms for shocked PETN chemistry are analyzed as consistent or inconsistent with the data presented here. Our experimental data suggests C≡O or N 2 O bond formation, nitrite formation, and absence of significant hydroxyl or amine concentrations in the initial chemistry steps in PETN shocked above 30 GPa.

Molecular Inclusion of Small Aging Products into the Hexanitrohexaazaisowurtzitane (CL‐20) Lattice: Part I, Infrared Spectra

“…A peak at 2243 cm −1 was attributed to N 2 O [23] as well as CO [12]. Either N 2 O or CO were also contemplated as the source of a peak at similar frequency in the IR spectrum of a nitrate ester (PETN) [52]. The peak at 2343 cm −1 in the CL‐20 ATR‐IR spectrum has been assigned to CO 2 [12, 22–23].…”

Predicted Reaction Mechanisms, Product Speciation, Kinetics, and Detonation Properties of the Insensitive Explosive 2,6-Diamino-3,5-dinitropyrazine-1-oxide (LLM-105)

“…The combination of the increased accuracy of interatomic potentials and the increased computational capabilities, as well as modern experimental capabilities, has recently enabled direct comparisons of chemistry at extreme conditions. Recent spectroscopic work on nitromethane, polyvinyl nitrate (PVN), and pentaerythritol tetranitrate (PETN) have been reproduced utilizing reactive MD simulations. − Bassett and Dlott performed optical pyrometry experiments to measure hotspot temperatures with a high temporal resolution for a variety of HE materials. − These measurements compare well to previously predicted temperatures from both atomistic and continuum studies. − Coupling ultrafast dynamic compression experiments and machine-learned reactive force fields also holds promise to elucidate the initial steps of soot formation during detonation …”

“…DFTB also scales as O­(N 3 ), but with a smaller prefactor accompanied with lower accuracy. Given its short-range nature, ReaxFF scales as O­(N) and much larger system sizes and longer timescales are achievable, with a tradeoff in the form of accuracy and/or transferability. ,, …”

“…Given its short-range nature, ReaxFF scales as O(N) and much larger system sizes and longer timescales are achievable, with a tradeoff in the form of accuracy and/or transferability. 30,31,34 DFT-MD simulations were conducted using the Perdew− Burke−Ernzerhof (PBE) 62 generalized gradient approximation functional with the Grimme D2 empirical correction (PBE +D2) 63 with PAW pseudopotentials, 64,65 implemented in VASP. 66 Classical trajectories over the Born−Oppenheimer surface were solved numerically with a 0.5 fs time step.…”

Mode-Selective Vibrational Energy Transfer Dynamics in 1,3,5-Trinitroperhydro-1,3,5-triazine (RDX) Thin Films

“…While there is rich literature investigating shockwave-induced processes in reactive and inert solids (see refs − for recent examples), little experimental work exists − on fundamental vibrational dynamics in solid, neat EMs that shed light on VET through strongly coupled, delocalized vibrational modes. Much of the existing VET research is performed in EM mimics and model systems (e.g., liquid nitromethane , ) or solvated energetics (see refs − for examples), where the importance of phonon-mediated long-range interactions that form the basis for a complete picture of VET in EMs is not captured, leading to observed dynamics that are not representative of the dynamics in solid EMs. , Furthermore, the molecular conformation of solvated energetics can be different from that in extended EM solids, resulting in differences in the nature of intermolecular interactions and vibrational coupling.…”