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

Thin films of pentaerythritol tetranitrate (PETN) were shock compressed from 3-55 GPa using the laser driven shock apparatus at Los Alamos National Laboratory. Spectra were measured during the first 250 ps using visible transient absorption (VIS) from 400 to 700 nm and mid-infrared transient absorption (MIR) from 1150 to 3800 cm^–1. VIS and MIR results suggest irreversible chemistry was induced in PETN at pressures above 30 GPa. Under reactive conditions, the antisymmetric NO₂ stretch mode had a significantly increased absorption while the symmetric NO₂ stretch did not. No increased absorption occurred in the frequency regions where new CH-, NH-, and OH- bond absorptions would be expected. A new absorption appeared 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 NO₂ related features provided the strongest contributions to the shocked infrared changes. Our experiments suggest C≡O or N₂O bond formation, nitrite formation, and absence of significant hydroxyl or amine concentrations in the initial chemistry steps in shocked PETN.