Coloumb Explosion Dynamics of Multiply Charged para-Nitrotoluene Cations*

Understanding the initial dissociation pathways of energetic materials can provide insight into the mechanisms of shock initiation and help design safer photoactive explosives. Strong field ionization (SFI) with intense femtosecond laser pulses coupled to mass spectrometry has been used to investigate molecular dissociation of nitro-organic molecules. More recently, this technique was able to identify coulomb explosion (CE) of multiply charged cations (MCCs) such as NO₂⁺ and NO⁺ in nitro-organic molecules. While the kinetic energy released by CE in these fragments has been measured, the CE reaction pathways and their dynamical timescales have yet to be elucidated. This work uses SFI coupled with mass spectrometry to explore CE dissociation pathways in MCCs of para-nitrotoluene (PNT), a model compound for nitroaromatic energetic molecules. Experiments indicate that metastable cations PNT²⁺ and PNT³⁺ both undergo CE to produce NO₂ ⁺ and NO⁺ and the kinetic energy (KE) release upon formation of the fragments was calculated. These results agree quantitatively with the KE release predicted by computations of the reaction pathways in PNT²⁺ and PNT³⁺ using density functional theory (DFT). Additional products from CE of highly charged PNT^q+ cations with q > 3 were identified with both DFT computations and mass spectrometry. The dynamical timescales required for direct CE of PNT²⁺ and PNT³⁺ to produce NO₂⁺ were estimated to be 200 and 90 fs, respectively, using ultrafast disruptive probing measurements.