Theoretical research of time-dependent density functional on initiated photo-dissociation of some typical energetic materials at excited state

Abstract

Nitro explosive is a main type of energetic material which can release a large amount of energy when detonated under extreme conditions. Further study of the excited state dynamics of photo-induced nitro explosive can provide an effective method to understand the complex process of ultrafast detonation physics. In this paper, the initial step of photodissociation at the first excited electron state of some typical nitro explosives including nitromethane (NM), cyclotrimethylenetrinitramine (RDX) and triaminotrinitrobenzene (TATB) is studied using the time-dependent density functional theory and the molecular dynamic method. The transient structures of energetic molecules and time evolutions of excited energy levels are observed. It is found that the structural relaxation of energetic molecules occurs immediately after the electronic excitation, and the entire photoexcitation process comes into being within a range of 200 fs. At the same time, the positions of molecular energy levels change to various degrees with the oscillations of different frequencies, such as the overlap between HOMO and LUMO, which is related to the obvious change of molecular configuration, indicating that the energy of excited carriers transfers to atoms in the form of heat through electron-phonon coupling, and the energy is redistributed through vibration relaxation in the initial stage of photodissociation which causes the chemical bonds of C—H, N—N and N—N to rupture, and the hydrogen atoms dissociated from methyl, methylene or amino groups, and the nearest nitro group to form some new intermediate states. In this process, the energy levels near the excited electron and hole energy also change significantly with time, suggesting that the coupling between electron and electron also plays a role in the dissociation process. Comparing with NM and RDX, the evolution of the excited energy level of TATB has obvious lower-frequency (phonon frequency) oscillations, showing that the coupling between electronic state and phonon of TATB is weak and thus makes it more difficult to dissociate. Our study can deepen the understanding of the structural relaxation of excited states and the time evolution of excitation energy levels in energetic materials, and provide a new understanding of the photoinduced reaction and the initial steps of laser ignition in energetic materials.