几种典型含能材料光激发解离的含时密度泛函理论研究

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

硝基类炸药作为主要的含能材料在极端条件下引爆可释放巨大能量,深入研究硝基类炸药光激发后的载流子动力学过程,将有助于揭示含能材料复杂的超快爆轰物理机制.本文利用含时密度泛函理论和分子动力学计算方法,研究了典型的硝基类炸药,包括硝基甲烷(NM)、环三亚甲基三硝胺(RDX)和三氨基三硝基苯(TATB)分子的光激发解离过程,观察了含能分子瞬时的结构变化和分子能级随时间的演化过程.结果显示,含能分子在200 fs范围内发生解离,激发载流子的能量通过电声耦合以热能的形式传输给原子,从而导致C-H,N-H或N-N化学键的断裂,从甲基、亚甲基或氨基中解离出的氢原子会与近邻的硝基形成新的基团.在这一过程中,激发电子和空穴附近的能级也随着时间演化发生明显变化,表明电子耦合也在解离过程中发挥了作用.我们的研究加深了含能材料激发态结构弛豫和激发能级演化的认识,并对光诱导反应及含能材料激光点火初始步骤提供了新的理解.

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.