Laser-induced Decomposition Mechanism Of Typical Energetic Compounds

The initiation of explosives by laser is a new initiation method. Compared with the traditional initiation methods, laser initiation has the characteristics of high reliability and high safety. It can be used as one of the alternative technologies for future initiation device. A microscopic understanding of the complex physical and chemical processes involved in the reaction process is essential for laser initiation. In order to give a better understanding of the mechanism of laser interaction with energetic materials, the systematic investigations for revealing dissociation processes have been carried on. Three categories of energetic compounds were selected, such as nitrate ester (PETN), nitramines (RDX and HMX) and aromatic nitro compounds (TNT and HNS). Preliminary study on the possible formation processes of sodium azide (NaN3) clusters under 532 nm laser ablation was carried out. The wavelengths of the laser were 532 nm and 1064 nm, respectively. A time of flight mass spectrometer (TOFMS) was used to detect the positive ions and the negative ions produced in the laser-induced dissociation processes. According to the possible components of the ions, possible dissociation paths of energetic compounds were proposed. The influences of the intensity and the delay time to the decomposition were also studied. Quantum chemistry method was used to optimize the geometric structure of intermediates and transition states, the vibration frequencies were also calculated. On the energy level of the molecule, the proposed dissociation processes were analyzed. The specific contents are as follows:(1) The reflectivity at 400 nm-1000 nm and 900 nm-1700 nm of five kinds of energetic compounds (PETN, RDX, HMX, TNT and HNS) mentioned above were tested. The experimental results showed that the reflectivity of these energetic compounds at 1064 nm is higher than those at 532 nm. The negative ions and positive ions of laser-induced dissociation reactions were detected by a time of flight mass spectrometer, the influence of delay time and laser energy to the intensity of the ions were also studied. The results showed that the attribution of the ions was similar for the same explosive at two different wavelengths, it indicated that the similar dissociation paths involved in the processes of explosives after irradiated by 532 nm and 1064 nm laser. For the same category of energetic compounds, the attribution of the ions was similar, that means the dissociation paths were similar. In the experimental laser energy range, the intensity of the ions increased gradually with the increasing of laser energy. Moreover, it was clear that fragmentation processes are highly dependent on the laser energy. Higher energy values resulted in an excess of internal energy and more efficient dissociation. In the experimental delay range, with the increasing of delay time, the intensity of main ions increased first and then decreased, indicating that different dissociation reactions occurred at different delay times.(2) Based on the molecular structure and the analysis of the attribution of the ions obtained by time of flight mass spectrometry, possible dissociation paths of different energetic compounds were proposed. By using density functional theory (DFT) of quantum chemical, the structure of the intermediates and the transition states were optimized and the vibration frequencies were calculated at the B3LYP/6-31+G (d, p) level. By comparing the energy barrier for dissociation path, heat release, and the stability of the product, the most favorable dissociation paths for different explosives were confirmed. For PETN, the path of successive elimination of four HONO to generate CO and CH4 was the dominant dissociation reaction. For RDX, after eliminating three HONO, the ring opened to generate three HCN, this dissociation path was most likely to occur. For HMX, after the elimination of four HONO, the ring opened to generate four HCN, it was the main dissociation path. For TNT, ortho nitro and para nitro eliminating three NO by isomerization reaction to form CO, O2 and C6H7 was an energy favor dissociation path. For HNS, the path that ortho nitro eliminate NO through isomerization reaction was prone to happen. We can conclude that if the structures of the energetic compounds were similar, the dissociation paths were similar.(3) Both positive and negative clusters produced by 532 nm laser ablation of NaN3 were detected, ion-molecule (atom) collision reaction can be used to explain the formation mechanism of clusters...