Infrared multiphoton dissociation of RDX in a molecular beam

Abstract

Infrared multiphoton dissociation (IRMPD) of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in a molecular beam has been performed in order to investigate the mechanism of RDX thermal decomposition. A beam of molecules was crossed by a pulsed TEA CO2 laser and velocity distributions of the various products were measured by the time-of-flight (TOF) technique as a function of the laboratory angle using a mass spectrometric detector. The dissociation channels, their branching ratios, and the translational energy distributions of the products were determined. In contrast to the conventional view of simple bond rupture through loss of NO2 as the dominant primary channel in RDX decomposition, it was found that the dominant primary channel is concerted symmetric triple fission to produce three CH2N2O2 fragments which subsequently undergo secondary concerted dissociation to produce HCN, H2CO, HONO (or HNO2), and N2O. A total of two primary and four secondary dissociation channels were observed. Concerted reactions predominate over simple bond rupture not only in the number of channels (four vs two) but also in the amount of products. A fair amount of translational energy release through concerted reaction channels was observed, which is significant for an explanation of the energies of RDX decomposition.