X-ray photoelectron spectroscopy (XPS) study of Hexahydro-1,3,5-Trinitro-S-Triazine (RDX)

Abstract

Energetic materials are used as explosives component of landmines, plastic explosives (C-4), and home made bombs, among others. Hexahydro-1,3,5-trinitro-s-triazine (RDX) is a secondary explosive, which is the main component of C-4. Structural studies of solid RDX have established the existence of two forms known as α−RDX and a less stable and less understood β−RDX. The structural conformation of α-RDX has a Cs symmetry in which two of its nitro groups are in an axial orientation while the third one is oriented equatorially. The β−RDX solid form has a C3v symmetry, which results from an all-axial or all-equatorial orientation of its three nitro groups. The ring breathing Raman frequency is centered at 882 and 878 cm-1 in α− and β−RDX, respectively. This work reports X-ray photoelectron spectroscopy (XPS) measurements on RDX deposits on a silicon surface as a substrate. Deposits from RDX/acetone solution on a silicon substrate were prepared and allowed to evaporate to open air at room temperature. XPS measurements performed on these deposits revealed a broad N 1s band between 403.4 and 405.8 eV for deposits that consist of small amounts of RDX while a second band at 410 eV was observed in deposits that had large amounts of RDX. The 403 eV and 410 eV N 1s peaks were associated with aliphatic or ring nitrogen atoms and the nitrogen atom in the nitro group of RDX, respectively. In an attempt to assign these bands to either β− or α−RDX, vibrational spectroscopy measurements were performed on the deposits. The ring breathing mode frequency on the deposits was determined to be between 883 and 887 cm-1, a value that allowed ruling out the presence of β−RDX in the deposits. A third type of RDX is proposed to exist to account for the difference in the coverage dependent of the N 1s XP data.