Vibrational Spectrum of HMX at CO2 Laser Wavelengths: A Combined DRIFT and LPAS Study

The vibrational spectrum of solid standard HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) was investigated. Two spectroscopic techniques were adopted for their different sensitivity and resolution. A preliminary survey of the absorption bands of the compound was performed in the 8000–400？cm？1 spectral range by employing the diffuse reflectance infrared Fourier transform (DRIFT) technique at room temperature. The high-resolution line spectrum of HMX was obtained in the 9.2–10.8？μm spectral range by laser photoacoustic spectroscopy (LPAS) method, using a line tuneable 10？W stabilised cw CO2 laser light source. By comparing the data collected with the two techniques in the common frequency range, a very good agreement was observed. 1. Introduction Nowadays the joined efforts at different levels from national and international authorities and governments to increase the homeland security are devoted to fight against the new threats arising from terror and organized crime. Here we take into account threats based on use of explosive materials where the detection capability is fundamental. The identification of hidden explosives is a very difficult task, mostly due to their very low vapour pressure at normal conditions, as well as due to the existence of a wide variety of energetic species with similar spectral features. Consequently, the identification and quantification of explosive traces require very sensitive instrumentation and the development of fast screening sensors which are not invasive and not destructive. Optical methods operating in the middle infrared (MIR) region have the potential capability to detect the fingerprint signatures of organic molecules. In principle, they can detect explosive species with high specificity. A high number of explosives contain –NO2 groups, having quite intense infrared active absorption bands whose vibration frequency strictly depends on the molecular structure and consequently constitutes a fingerprint [1]. This datum is particularly important when dealing with solid materials, since bandwidths are much more sharp than in vapour state. A further difficulty of the detection of gaseous molecules is due to the decomposition of the considered molecules at the temperature required to gain the sufficient vapour pressure for obtaining a readable spectrum, which would contain traces of the decomposition products. In the frame of a systematic study of solid energetic materials we combined two noninvasive infrared spectroscopic techniques not requiring manipulation of the sample: the diffuse reflectance infrared