Long-Wave, Infrared Laser-Induced Breakdown (LIBS) Spectroscopy Emissions from Energetic Materials Clayton S.-C. Yang, a Ei E. Brown, b Uwe Hommerich, b Feng Jin, c Sudhir B. Trivedi, c Alan C. Samuels, d A. Peter Snyder d, * a Battelle Eastern Science and Technology Center, Aberdeen, MD 21001, USA b Department of Physics, Hampton University, Hampton, VA 23668, USA c Brimrose Corporation of America, Baltimore, MD 21152, USA d Edgewood Chemical Biological Center, Aberdeen Proving Ground, MD 21010, USA Laser-induced breakdown spectroscopy (LIBS) has shown great promise for applications in chemical, biological, and explosives sensing and has significant potential for real-time standoff detection and analysis. In this study, LIBS emissions were obtained in the mid-infrared (MIR) and long- wave infrared (LWIR) spectral regions for potential applications in explosive material sensing. The IR spectroscopy region revealed vibrational and rotational signatures of functional groups in molecules and fragments thereof. The silicon-based detector for conventional ultraviolet–visible LIBS operations was replaced with a mercury– cadmium–telluride detector for MIR–LWIR spectral detection. The IR spectral signature region between 4 and 12 lm was mined for the appearance of MIR and LWIR–LIBS emissions directly indicative of oxygenated breakdown products as well as dissociated, and/or recombined sample molecular fragments. Distinct LWIR–LIBS emission signatures from dissociated–recombination sample molecular fragments between 4 and 12 lm are observed for the first time. Index Headings: Laser-induced breakdown spectroscopy; LIBS; Long- wave infrared spectra; LWIR; Energetic substances. INTRODUCTION There is interest in an active standoff system that can detect and characterize surfaces that have been contaminated with chemical, biological, and explosive materials. Laser-induced breakdown spectroscopy (LIBS) has shown great promise for applications in chemical and biological sensing 1–6 and has significant potential for real-time standoff detection and analysis. LIBS relies on short-lived (nanoseconds) micro- plasma, generated by an intense laser pulse to dissociate, atomize, and ionize target molecules. The emission is characteristic for excited-state relaxation transitions from atoms, ions, and molecular fragments 7 generated by the LIBS microplasma. Important information concerning the identifica- tion, composition, and concentrations of trace elements can be derived from the analysis of a conventional ultraviolet–visible (UV–Vis) LIBS emission spectra. The atomic, ionic, and molecular emissions from LIBS experiments have been employed to analyze a wide range of materials including gasesgasses, metals, soils, aerosols, energetic materials, and biological substances. 1–7 However, nearly all previous LIBS experiments were limited to spectral measurements in the UV–Vis and near-infrared (NIR) regions (;200–980 nm) by using silicon-based detectors. It is well known, however, that molecules exhibit spectroscopic signatures or ‘‘fingerprints’’ in the mid-IR (MIR) and long-wave IR (LWIR) regions because of vibrational and rotational transitions. Therefore, an extension of LIBS to the IR region augurs the potential to provide additional information concerning the identification and classification of substances that might complement results obtained from conventional UV–Vis–LIBS measurements. This concept necessitates the replacement of the silicon-based detecting system with appropriate IR optics and detectors, such as indium antimony (InSb) for MIR and mercury–cadmium–telluride (MCT) detector for MIR–LWIR detection, to adequately capture the IR spectra. Flame emission studies produced IR emission features from various types of molecular breakdown products including hydrogenated fragments, oxygenated fragments, and dissociat- ed and/or recombination molecular fragments of the target compound. 8 Several studies have also suggested the existence of more complicated molecular breakdown fragments generat- ed from laser-induced plasma. In the LIBS studies of various organic compounds, UV–Vis emission features from C 2 are found to be strongly correlated to the presence of aromatic rings 9 and aromatic ring arrangements 10 in the analyte material. Studies using laser desorption mass spectroscopy have also shown the laser-induced vaporization (ablation) process to provide adequate molecular fragments for identification of a variety of chemical and biological species. 11 Recently, we demonstrated the first MIR–LIBS emission signatures in the 2 to 5 lm spectral region. 12–16 Spectral and time-resolved MIR–LIBS studies were performed on several solid substances and revealed emission signatures from atomic and molecular relaxation processes. For example, MIR–LIBS measurements on carbon-containing substances revealed vi- brational emission features centered at ;4.4 lm, resulting from the CO 2 - and CO-oxygenated products and features around 3 lm from the H 2 O-oxygenated product. The CO 2 , CO, and H 2 O molecules were derived from atmospheric oxygen interaction with the sample carbon and hydrogen atoms. 12 Also reported was the observation of MIR–LIBS atomic emissions from alkali metal halide tablets resulting from the higher-lying Rydberg transitions of the alkali metal atoms from the sample. 14 Recent concentration-dependent LIBS studies were per- formed on the 2.7 lm IR emission from potassium chloride, and a limit of detection of 0.19% (wt/wt) was obtained. 16 Although this result is not as sensitive as the limit of detection values reported for near-IR (NIR)–LIBS measurements, 17 it is Received 24 April 2012; accepted 20 August 2012. * Author to whom correspondence should be sent. E-mail: arnold.p. snyder.civ@mail.mil. DOI: 10.1366/12-06700 Volume 66, Number 12, 2012 APPLIED SPECTROSCOPY 1397 0003-7028/12/6612-1397/0 Ó 2012 Society for Applied Spectroscopy