Scott W. Reeve

Standoff Methods for the Detection of Threat Agents: A Review of Several Promising Laser-Based Techniques

Detection of explosives, explosive precursors, or other threat agents presents a number of technological challenges for optical sensing methods. Certainly detecting trace levels of threat agents against a complex background is chief among these challenges; however, the related issues of multiple target distances (from standoff to proximity) and sampling time scales (from passive mines to rapid rate of march convoy protection) for different applications make it unlikely that a single technique will be ideal for all sensing situations. A number of methods for spanning the range of optical sensor technologies exist which, when integrated, could produce a fused sensor system possessing a high level of sensitivity to threat agents and a moderate standoff real-time capability appropriate for portal screening of personnel or vehicles. In this work, we focus on several promising, and potentially synergistic, laser-based methods for sensing threat agents. For each method, we have briefly outlined the technique and report on the current level of capability.

Sensing and characterization of explosive vapors near 700 cm-1

One of the technological challenges associated with trace vapor detection of explosive materials are the relatively low vapor pressures exhibited by most energetic materials under ambient conditions. For example, the vapor pressure for TNT is ~10 ppbv at room temperature, a concentration near the Limit of Detection for many of the technologies currently being deployed. In the case of improvised explosive devices, the clandestine nature of the device further serves to exacerbate the vapor pressure issue. Interestingly, the gold standard in explosives detection remains the trained canine nose. While there is still some debate as to what the dog actually smells, recent studies have indicated the alert response is triggered, not by the vapor presence of a specific explosive compound but, by a characteristic bouquet of odors from chemical impurities used to manufacture and process the explosives. Here we present high resolution infrared data for several of these volatile organic compounds in the 700 cm-1 region required for real time optical sensing of energetic materials.

Infrared laser spectroscopy of jet cooled cobalt tricarbonyl nitrosyl.

Rotationally resolved infrared absorption spectra for the 1(0)(1) band of jet cooled cobalt tricarbonyl nitrosyl have been observed and analyzed. Several longitudinal modes of a Pb-salt diode laser were utilized to measure 105 rovibrational transitions for this particular vibrational band centered near 2112 cm(-1). Spectra were optimized using both argon and helium carrier gases and these experiments eventually led to rovibrational transitions being assigned to four different K subbands, specifically the K = 0, 3, 6, and 9 subbands. An iterative least-squares analysis of the spectroscopic data yielded the following molecular parameters nu0 = 2111.7457(9) cm(-1), B0 = 0.034747(12) cm(-1), B1 = 0.034695(15) cm(-1), C1 = 0.03380(9) cm(-1), and D1K = 6.3(9) x 10(-6) cm(-1) (where 3sigma uncertainties are listed in parenthesis).

Spectral signatures for volatile impurities in TNT and RDX based explosives

Vapor phase sensing and detection of TNT-based explosives is extremely challenging due in part to the low vapor pressure of TNT. We believe one effective strategy for optically based sensing of TNT-based explosives involves focusing not on the spectral signature for pure TNT, but rather on a more volatile series of compounds that are present in TNT as impurities. To date we have catalogued and reported a number of rotationally resolved infrared transition frequencies for nitrobenzene, toluene, o-nitrotoluene, and m-nitrotoluene in the 14 micron region. Here we describe the use of an in-house spectral calibration program that while designed for calibration of Pb-salt diode laser spectra, is quite general and could be utilized for many spectroscopic detection and/or analysis applications. Finally, a sensing measurement for a volatile organic impurity related to RDX-based explosives such as C4 is presented and discussed.

Quantum cascade laser FM spectroscopy of explosives

Polyisobutylene is an industrial polymer that is widely used in a number of applications including the manufacture of military grade explosives. We have examined the vapor emanating from a series of different molecular weight samples of polyisobutylene using high resolution Quantum Cascade Laser FM spectroscopy. The vapor phase spectra all exhibit a rovibrational structure similar to that for the gas phase isobutylene molecule. We have assigned the structure in the 890 cm-1 and 1380 cm-1 regions to the isobutylene ν28 and ν7 fundamental bands respectively. These spectroscopic signatures may prove useful for infrared sensing applications. Here we will present the infrared signatures along with recent GCMS data from a sample of C4, utilizing solid-phase microextraction vapor collection fibers, which confirm the presence of isobutylene as one of the volatile bouquet species in RDX-based explosives.

Optical detection of explosives: spectral signatures for the explosive bouquet

Research with canines suggests that sniffer dogs alert not on the odor from a pure explosive, but rather on a set of far more volatile species present in an explosive as impurities. Following the explosive trained canine example, we have begun examining the vapor signatures for many of these volatile impurities utilizing high resolution spectroscopic techniques in several molecular fingerprint regions. Here we will describe some of these high resolution measurements and discuss strategies for selecting useful spectral signature regions for individual molecular markers of interest.

Measurement of ammonia skin gas using a mid-infrared Pb-salt tunable diode laser

The concentration of Ammonia excreted through human skin has recently been measured using a gas chromatograph equipped with a flame ionization detector (GC-FID) by a group from Nagoya, Japan. These emissions, referred to as ammonia skin gas, were determined to be 1.7±.4 ng/cm3 for healthy subjects in the study. To achieve greater molecule specificity, sensitivity, as well as add a real time capability, we are investigating the potential of a mid IR laser spectrometer, consisting of a Pb-salt diode laser coupled with a low volume 75 meter Herriott gas sample cell, to perform real time ammonia diagnostic measurements. Here we will present a series of preliminary ammonia skin gas measurements obtained with this mid IR laser system.

Vibrational band assignment of 2-ethyl-1-hexanol

The vibrational structure of 2-ethyl-1-hexanol is of great interest because of its industrial and military applications. However, detailed spectral analysis is challenging due to its flexibility. This paper reports a detailed analysis of the gas and liquid phase vibrational spectra of 2-ethyl-1-hexanol using the Fourier transform infrared spectroscopy and Raman experimental data. By performing a detailed exploration of the conformational space in this work, the theoretical spectra reproduced almost all experimental details observed, and assigned internal valence coordinates to all of the experimentally observed bands in the floppy 2-ethyl-1-hexanol molecule. Relative contributions from the various internal valence coordinates to the experimental vibrational bands are directly compared between the liquid phase Raman band and the gas and liquid phase infrared band.

FTIR monitoring of methane from a local landfill

From anthropogenic sources to natural oceanic emissions, the concentration of methane in the atmosphere has more than doubled in the last 200 years. Since methane represents a global warming potential 23 times an equivalent mass of carbon dioxide, monitoring this species is of great interest. In terms of anthropogenic emissions, landfills represent a significant source of atmospheric methane. We have developed an in-house algorithm for extracting methane concentrations directly from FTIR spectra for gas samples from a local landfill. In this work, we will describe the method and present some preliminary measurements.

Tunable picosecond spectroscopy for detection of nitric oxide

Nitric oxide (NO) is a major chemical byproduct of many photochemically active nitrogen-containing compounds. As a prototypical free radical with a very well characterized high-resolution spectrum, NO provides a standard spectroscopic fingerprint for indirect quantitative analysis and detection of a number of low vapor pressure nitroaromatic compounds in air through either direct photochemical decomposition of a parent molecule or from its relatively high vapor pressure chemical constituents. In this paper, we will discuss applications of picosecond laser spectroscopy for measurements and detection of NO and the nascent NO generated from photolysis of nitrobenzene. We will give a general overview of our tunable picosecond laser and detection system that we routinely use for probing and exciting the NO gamma band. This broad wavelength tuning capability of our laser allows us to set up pump-probe type experiments for detecting blue shifted rovibronic bands and probing the relative population distribution for NO. In all cases, experiments were performed using UV laser pulses of duration less than 20 ps. Also, we studied the effect of N2 collisions on the photoframentation spectrum of nitrobenzene in 1000 mbar of N2 buffer gas.