Detecting Trace Levels of Explosives Using Vibrational Sum Frequency Spectroscopy

February 1, 2015

Office of Naval Research, Arlington, Virginia

The threat of improvised explosive devices (IEDs) to human life is grave, and countering this threat is a high priority for force protection during military operations. Remote, standoff detection of in-place IEDs would be a significant step forward in mitigating the threat posed by these weapons.

Because of the low vapor pressures of most high explosives (HEs), it is extremely difficult to detect their presence in the gas phase using methods that might be adapted as standoff systems. For example, although cavity ring-down laser spectroscopy (CRLS) has the sensitivity to detect explosives in the gas phase, it is not amenable to standoff application. However, explosives are known to adsorb on surfaces due to their high electronegativity and low vapor pressure, and methods relying on detecting their presence on surfaces show promise for remote-sensing applications.

Standoff detection of trace levels of explosives would be of great benefit in identifying the location of hidden explosive devices and locations where these munitions are assembled. Previous research investigated the use of the nonlinear optical technique vibrational sum frequency spectroscopy (VSFS) for standoff detection of trace levels of explosives on surfaces.

VSFS combines a visible laser beam and a tunable infrared laser beam at the interface with the energy range of the tunable IR laser overlapping with the energies of vibrational modes of molecules present at the interface. By scanning the energy of the IR laser and monitoring the generated sum frequency signal, one obtains a vibrational spectrum of the interfacial molecules.

VSFS can detect 2,4,6-trinitrophenol (picric acid), 2,4,6-trinitrotoluene (TNT) and 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) at surface concentrations as low as 300 ng cm^-2. Because these surface concentrations are typical of what might be found on surfaces containing adventitious contamination of explosives, these laboratory results indicate that VSFS could be used as a remote-sensing probe for detecting trace levels of explosives. However, in order for a method to be useful for operation detection of explosives, it must be demonstrated that the signal generated by explosives can be detected in the presence of environmental contamination on a variety of substrates.

The objectives of this work were to understand the nonlinear optical response of explosives on surfaces that are typically encountered in urban environments, determine if environmental contaminants produce signatures that would mimic those from explosives, and demonstrate that VSFS signals can be detected at standoff distances of up to five meters.

As a trace detection method, VSFS has the advantages of being non-contact and non-destructive with sub-second detection times. Therefore, a high explosives detection method for detecting IEDs or portal defense based on VSFS could provide standoff trace detecting for IEDs, and increase the throughput of package screening (in terms of objects scanned per minute) for portal defense. Furthermore, because VSFS does not degrade contaminants on surfaces, a positive detection result leaves any explosives detected in place for subsequent forensic analysis such as fingerprint identification. This research has shown that VSFS provides high chemical selectivity for nitro-containing HEs in the presence of environmental chemical contamination.

Experiments have demonstrated that the VSFS response of nitro-containing explosive crystals adsorbed on surfaces is largely independent of surface contamination or chemical complexity. Range-insensitive optical configurations for performing VSFS measurements are possible, and detection using the method at standoff distances of up to 2.2 meters has been demonstrated. Detection time using VSFS is rapid, and the method can detect trace levels of urea from suitcases moving on a baggage carousel (see figure). In the case of RDX, preparation of samples via recrystallization from solvents produces different crystal structures than found in operational explosive samples, and this affects VSFS signal response.

This work was done by William Asher of the University of Washington Applied Physics Laboratory for the Office of Naval Research. ONR-0033

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