Typical spectral signals obtained from NaNO3 in ULIBS using 10mJ, 100 fsec laser pulses at different laser diameters on target: A) 0.608 mm, B) 0.628 mm, C) 0.740 mm, and D) 0.896 mm. The prominent broadband feature between 400 nm to 420 nm for both NaNO3 and KNO3 for various laser energies, pulse lengths, and spot diameters.
Typical spectral signals obtained from NaNO3 in ULIBS using 10mJ, 100 fsec laser pulses at different laser diameters on target: A) 0.608 mm, B) 0.628 mm, C) 0.740 mm, and D) 0.896 mm. The prominent broadband feature between 400 nm to 420 nm for both NaNO3 and KNO3 for various laser energies, pulse lengths, and spot diameters.

Ultrashort Laser Induced Breakdown Spectroscopy (ULIBS) was used to detect the emission radiation from the breakdown of surface contaminants by an ultrashort laser pulse. This study focused on the detection of radiation signatures from molecular fragments of the Nitro (NO) group present in the breakdown plasma, where target chemicals of Potassium Nitrate (KNO3) and Sodium Nitrate (NaNO3) were used. The results of this study could lead to the early detection of residues of nitro-group explosive materials at standoff distances. Spectral signatures at a wavelength region around 410 nm were observed for both KNO3 and NaNO3, and were identified as the fluorescence transitions of the NO-molecular structures. The signatures obtained were systematically analyzed and studied as functions of laser parameters. It is shown that for laser parameters used in this study, laser pulse durations ≥1 psec were not as effective as shorter pulses in generating these signatures.

Background: Laser induced breakdown spectroscopy (LIBS) is a robust technique of analyzing chemical compounds because it can be performed with minimal target material in a non-contact environment. However, conventional LIBS using relatively long (nanosecond) laser pulses has the deficiency of low discrimination among chemicals with similar atomic compositions. Using ultrashort (femtosecond) laser pulses could partially breakdown the chemical into molecular fragments for identification. In addition, nonlinear effects and group velocity dispersion (GVD) compression in the atmosphere allow a femtosecond laser pulse to be projected at a target location at standoff distances.

Accomplishment: Successfully identified two nitrate compounds by detecting the emission signature of the nitro-radicals formed during the laser induced breakdown by the ultrashort laser pulse. The laser parameters for such partial breakdown were also optimized.

Signficance: Since most explosives are nitro-based, they could similarly be partially disintegrated by the ultrashort pulse laser into their nitro components for identification. Depending on the complexity of the fragments where other chemical radicals may still be attached to the nitro-group, discrimination between different explosives could be achieved with this technique.

Application: This technique could be used for standoff detection of explosives at distances of over 100 meters or more since ultrashort laser pulses have been demonstrated to GVD compress at such distances.