Bernhard Zachhuber

Stand-off spatial offset Raman spectroscopy for the detection of concealed content in distant objects.

A pulsed (4.4 ns pulse length) frequency-doubled Nd:YAG laser operated at 10 Hz was used to generate Raman scattering of samples at a distance of 12 m. The scattered light was collected by a 6 in. telescope, and the Raman spectrum was recorded using an Acton SP-2750 spectrograph coupled to a gated intensified charge-coupled device (ICCD) detector. Applying a spatial offset between the point where the laser hit the sample and the focus of the telescope on the sample enabled collection of Raman photons that were predominantly generated inside the sample and not from its surface. This is especially effective when the content of concealed objects should be analyzed. High-quality Raman spectra could be recorded, within 10 s of data acquisition, from a solid (NaClO(3)) as well as a liquid (isopropyl alcohol) placed inside a 1.5 mm thick opaque low-density polyethylene (LDPE) plastic bottle. The applied spatial offset was also advantageous in cases where the surface of the container was highly fluorescent. In such a situation, Raman spectra of the sample could not be recorded when the sampling volume (telescope observation field) coincided with the focus of the excitation laser. However, with the use of a spatial offset of some millimeters, a clear Raman spectrum of the content (isopropyl alcohol) in a strongly fluorescent plastic container was obtained.

Depth profiling for the identification of unknown substances and concealed content at remote distances using time-resolved stand-off Raman spectroscopy.

Time-resolved stand-off Raman spectroscopy was used to determine both the position and identity of substances relative to each other at remote distances (up to tens of meters). Spectral information of three xylene isomers, toluene, and sodium chlorate was obtained at a distance of 12 m from the setup. Pairs and triplets of these samples were placed at varying distances (10–60 cm) relative to each other. Via the photon time of flight the distance between the individual samples was determined to an accuracy of 7% (corresponding to a few cm) of the physically measured distance. Furthermore, at a distance of 40 m, time-resolved Raman depth profiling was used to detect sodium chlorate in a white plastic container that was non-transparent to the human eye. The combination of the ranging capabilities of Raman LIDAR (sample location usually determined using prior knowledge of the analyte of interest) with stand-off Raman spectroscopy (analyte detection at remote distances) provides the capability for depth profile identification of unknown substances and analysis of concealed content in distant objects. To achieve these results, a 532 nm laser with a pulse length of 4.4 ns was synchronized to an intensified charge-coupled device camera with a minimum gate width of 500 ps. For automated data analysis a multivariate curve resolution algorithm was employed.

Stand off spatial offset Raman spectroscopy: a distant look behind the scenes

A pulsed (4.4 ns pulse length) frequency doubled Nd:YAG laser, operating at 10 Hz, was used to generate Raman scattering from samples at a distance of 12 m. The scattered light was collected by a 6 inch telescope and the Raman spectrum recorded using an Acton SP-2750 spectrograph coupled to a gated ICCD detector. To extend the potential applications further, employing a spatial offset between the point where the laser hit the sample and the focus of the telescope on the sample, enabled collection of Raman photons that were predominantly generated inside the sample and not from its surface. This is especially effective when the content of concealed objects should be analysed. Raman spectra of H2O2 in a 1.5 mm thick, fluorescent HDPE plastic bottle were recorded at a distance of 12 m. From the recorded spectra it was possible to determine the H2O2 concentration in the concentration range from 2-30%. Stand-off Raman spectra of eleven potentially dangerous chemicals (commercial and improvised explosives) were recorded at a distance of 100 m.

Spatial Offset Stand Off Raman Scattering

Identification and quantification of potentially harmful substances concealed in fluorescent containers were achieved at a distance of 12 metres using stand off spatial offset Raman scattering.