Vibrational sum-frequency spectroscopy (VSFS) has been used for some time as a laboratory-based surface chemical analytical tool. Here, theoretical considerations in applying the method as a remote-sensing probe for detecting trace levels of chemicals adsorbed on surfaces are presented. Additionally, a VSFS instrument is configured to operate at a stand-off distance of 2.2 m using near-nadir incidence angles. This system was used to measure VSFS spectra for films of pure 1-amino-4-nitrobenzene (<i>p</i>-nitroaniline, PNA) and pure 2-hydroxy-1,3,5-trinitrobenzene (picric acid, PA) adsorbed on polished T-6061 aluminum alloy. These spectra are used to investigate the effect of optical polarization on the sum-frequency response of these compounds at nadir optical geometries. Detection limits for each compound are also estimated and found to be 0.51 μg cm<sup>2</sup> for PNA and 0.89 μg cm<sup>2</sup> for PA. The implications of these results regarding remote sensing applications of VSFS are discussed.
William E. Asher and Ella Willard-Schmoe, "Vibrational Sum-Frequency Spectroscopy for Trace Chemical Detection on Surfaces at Stand-Off Distances," Appl. Spectrosc. 67, 253-260 (2013)