Tunable QCL emission profile between 7 and 9 μm (1,430 cm−1 to 1,130 cm−1), shown in Fig. 3a, has been used for generating the PT signals following absorption by molecular species on the target. Nitrobenzene has been used to illustrate the principle of PT detection with QTF. The raw absorption of nitrobenzene (NB) molecule is recorded in Fig. 3b using the QTF detector and the normalized absorption spectrum shown in Fig. 3c. The normalization is done by dividing the spectrum of nitrobenzene by spectrum of laser profile. The normalization takes care of absorption by water molecules present in the atmosphere. The observed NB absorption peak between 1,360 and 1,370 cm−1 is in agreement with its reported value in the literature corresponding to symmetric NO2 vibration of C-NO2 moiety in the molecule18.
Using the experimental setup of Fig. 1 in point detection, the PT spectrum of NB molecule is plotted in Fig. 3d. The short standoff (~ 1.0 m) PT spectra of RDX, TNT and Acetone were recorded using the setup shown in Fig. 2. In these experiments, slight adjustment of the distance between telescope mirror and the detector had to be made along the axis as the focal spot slightly shifts away from the mirror as we take the target closer to the telescope. The spectra thus recorded from the short distance are shown in Figs. 4a, 5a, and 6a respectively.
The experimental setup of Fig. 2 has been used to record the standoff PT spectra of RDX, TNT, and Acetone from 25 m. These spectra are shown in Figs. 4b, 5b, and 6b respectively. Figures 4c, 5c, and 6c respectively depict the spectra recorded when there was no sample present at the target surface (or inside the aerosol cell). The scattered probe laser radiation from the target is collected by a spherical mirror of 12 cm diameter, coupled to a pinhole as shown in Fig. 2. The probe laser beam, incident on the pinhole, is focused by a lens on the QTF detector to generate the PT signal as a function of wavelength when QCL is scanned over its spectral profile. As described earlier the QCL radiation is blocked, from hitting the QTF, by the 532 nm interference filter. The strongest peaks are resolved at 1,268 cm−1 for RDX molecule, and at 1,364 cm−1 of TNT molecule in agreement with the literature19.
Acetone is the precursor of TATP explosive and its PT spectra, shown in Fig. 6, exhibit two strong peaks at 1,218 cm−1 and at 1,230 cm−1 in agreement with the normal modes of vibration corresponding to ‘CCC bend + OCO bend’ and ‘CC stretch + CCO bend’ respectively20.
It is apparent from the vibrational assignments described above18,19,20, that the molecular species are clearly detected by the PT sensor. The strength of noise signal, corresponding to the baseline (spectrum from the clean surface, without sample) of Figs. 4, 5 and 6, was about 75 mV while a value of 150 mV for the PT signal was taken as the detection limit and this gives a SNR of about 2. Noise level was taken as the average height of the spectra of clean surfaces (without sample). For calculating noise level, those spectral ranges were used where the noise were maximum. The detection limit is found to be 200 nl concentration for Acetone and 5 μg/cm2 for TNT and RDX with a SNR ~ 2 at a distance of 25 m.
The experimental setup similar to that of Fig. 2 was used to record the standoff photoacoustic (PA) spectra of TNT by using only the QCL source as described for the QE-LPAS technique6. In this case a gold coated mirror collecting mirror was used instead of enhanced aluminium coated mirror and a ZnSe focussing lens was used instead of glass lens. The variation of PA signal for TNT, with change in standoff distance is shown in Fig. 7a. It is found that the limit of PA signal detection is reached at 15 m for TNT residue concentration of 5 μg/cm2. In a parallel experiment of standoff photothermal (PT) detection; using a similar target as for the PA detection, the PT signal variation with distance is shown in Fig. 7b. It is found that the limit of detection for the PT signal is reached at ~ 25 m instead of 15 m (as in PA detection) for TNT concentration of 5 μg/cm2 on the target surface. A comparison of Fig. 7a and b shows that both the PA and the PT signals decay exponentially with increasing standoff distance. It is evident from these investigations that the PT detection, using the pump-probe laser technique, can be successfully employed at longer standoff distance in comparison to the QE-LPAS technique.