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Laser Optogalvanic Spectroscopy (LOGS) is an extremely sensitive detection technique based on the 'Optogalvanic Effect'. The impedance change of gaseous discharge following the absorption of resonant laser light by the discharge species has proved to be a powerful spectroscopic tool to investigate all kinds of matter in vapour phase in discharge plasmas and flames. LOGS does not require any optical detector and the signal-to-noise ratios are often greater than 10-'. The advent of tunable lasers has pushed LOGS as a unique detection technique, efficient over a wide frequency spectrum from UV through visible to IR. As a simple and most sensitive detection technique, LOGS can have diverse practical applications in science and technology. The experimental details with different discharge excitations and the potential applications of LOGS are briefly described. Some prominent practical applications like wavelength calibration, laser linewidth determination, trace elel11ent detection, isotope analysis, material characterisation, laser frequency and power stabilisation, Rydberg atom spectroscopy and combustion and plasma diagnostics are briefly discussed. the spectroscopy of even the transient species with many orders of magnitude more sensitivity and selectivity than was possible previously. High spectral density and purity of lasers have enabled the development of important nonlinear and sub-Doppler spectroscopic techniques which. have achieved unprecedented precision measurements. But, the advent of dye-Iasers has given a new impetus to almost all the spectroscopic techniques, particulary laser-induced fluorescence (LIF), laser optoacoustic detection (LOAD) and laser optogalvanic spectroscopy (LOGS) techniques. With all the above-mentioned qualities, the laser spectroscopic techniques provide us with a unique opportunit)' for accurate non-intrusive measurements with high spatial and temporal resolution even on extremely hostilc media constituted by burning or exploding gases. OPTOGAL V ANIC EFFECT AND ADV ANT AGES OF LOGS

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Laser based time-gated Raman Spectroscopy experiments for detection and identification of nitro-aromatic sample which premixed with soil and sand in different concentrations are conducted. A back-scattered stand-off Raman spectroscopy set-up using a frequency double Nd:YAG pulsed laser (532 nm, 8 ns, 10 Hz) was employed to evaluate samples made up of standard nitro-aromatic compounds, p-nitro Benzoic acid (PNBA) premixed with soil and sand at different concentrations. A sensitive ICCD detector was used to capture the weak Raman signals buried in high background noise at optimised experiments parameters like laser pulse energy, ICCD gate width, background subtraction, number of laser pulses averaging etc. p-nitro benzoic acid up to 5 per cent (v/v) concentration in sand with appreciable signal to noise ratio has been detected. This feasibility study also becomes relevant to evaluate the remnants for post-blast detection of explosives in terrorist attack as most of the explosives used in such attacks contain nitro-aromatic based high energetic explosives.

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