Mid-Infrared Pumped Laser-Induced Thermal Grating Spectroscopy for Detection of Acetylene in the Visible Spectral Range

We present mid-infrared laser-induced thermal grating spectroscopy (IR-LITGS) using excitation radiation around 3 mu m generated by a simple broadband optical parametric oscillator (OPO). Acetylene as a typical small hydrocarbon molecule is used as an example target species. A mid-infrared broadband OPO pumped by the fundamental output of a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser was used to generate the pump beams, with pulse energies of 6-10mJ depending on the wavelength. The line width of the OPO idler beam was approximate to 5cm(-1), which is large enough to cover up to six adjacent acetylene lines. The probe beam was the radiation of a 532nm cw solid state laser with 190 mW output power. Signals were generated in atmospheric pressure gas flows of N-2, air, CO2 and Ar with small admixtures of C2H2. A detection limit of less than 300ppm was found for a point measurement of C2H2 diluted in N-2. As expected, the oscillation frequency of the IR-LITGS signal was found to have a large dependency on the buffer gas, which allows determination of the speed of sound. Moreover, the results reveal a very strong collisional energy exchange between C2H2 and CO2 compared to the other gases. This manifests as significant local heating. In summary, the MIR-LITGS technique enables spectroscopy of fundamental vibrational transitions in the infrared via detection in the visible spectral range.