Combustion is a very complicated phenomenon. The burning of even a simple hydrocarbon involves several hundred coupled chemical reactions. For this reason, a thorough understanding of the combustion process does not yet exist. Besides the intrinsic interest in understanding the physics and chemistry of this prevalent natural phenomenon, an understanding of the combustion would presumably lead to the design of highly efficient and/or nonpolluting engines.

The theoretical models of combustion, in general, need to be experimentally verified, and, at times, even to build a theoretical model one needs the experimental data. Hence there is a need for diagnostic techniques, and in general one is interested in measurements of majority (such as H2O, CO2 ) and minority species (such as OH, CH, NO, etc.) concentrations, local temperatures, and flow velocity. Over the past several years we have demonstrated that photothermal deflection spectroscopy is an ideal technique for combustion diagnostics. The technique can be used to measure all three parameters of interest, that is, species concentration, temperature, and flow velocity simultaneously using the data obtained in a single laser pulse. We are engaged in experiments to demonstrate that this indeed is possible, and will then use this technique to make measurements of several minority species in a variety of flames.

The basic ideas involved in using PTDS for combustion diagnostics are as follows: A dye-laser beam (pump beam) passes through the flame. The dye laser is tuned to one of the absorption lines of the molecules that is to be detected, and the molecules absorb the optical energy from the laser beam. Due to fast quenching rates in a flame, most of this energy quickly appears in the rotational- translational modes of the flame molecules. Thus the dye laser-irradiated region gets slightly heated, leading to changes in the refractive index of the medium in that region. Now if a probe-laser beam overlaps the pump beam, the probe beam is deflected due to the variations in the refractive index of the medium created by the pump beam. The species concentration, temperature, and the flow velocity can be derived from the amplitude, width, and the time-of-flight of the deflection signal, respectively.