Methods for enhancing radar REMPI sensitivity

Radar REMPI is a promising technique and alternative to other laser diagnostics such as laser-induced fluorescence, Raman scattering or coherent anti-Stokes Raman scattering. While measurements in nitric oxide illustrated the ability of the technique to capture trace species, the sensitivity ultimately depends upon the experimental configuration and background interference from other species with overlapping transitions. 2+2 radar REMPI measurements performed in molecular nitrogen were limited to densities at or above 10¹⁷ cm^-3. This paper looks at methods for increasing the sensitivity of the technique. Near-field enhancement of the scattered signal was observed to produce a factor of 20 signal enhancement. However, this enhancement was found to be very sensitive to alignment and was difficult to achieve consistently. Alternatively, a double pass configuration was used and found to increase the signal level. Therefore, a multi-pass configuration has the potential for offering an increase in the sensitivity of the technique. This approach may be useful for measurements of trace species in an atmospheric flame, where remote detection is not required.