DFWM spectroscopy as a tool to detect products from low-density discharges and nozzle expansions

Degenerate four-wave mixing spectroscopy at 226 nm has been used to determine state selectively parallel and perpendicular density profiles of nitric oxide (NO) in continuous-flow, low-density nozzle beam expansions. In contrast to conventional techniques, this nonlinear optical method facilitates obtaining quantitative data close to the nozzle orifice for unusual nozzle expansions (“wind tunnels”) and for small absolute densities down to 1013 molecules per cm3 in a particular quantum state. The potential of the method to determine molecular densities in environments with high background light levels is demonstrated by applying it to a nozzle beam of NO molecules generated in a microwave discharge in a O2/N2 mixture.