Nitric oxide laser-induced fluorescence using the fifth harmonic of a broad-band Nd:YAG laser

We explore the prospects of laser-induced fluorescence diagnostics of nitric oxide (NO) using non-tunable fifth-harmonic radiation of a broad-band, ns-pulsed Nd:YAG laser at λ=213 nm. Typically, 2–5 mJ/pulse of 213-nm radiation is produced by a commercial harmonic generator in this study, with an efficiency of about 1–3% (relative to the input pulse energy). We present spectral results obtained in various environments, ranging from air-based combustion processes at room conditions up to elevated pressure and temperature environments, the latter resembling conditions typical for compression-ignition internal combustion engines. In all cases, the laser-induced fluorescence spectrum shows clear signatures of the NO spectrum, mostly on transitions in the γ-band system (A²Σ⁺→X²Π). At higher fluences, multi-photon absorption also gives rise to blue-shifted fluorescence. The fluorescence yield increases with increasing pressure, allegedly due to non-resonant excitation, the efficiency of which increases with increasing pressure broadening. When applied to air-based combustion processes, interference by (hot) oxygen needs to be taken into account. We conclude that the method is a relatively straightforward option to visualize the NO distribution in a broad variety of applications.