HIGH-SPATIAL-RESOLUTION OH ROTATIONAL TEMPERATURE-MEASUREMENTS IN AN ATMOSPHERIC-PRESSURE FLAME USING AN INDIUM-BASED RESONANCE IONIZATION DETECTOR

The use of a resonance ionization photon detector (RID) is described f or the measurement of flame temperatures with a spatial resolution of less than 100 mu m. The detector, based on the two-step excitation of indium atoms, with subsequent collisional ionization, was used to reco rd rotational excitation scans of OH in an atmospheric-pressure acetyl ene/air flame. The OH excitation spectra were recorded by scanning an ''excitation'' laser in the A(2) Sigma(+) <-- X(2) Pi(i) (1, 0) vibron ic band in the wavelength range, 281-288 nm, while simultaneously illu minating the same flame region with the ''detection'' laser, tuned to the 6p(2)P(3/2) --> 10d(2) D-5/2 excited-state transition of In at 786 .44 nm. The excitation and detection laser beams were made orthogonal in the flame, defining the resolution to be limited by the waist of th e excitation beam (100 mu m), whose diameter was always smaller than t he detection laser beam. A temperature profile of the flame is recorde d with the use of both the RID approach and a more conventional laser- induced fluorescence (LIF) approach for comparison. A more structured temperature profile is recorded with the RID owing to its high spatial resolution, whereas the LIF method, which is inherently a line-of-sig ht method, produces a rather featureless temperature distribution acro ss the flame. Anomalously high flame temperatures were recorded at the flame edge with the RID. The cause of these high flame temperatures h as not been determined.