High-Spatial-Resolution OH Rotational Temperature Measurements in an Atmospheric-Pressure Flame Using an Indium-Based Resonance Ionization Detector

Abstract

The use of a resonance ionization photon detector (RID) is described for the measurement of flame temperatures with a spatial resolution of less than 100 μm. The detector, based on the two-step excitation of indium atoms, with subsequent collisional ionization, was used to record rotational excitation scans of OH in an atmospheric-pressure acetylene/air flame. The OH excitation spectra were recorded by scanning an “excitation” laser in the A²σ⁺ ← X²II _i (1, 0) vibronic band in the wavelength range, 281–288 nm, while simultaneously illuminating the same flame region with the “detection” laser, tuned to the 6p²P_3/2 → 10d². 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 the excitation beam (100 μm), whose diameter was always smaller than the detection laser beam. A temperature profile of the flame is recorded 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-sight method, produces a rather featureless temperature distribution across the flame. Anomalously high flame temperatures were recorded at the flame edge with the RID. The cause of these high flame temperatures has not been determined.