Simultaneous Visualization of Hydrogen Peroxide and Water Concentrations Using Photofragmentation Laser-Induced Fluorescence

Abstract

A concept based on photofragmentation laser-induced fluorescence (PFLIF) is for the first time demonstrated for simultaneous detection of hydrogen peroxide (H₂O₂) and water (H₂O) vapor in various mixtures containing the two constituents in a bath of argon gas. A photolysis laser pulse at 248 nm dissociates H₂O₂ into OH fragments, whereupon a probe pulse, delayed 100 ns and tuned to an absorption line in the A²Σ⁺ (v = 1) ← X²Π(v = 0) band of OH near 282 nm, induces fluorescence. The total OH fluorescence reflects the H₂O₂ concentration, while its spectral shape is utilized to determine the H₂O concentration via a model predicting the ratio between the fluorescence intensities of the A²Σ⁺ (v = 1) → X²Π(v = 1) and the A²Σ⁺ (v = 0) → X²Π(v = 0) bands. The H₂O detection scheme requires that the bath gas has a collisional cross-section with OH(A) that is significantly lower than that of H₂O, which is the case for argon. Spectrally dispersed OH fluorescence spectra were recorded for five different H₂O₂/H₂O/Ar mixtures; the H₂O₂ concentration in the range of 30–500 ppm and the H₂O concentration in the range of 0–3%. Fluorescence intensity ratios predicted by the model for these mixtures agree very well with corresponding experimental data, which thus validates the model. The concept was also demonstrated for two-dimensional imaging, using two intensified charge-coupled device (CCD) cameras for signal detection. Water content was here sensed through the different temporal characteristics of the two fluorescence bands by triggering the two cameras so that one captures the total OH fluorescence while the other one captures only the early part, which mainly stems from A²Σ⁺ (v = 1) → X²Π(v = 1) fluorescence. Hence, the H₂O₂ concentration is reflected by the image of the camera recording the total OH fluorescence, whereas H₂O concentration is extracted from the ratio between the two camera images. Quantification of the concentrations was carried out based on calibration measurements performed in known mixtures of H₂O₂ (30–500 ppm) and H₂O (0–3%) in bulk argon. The detection limits for single-shot imaging are estimated to be 20 ppm for H₂O₂ and 0.05% for H₂O. The authors believe that the concept provides a valuable asset in, for example, pharmaceutical or aseptic food packaging applications, where H₂O₂/H₂O vapor is routinely used for sterilization.