A non-intrusive, optical technique for measuring absolute concentration of formaldehyde has been developed. This method uses tunable, infrared diode laser absorption measurements to infer formaldehyde concentration along the optical path length. The technique was applied to three different experimental systems to demonstrate the capabilities and limitations of the method. Time resolved measurements were made on dilute mixtures of formaldehyde in N_2 in a stainless steel, heated test cell at T = 650 K and pressures less than 100 Torr. These measurements were used to determine the rate of formaldehyde decomposition. The measured rate of decay, approximately 3 times 10^3 cm^3 /mol-s, is over 10 orders of magnitude greater than the rate predicted at this temperature from correlations of high temperature, gas-phase kinetic data. Diode laser concentration measurements were also made on a stream of air seeded with formaldehyde at atmospheric pressure and room temperature. The average concentration of formaldehyde inferred from analysis of the absorption profile was 0.74%. Finally, the diode laser system was used to make absorption measurements on combustion gases in a premixed, methane-air flame. Three spectral regions determined to be the most promising for formaldehyde absorption were investigated. Absorption features were observed in these regions, however, it was not clear that the features were the result of formaldehyde absorption. Failure to detect formaldehyde in the methane-air flames is probably due to the large beam diameter (2 mm) relative to the limited region of high formaldehyde concentration in an atmospheric pressure flame (~0.2 mm). The minimum fractional absorption (I/I^circ ) detectable averaging 1500 laser sweeps recorded at 30 sweeps per second was estimated to be 0.05%. Spectral line parameters of formaldehyde, integrated absorption and broadening coefficients, required to analyze the absorption data were also studied. Results of the integrated absorption measurements indicate that values inferred from line intensity data in the literature may be in error by as much as 27%. Broadening coefficients measured here are in good agreement with measurements and calculations reported by other investigators. Absorption measurements at temperatures from 298 to 650 K were used to test the accuracy of theoretical expressions for the temperature dependence of integrated absorption, and infer the temperature dependence of N_2 -broadening. Values of integrated absorption calculated from the theoretical expressions agreed with measured values within 5%.
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