High-Temperature Absorption of the 3.39 μm He-Ne Laser Line by Acetylene, Ethylene, and Propylene

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Abstract
Absorptivity measurements for the 3.39 μm He-Ne laser line are reported for acetylene, ethylene, and propylene over the temperature ranges 1500 to 3000 K (acetylene), 1000 to 2000 K (ethylene), and 700 to 2300 K (propylene). In contrast to the behavior of other hydrocarbons, the absorptivities of acetylene and ethylene increase with increasing temperature. The conventional spectroscopic analysis of line positions based upon high-resolution infrared spectroscopy at room temperature and approximate line-broadening theory indicate that no absorption at all would be expected for acetylene or ethylene. The time-resolved absorption profiles and computer models of the chemical behavior expected for shock-heated test gas confirm that the absorption is really due to these molecules, but must be attributed to some heretofore unidentified absorption process.