Determination of infrared optical constants for single component hydrocarbon fuels

The objective of this investigation was to develop an experiment that could determine the infrared optical constants of single component hydrocarbon fuels from 2 - 15 pm (5000 - 667 cm-1). The optical constants to be determined were the Lambert coefficient of absorption and the real and imaginary parts of the complex index of refraction. The coefficient of absorption and the imaginary part of the index of refraction (extinction coefficient) were determined directly from transmittance measurements. The real part of the index of refraction (refractive index) was calculated using the absorption coefficient and Kramers-Kronig optical dispersion relations. Since Kramers-Kroning relations require knowledge of the absorption coefficient across the entire spectrum of frequencies (wave numbers 0 to ∞), assumptions about the behavior of the absorption coefficient were modeled from existing transmittance data outside the experimental region. Optical constants were determined for nine single component liquid hydrocarbon fuels. The fuels investigated were: iso-octane, iso-pentane, n-heptane, n- hexane, n-nonane, n-decane, 1-hexene, o-xylene, and toluene. Because of the availability of accurate published data, water was used to validate the experimental set-up. The determined absorption coefficient curve for iso-octane showed excellent agreement with the absorption coefficient curve produced from American Petroleum Institute (API) data. Additionally, the calculated value for the absorption coefficient and extinction coefficient for iso-octane at 3.39 µm (2950 cm-1) is in very good agreement with existing results from reflectance measurements --Abstract, page iii