Temperature-dependent mid-IR absorption spectra of gaseous hydrocarbons

Abstract Quantitative mid-IR absorption spectra (2500–3400 cm −1 ) for 12 pure hydrocarbon compounds are measured at temperatures ranging from 25 to 500 °C using an FTIR spectrometer. The hydrocarbons studied are n -pentane, n -heptane, n -dodecane, 2,2,4-trimethyl-pentane ( iso -octane), 2-methyl-butane, 2-methyl-pentane, 2,4,4-trimethyl-1-pentene, 2-methyl-2-butene, propene, toluene, m -xylene, and ethylbenzene. Room-temperature measurements of neat hydrocarbon vapor were made with an instrument resolution of both 0.1 and 1 cm −1 (FWHM) to confirm that the high-resolution setting was required only to resolve the propene absorption spectrum while the spectra of the other hydrocarbons could be resolved with 1 cm −1 resolution. High-resolution (0.1 cm −1 ), room-temperature measurements of neat hydrocarbons were made at low pressure (∼1 Torr, 133 Pa) and compared to measurements of hydrocarbon/N 2 mixtures at atmospheric pressure to verify that no pressure broadening could be observed over this pressure range. The temperature was varied between 25 and 500 °C for atmospheric-pressure measurements of hydrocarbon/N 2 mixtures ( X hydrocarbon ∼0.06–1.5%) and it was found that the absorption cross section shows simple temperature-dependent behavior for a fixed wavelength over this temperature range. Comparisons with previous FTIR data over a limited temperature range and with high-resolution laser absorption data over a wide temperature range show good agreement.