Abstract

The combination of collisional line broadening and limited instrumental resolution can have peculiar and unexpected effects on the intensities of infrared spectra. An illustrative example is provided by the series of <i>Q</i> branches in the <i>v</i><sub>7</sub> band of ethane. Conventional FT-IR spectra, with a resolution of ~0.25 cm<sup>−1</sup>, show a dramatic increase in the absorbance of these branches as a function of increasing N<sub>2</sub> partial pressure, an effect which could be construed as collisional narrowing. In contrast, high-resolution diode laser spectroscopy reveals a monotonic increase in branch width (0.14 cm<sup>−1</sup>/atm), and a concomitant decrease in intensity, as the N<sub>2</sub> pressure is raised. The present work shows that absorption measurement of the inherently very narrow ethane <i>Q</i> branches with limited instrumental resolution and collisional line broadening conspire to give the appearance that the absorbance increases with N<sub>2</sub> pressure. This effect in general, as well as its implications for the use of FT-IR spectroscopy to measure molecular concentrations in gaseous mixtures, is discussed.

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