Anomalous Pressure Dependence of Infrared Spectra Due to the Combination of Collisional Broadening and Instrument Resolution

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 Q branches in the v₇ band of ethane. Conventional FT-IR spectra, with a resolution of ∼0.25 cm⁻¹, show a dramatic increase in the absorbance of these branches as a function of increasing N₂ partial pressure, an effect which could be construed as collisional narrowing. In contrast, high-resolution diode laser spectroscopy reveals a monotonie increase in branch width (0.14 cm⁻¹/atm), and a concomitant decrease in intensity, as the N₂ pressure is raised. The present work shows that absorption measurement of the inherently very narrow ethane Q branches with limited instrumental resolution and collisional line broadening conspire to give the appearance that the absorbance increases with N₂ 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.