High-temperature Pressure Broadening of Water: Experimental Evidence for Negative Temperature Exponents

Kurzfassung

Fourier transform spectra of H2O-air mixtures have been measured at 296, 742, and 980 K for total pressures of 0.5 and 1.0 atm with a H2O VMR of 0.5% in the ƒÞ2 band (range 1550-2300 cm-1) at a spectral resolution of 0.01 cm-1. These measurements were initially carried out in the frame of two EC projects aiming at a spectroscopic database for non-intrusive aircraft exhaust measurement. A special cell for high temperatures, good temperature homogeneity, 16 cm absorption path, equipped for continuous gas flow, has been developed to be operated stand-alone and inside the evacuated sample compartment of the Bruker IFS 120 HR FT spectrometer. Thermal emission compensation and proper transmittance calculation required high resolution sample and reference spectra for the IR source switched on and off. Temperatures were deduced from CO line intensity measurements. Line positions, linestrengths and Lorentzian halfwidths have been retrieved from the spectra. For the following analysis only air-broadening for the doublet lines with Kc=J is considered. From these data and previous experimental determinations, line-broadening parameters ƒ×(T) are obtained at all three temperatures for the doublet lines from J=0 up to J=16. These values are then used to deduce the temperature dependence exponent n, such that . The values of n obtained using the two elevated temperatures are consistent and show that line widths decrease with T for small J values (n „l +0.8), are almost temperature independent on T for J„l10, and increase with T for high J lines (n „l -0.3 for J„l15). This original result, which confirms theoretical predictions made about fifteen years ago with a semi-classical approach [J Chem Phys 1987;86:144-56], is analyzed here using an improved version of the model.