The Boltzmann constant from the shape of a molecular spectral line

We report on our recent determination of the Boltzmann constant, k(B), by means of Doppler broadening thermometry. This relatively new method of primary gas thermometry was implemented by using a pair of offset-frequency locked extended-cavity diode lasers at 1.39 mu m, to probe a particular vibration-rotation transition of the (H2O)-O-18 molecule. Adopting a rather sophisticated and extremely refined line shape model in the spectral analysis procedure, we were able to determine the Doppler width from high-quality absorption spectra with unprecedented accuracy. Our spectroscopic determination of kB exhibits a combined (type A plus type B) uncertainty of 24 parts over 10(6). The complete uncertainty budget is presented and discussed

Speed dependence of collision parameters in the H2O18 near-IR spectrum: Experimental test of the quadratic approximation

A laser absorption technique, based upon a pair of offset-frequency-locked extended-cavity diode lasers, was employed to perform line-shape measurements for H218O vibration-rotation transitions at 1.38 μm, with an extremely high spectral fidelity. This important feature was mostly ensured by the highly accurate, absolute, and repeatable frequency axis characterizing each acquired spectrum. We present the outcomes of a specific study of the speed dependence of collision parameters influencing the shape of the H218O absorption lines. More particularly, the performance of the quadratic approximation, either used alone or considered in conjunction with the Dicke narrowing effect, was investigated and compared to that of the confluent hypergeometric dependence. Hence, experimental evidences of the partial failure of the quadratic approximation are given and discussed. Finally, we investigate the possible influence of the choice of the speed-dependent line-shape model on the retrieved collisional parameters

Experimental test of the quadratic approximation in the partially correlated speed-dependent hard-collision profile

We report on the outcomes of a specific study on the quadratic approximation in the partially correlated speed-dependent hard-collision (pC-SDHC) model, which is currently the recommended profile to replace the Voigt convolution for the shape of an isolated line, when perturbed by neutral gas-phase molecules. In particular, we compared the quadratic approximation with the hypergeometric dependence of the collisional relaxation rate on the absorber velocity by using high-quality H2 18O absorption spectra, in coincidence with three vibration-rotation transitions of the ν1 + ν3 band, at 1.39 μm, also by looking for possible differences in the retrieved parameters. The pC-SDHC profile was found to be quite robust, regardless of the choice of the particular speed dependence. The pressure broadening and shifting parameters, retrieved by using the quadratic and hypergeometric versions, were found to be fully consistent, provided that the velocity-changing collision frequency νvc was considered as a free parameter. Similarly, the integrated absorbance was found to be completely unaffected by the choice of the speed dependence, in the entire pressure range we have explored. It must be noted, however, that the velocity-changing collision frequency resulted to be largely overestimated compared to the expected one when using the quadratic approximation. Moreover, the νvc values from the quadratic approximation are always significantly larger than those of the hypergeometric model

Recommended isolated-line profile for representing high-resolution spectroscopic transitions (IUPAC Technical Report)

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