Christof Janssen | Searcharxiv

arXiv2018

First pressure shift measurement of ozone molecular lines at 9.54 $μ$m using a tunable quantum cascade laser

Using a free-running distributed-feedback quantum cascade laser (QCL) emitting at 9.54 $μ$m, the pressure shift parameters of four intense rovibrational transitions in the $ν_3$ fundamental band of ozone induced by oxygen (O$_2$), air and the noble gases helium (He), argon (Ar), and xenon (Xe) are obtained by employing second harmonic detection. The experimental analysis comprises a full uncertainty budget and provides line shift data which are traceable to SI. The high density of transitions in the $ν_3$ spectral region of ozone make this region particularly difficult to study with more commonly used techniques such as Fourier transform spectroscopy. The comparatively high spectral resolution of the QCL in the MHz range, on the contrary, allows to measure molecular shifts at relatively low pressures (from 2 to 70 hPa), thus reducing the impact of spectral congestion due to pressure broadening of molecular lines. The comparison of our results with published data shows that presently recommended values for the pressure shift are too low in this region. This observation is corroborated by semi-classical calculations using the Robert-Bonamy formalism. A slight negative $J$ dependence, already observed in other ozone vibrational bands, is predicted. Systematic use of our technique could be very useful to support this hypothesis and to make up for the lack of shift parameters for ozone $ν_3$ transitions in molecular spectral databases. A subsequent stabilization of the QCL onto an optical frequency comb will open up possibilities to perform metrological measurements of Doppler-free molecular lines.

physics.atom-ph↗

arXiv2014

On the gas dependence of thermal transpiration and a critical appraisal of correction methods for capacitive diaphragm gauges

Thermal transpiration effects are commonly encountered in low pressure measurements with capacitance diaphragm gauges. They arise from the temperature difference between the measurement volume and the temperature stabilised manometer. Several approaches have been proposed to correct for the pressure difference, but surface and geometric effects usually require that the correction is determined for each gas type and gauge individually. Common (semi) empirical corrections are based on studies of atoms or small molecules. We present a simple calibration method for diaphragm gauges and compare transpiration corrections for argon and styrene at pressures above 1 Pa. We find that characteristic pressures at which the pressure difference reaches half its maximum value, are compatible with the universal scaling p_{1/2} = 2 \η \cdot \{v_{th}} / d, thus essentially depending on gas viscosity η, thermal molecular speed v_{th} and gauge tubing diameter d. This contradicts current recommendations based on the Takaishi and Sensui formula, which show an unphysical scaling with molecular size. Our results support the Miller or Šetina equations where the pressure dependency is basically determined by the Knudsen number. The use of these two schemes is therefore recommended, especially when thermal transpiration has to be predicted for new molecules. Implications for investigations on large polyatomics are discussed.

physics.chem-ph↗