RESUMEN
Information about molecular collisions is encoded in the shapes of collision-perturbed molecular resonances. This connection between molecular interactions and line shapes is most clearly seen in simple systems, such as the molecular hydrogen perturbed by a noble gas atom. We study the H2-Ar system by means of highly accurate absorption spectroscopy and ab initio calculations. On the one hand, we use the cavity-ring-down-spectroscopy technique to record the shapes of the S(1) 3-0 line of molecular hydrogen perturbed by argon. On the other hand, we simulate the shapes of this line using ab initio quantum-scattering calculations performed on our accurate H2-Ar potential energy surface (PES). In order to validate the PES and the methodology of quantum-scattering calculations separately from the model of velocity-changing collisions, we measured the spectra in experimental conditions in which the influence of the latter is relatively minor. In these conditions, our theoretical collision-perturbed line shapes reproduce the raw experimental spectra at the percent level. However, the collisional shift, δ0, differs from the experimental value by 20%. Compared to other line-shape parameters, collisional shift displays much higher sensitivity to various technical aspects of the computational methodology. We identify the contributors to this large error and find the inaccuracies of the PES to be the dominant factor. With regard to the quantum scattering methodology, we demonstrate that treating the centrifugal distortion in a simple, approximate manner is sufficient to obtain the percent-level accuracy of collisional spectra.
RESUMEN
Absolute frequencies of unperturbed (12)C(16)O transitions from the near-infrared (3-0) band were measured with uncertainties five-fold lower than previously available data. The frequency axis of spectra was linked to the primary frequency standard. Three different cavity enhanced absorption and dispersion spectroscopic methods and various approaches to data analysis were used to estimate potential systematic instrumental errors. Except for a well established frequency-stabilized cavity ring-down spectroscopy, we applied the cavity mode-width spectroscopy and the one-dimensional cavity mode-dispersion spectroscopy for measurement of absorption and dispersion spectra, respectively. We demonstrated the highest quality of the dispersion line shape measured in optical spectroscopy so far. We obtained line positions of the Doppler-broadened R24 and R28 transitions with relative uncertainties at the level of 10(-10). The pressure shifting coefficients were measured and the influence of the line asymmetry on unperturbed line positions was analyzed. Our dispersion spectra are the first demonstration of molecular spectroscopy with both axes of the spectra directly linked to the primary frequency standard, which is particularly desirable for the future reference-grade measurements of molecular spectra.
