FROM AB INITIO CALCULATIONS TO SPECTRAL LINE SHAPE PARAMETRISATION FOR SPECTROSCOPIC DATABASES
Résumé
Hydrogen molecule in its ground electronic state perturbed by the helium atom constitutes the simplest system of molecule perturbed by atom. This gives possibility to make a link between the experiment and the theory from first principles. We utilize highly accurate cavity ring-down spectroscopy to study the H2-He collisions and interactions. In contrast to most of the previous studies, we directly superimpose theoretical profiles, originating from our ab initio calculations, on the experimental spectra without fitting any of the line-shape parameters. Within this approach not only the shapes of experimental lines are reliably reproduced, but also the underlying physics of molecular collisions can be traced. Besides the analysis of the basic line-shape effects (such as relaxation or phase changes of the internal states of the molecule), we also study the more sophisticated ones such as speed-dependent effects or velocity-changing collisions (complex Dicke narrowing parameter), which are particularly pronounced for the H2-He system. We achieved agreement at the 1% level between experimental data and ab initio calculations not reached before for any real system. In addition, we show that measured spectral line shapes can be interpolated within similar accuracy by Hartman-Tran line shape profile, which has been implemented as a standard for HITRAN database with β correction applied.