He-perturbed H$_{2}$ spectra: unprecedented agreement between ab initio theory and experimental data

Hydrogen molecule perturbed by the helium atom constitutes the simplest benchmark system for performing the tests of the ab initio quantum scattering theory on the ultra-accurate experimental spectra. Here, we report a full description of the collision-perturbed shapes of rovibrational lines for this particular system. We demonstrate, for the first time, agreement between measured and ab initio computed collision-perturbed shapes of molecular lines at the subpercent level: the root-mean-square difference between experimental and theoretical profiles is smaller than one-hundredth of the profile amplitude. In the analysis described here, we employed the state-of-the-art statistical model of the collision-perturbed shape of molecular lines, we obtained all the parameters of this model from quantum scattering calculations, and the dynamical calculations were performed on the most accurate potential energy surface to date.