Strong competition between velocity-changing and phase- or state-changing collisions in ${\mathrm{H}}_2$ spectra perturbed by Ar

A collisional inhomogeneous broadening of the ${\text{H}}_2$ $Q\left(1\right)$ line perturbed by Ar was observed for the first time 25 years ago. Several attempts were made to explain the line broadening from ab initio calculations, which, however, resulted in fundamental discrepancies between the theory and experiment. To resolve this problem we investigate two possible sources of these differences. First, we repeat the ab initio calculations of the broadening and shifting speed dependence using in the scattering calculations a highly accurate ab initio ${\text{H}}_2\text{-Ar}$ interaction potential. Second, we replace the previous phenomenological models of velocity-changing collisions with a more physical one based on the interaction potential. This allows us to properly reproduce the experimental broadening over a wide range of temperatures and pressures. We show that this abnormal broadening is affected by strong competition between the velocity-changing collisions and speed-dependent collisional shifting, especially at high pressures.

Figure 1

Speed-dependent collisional broadening $\gamma \left(v\right)$ and shifting $\delta \left(v\right)$ vs the speed of the active molecule for the isotropic Raman $Q\left(1\right)$ line of the 0–1 band of ${\text{H}}_2$ perturbed by Ar. The blue (dark gray) and red (light gray) lines correspond to $T=296$ K and $T=795$ K, respectively. The vertical dashed lines indicate the most probable speeds at these two temperatures. The blue (dark gray) and red (light gray) dotted lines represent the Maxwellian speed distribution for $T=296$ K and $T=795$ K, respectively.

Figure 2

The apparent half width at half maximum (HWHM) and the shift (center of gravity) of the spectral line shape as a function of number density for the isotropic Raman $Q\left(1\right)$ transition of the 0–1 band of ${\text{H}}_2$ perturbed by Ar. The solid lines represent the results of our calculations using SDBBP [18], while the points correspond to the experimental values [2, 3]. The dashed and dotted lines represent our theoretical calculations in which the velocity-changing-collision model was replaced with phenomenological soft- and hard-collision models, respectively. The concentration of Ar was $95\%$ (in the experiment as well as in our calculations).