Velocity-changing collisions in pure H2 and H2-Ar mixture

Abstract

We show how to effectively introduce a proper description of the velocity-changing collisions into the model of isolated molecular transition for the case of self-and Ar-perturbed H 2 . We demonstrate that the billiardball (BB) approximation of the H 2 -H 2 and H 2 -Ar potentials gives an accurate description of the velocitychanging collisions. The BB model results are compared with ab initio classical molecular dynamics simulations (CMDS). It is shown that the BB model correctly reproduces not only the principal properties such as frequencies of velocity-changing collisions or collision kernels, but also other characteristics of H 2 -H 2 and H 2 -Ar gas kinetics like rate of speed-changing collisions. Finally, we present line-shape measurement of Q (1) line of the first overtone band of self-perturbed H 2 . We quantify the systematic errors of line-shape analysis caused by the use of oversimplified description of velocity-changing collisions. These conclusions will have significant impact on recent rapidly developing ultra-accurate metrology based on Doppler-limited spectroscopic measurements like Doppler-width thermometry, atmosphere monitoring, Boltzmann constant determination or transition position and intensity determination for fundamental studies.

Recent advances in collisional effects on spectra of molecular gases and their practical consequences

“…Figure 7 shows a comparison between velocity collision kernels computed for H 2 -H 2 interactions using CMDS, the BB and KS models. As demonstrated in [159] and shown by this plot, for H 2 -H 2 (and also H 2 -Ar), the kernels calculated with the BB model are in excellent agreement with those predicted by CMDS, while the KS function leads to values which are independent of the relative orientation change between the velocities before and after the collision.…”

“…The same value (dashed black line) for the three angles is obtained with the KS model). After [159].…”

Strong competition between velocity-changing and phase- or state-changing collisions inH2spectra perturbed by Ar

“…Following Refs. [16,17], we demonstrate that even the use of the simplest hardsphere approximation of the ab initio potential in the speed-dependent billiard-ball profile (SDBBP) [18] leads to a correct reproduction of the experimental spectra.…”

“…[7][8][9][10], first, it is crucial to realize that this narrowing increases for larger changes in the molecule speed under the collisions. It was shown [16,17] (by the comparison of the results with those obtained from CMDS) that the changes of the speed are significantly overestimated by the simple phenomenological models such as hard-or soft-collision models [26,42]. Therefore, when analyzing their experimental data with a hard-collision model, Berger et al [2] introduced an extra parameter to reduce the effective rate of speed changes.…”

“…Note that the potential-well depth is much smaller than ǫ. The hardsphere approximation for H 2 -Ar was verified by comparing hard-sphere collision kernels and relaxation times with those obtained from CMDS [17].…”