Ab Initio Computation of Dynamical Properties: Pressure Broadening

Rotational spectroscopy of polar molecules is the main observational tool in many areas of astrophysics, for gases of low densities (n ∼ 10² − 10⁸ cm⁻³). Spectral line shapes in astrophysical media are largely dominated by turbulence-induced Doppler effects and natural line broadening are negligible. However line broadening remains an important tool for denser gases, like planetary high atmospheres. Understanding the excitation schemes of polar molecules requires the knowledge of excitation transfer rate due to collisional excitation, between the polar molecule and the ambient gas, usually H₂. Transport properties in ionized media also require a precise knowledge of momentum transfer rates by elastic collisions.

In order to assess the theoretically computed cross section and energy/momentum transfer rates, direct absolute experiments are scarce. The best way is to measure not individual scattering events but rather the global effect of the buffer gas, thanks to the pressure broadening cross sections, whose magnitude can be measured without any scaling parameters. At low temperatures, both elastic and inelastic scattering amplitudes are tested. At higher temperature, depending on the interaction strength, only inelastic scattering cross section are shown to play a significant role ^1 ,2.

Thanks to the advances of computer capabilities, it has become practical to compute spectral line parameters fromab initio quantum chemistry. In particular, the theory of rotational line broadening is readily incorporated into scattering quantum dynamical theory, like close-coupling schemes. The only approximations used in the computation are the isolated collision/isolated line approximations. We compute the non-binding interaction potential with high precision quantum chemistry and fit the resulting ab initio points onto a suitable functional.

We have recently computed several such systems, for molecules in H₂ buffer gas: H₂O,³ H₂CO,⁴ HCO⁺ .⁵ Detailed computations taking into account the ortho or para state of H₂ were performed, at temperatures ranging from 10 K to 100K, typically.

Reliable results are found, that compare favorably to experiments. In particular, the water-molecular hydrogen system has been thoroughly computed and successfully experimentally tested ⁶.

New projects consider other simple molecules as well as heavier systems, relevant for cometary comae and planetary high atmospheres.