Vol. 102, Iss. 2 — August 2020

DOI: https://doi.org/10.1103/PhysRevA.102.023104

On the shape of absorption bands inherent to symmetrical molecules in an external field

The main constituents of the known planetary atmospheres are symmetric molecules. In a rarefied gas, i.e. as long as the molecules are well separated from each other, symmetry restrictions prohibit their absorption of radiation within typically broad wavelength ranges. However, under the action of an external field, whether caused in collisions with other molecules or on application of a static electric field, very weak absorption manifests itself in the nominally "forbidden" spectral ranges. These weak absorption bands are commonly called "induced" because they appear as a result of a perturbation induced in a molecule by an external field. Despite their weakness these induced absorption bands are known to be capable to play an important role in various astrophysical gas-phase media. Induced spectra recorded in laboratory-controlled conditions show in some cases the traces of a smooth structure, the nature of which was not understood until the present time. In our work a theory is developed that proposes an explanation for the observed unconventional structure. Taking a linear CO2 molecule as an example, it is shown that the origin of the unconventional shape of an induced absorption band is caused by the combined effect of the anharmonicity of the vibrations and an off-axial perturbation of the molecular electronic cloud.