Vibrational magnetism and the strength of magnetic dipole transition within the electric dipole forbidden ν2 + ν2 absorption band of carbon dioxide

Abstract

The theory of vibrational magnetism in symmetric linear molecule is reviewed and extended taking an isolated CO molecule as example. The order of magnitude of the magnetic-dipole transition moment is evaluated for the nominally forbidden absorption band. Our consideration is inspired by a recent observation of an unconventional series of spectral lines obeying selection rules in the Martian atmosphere near 3.3 μm [A. Trokhimovskiy, V. Perevalov, O. Korablev, A.A. Fedorova, K.S. Olsen, J.-L. Bertaux, A. Patrakeev, A. Shakun, F. Montmessin, F. Lefevre, and A. Lukashevskaya, Astron. Astrophys. 639, A142 (2020)] assigned to magnetic transitions within absorption band in CO. Here we adduce arguments that the assignment of these lines requires further examination. It is shown that the integrated intensity of band derived from either observation in the Martian atmosphere or in the laboratory controlled conditions exceeds notably the value which might be theoretically expected for the magnetic-dipole transition.