Physical assessment of vibrationally induced magnetic-dipole transitions in polyatomic molecules

A weak magnetic-dipole ${\mathrm{CO}}_{2}$ absorption band has been recently identified in the Martian atmosphere. This unprecedented discovery raised a question of whether similar magnetic transitions could be found in other polyatomics as well. The present work proposes a simplified, but physically well-grounded approach, which is intended to help to evaluate the strength of magnetic-dipole transitions induced in the course of molecular vibrations. The suggested theory is developed assuming a pure vibrational magnetic moment arising within a combination band. Our estimate for the ${\ensuremath{\nu}}_{2}+{\ensuremath{\nu}}_{3} {\mathrm{CO}}_{2}$ electric-dipole-forbidden band demonstrates good agreement of the transition matrix element with the value derived previously from both laboratory experiments and in-depth ab initio based consideration. The possibility is shown to observe magnetic transitions owing to a magnetic moment that is linear with respect to momentum. This prediction is discussed taking a molecule of type HCN as an example.