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A new class of absorption lines in the water vapor spectrum

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The water molecule, as the vast majority of molecular species in the universe, has its infrared rotational-vibrational spectrum composed of electric-dipole transitions. The electric quadrupole absorption lines are about a million times weaker than their dipole counterparts and thus mostly obscured by much stronger and broader dipole lines. Electric-quadrupole infrared transitions are detected in water vapor at room temperature. This is the first direct detection of quadrupole transitions in a polyatomic molecule. Here, using a newly developed, high-accuracy variational approach for theoretical predictions, several weak quadrupole transitions of H2O were identified in a highly sensitive cavity-ring-down spectrum of water vapour near 1.3 µm, in full agreement with the computational predictions.

This new category of absorption lines is missing in current spectroscopic databases. Our results demonstrate that these databases need to be updated for this new class of transitions in order to allow for accurate remote sensing applications. Generally, our discovery opens up new perspectives for most polyatomic molecules, in particular CO2, with potential impact on atmospheric and astronomical sciences.


View online : Observation of electric-quadrupole infrared transitions in water vapor Alain Campargue, Samir Kassi, Andrey Yachmenev, Aleksandra A. Kyuberis, Jochen Küpper, and Sergei N. Yurchenko, Phys. Rev. Research April 2020