Abstract
Molecular absorption of infrared radiation is generally due to ro-vibrational electric-dipole transitions. Electric-quadrupole transitions may still occur, but they are typically a million times weaker than electric-dipole transitions, rendering their observation extremely challenging. In polyatomic or polar diatomic molecules, ro-vibrational quadrupole transitions have never been observed. Here, we report the first direct detection of quadrupole transitions in water vapor. The detected quadrupole lines have intensity largely above the standard dipole intensity cut-off of spectroscopic databases and thus are important for accurate atmospheric and astronomical remote sensing.
Highlights
Molecular absorption of infrared radiation is generally due to rovibrational electric-dipole transitions
Electric-quadrupole transitions are used for remote sensing of important diatomic molecules, such as hydrogen, oxygen, and nitrogen, in spectra of Earth’s atmosphere and other environments [4,6,7]
In practice the detection of quadrupole lines is made difficult by the fact that, even if transition frequencies differ, the quadrupole lines are drowned in the line profile of the much stronger dipole lines
Summary
Molecular absorption of infrared radiation is generally due to rovibrational electric-dipole transitions. Quadrupole electronic and rovibrational transitions have been detected in a number of atoms [1,2] and homonuclear diatomic molecules [3,4,5], respectively, for states that cannot undergo electric dipole transitions. In most heteronuclear diatomic and polyatomic molecules the rovibrational states can be excited both via dipole and quadrupole transitions.
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