| Summary: | Molecular absorption of infrared radiation is generally due to rovibrational 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, rovibrational quadrupole transitions have never been observed. Here, we report the direct detection of quadrupole transitions in water vapor. The detected quadrupole lines have intensity largely above the standard dipole intensity cutoff of spectroscopic databases and thus are important for accurate atmospheric and astronomical remote sensing.
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