  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.
