The gaps between the bands define the infrared window in the Earth's atmosphere. Standard (atmospheric optical) codes are assigned to absorption bands as follows. ![]() The presence of water vapor in the atmosphere is important for atmospheric chemistry especially as the infrared and near infrared spectra are easy to observe. Combination bands, such as ν 2 + ν 3 are also easily observed in the near infrared region. ν 3 has a series of overtones at wavenumbers somewhat less than n ν 3, n=2,3,4,5. In the gas phase all three bands show extensive rotational fine structure. Both symmetric stretching and bending vibrations have A 1 symmetry, but the frequency difference between them is so large that mixing is effectively zero. ![]() The asymmetric stretching vibration, of B 2 symmetry in the point group C 2v is a normal vibration. The O-H stretching vibrations give rise to absorption bands with band origins at 3657 cm −1 (ν 1, 2.734 μm) and 3756 cm −1 (ν 3, 2.662 μm) in the gas phase. The water molecule has three fundamental molecular vibrations.
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