Water Vapor, Water Vapor Everywhere and It All Absorbs in Air

Tweet Elementary quantum mechanics quickly shows that harmonic oscillators (shown by the dotted line in the figure to the right) can only absorb or emit light associated with a transition between neighboring quantum levels such as between v = 0 and 1.  To the extent that molecular vibrational motion is harmonic, this is an absolute rule. Oh yes, there also has to be a change in the dipole moment between the two levels of the transition which explains why homonuclear diatomics (N 2  , O 2 , H 2 ) don’t absorb in the infrared

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Water Vapor, Water Vapor Everywhere and It All Absorbs in Air