Rotation spectra result from radiation by molecules having intrinsic electric dipole moments upon undergoing a change in rotational quantum number. Rotational transitions have small energy, as so occur in the far infrared (1 mm) to microwave (1 cm) regions of the electromagnetic spectrum.
The rotational energy of a molecule in quantum state J is given by
where L is the rotational angular momentum, I is the moment of inertia, for J = 0, 1, 2, ... is the
rotational quantum number, and is h-bar. The quantum mechanical selection rule for rotation is
References
Burshtein, A. I. and Temkin, S. I. Spectroscopy of Molecular Rotation in Gases and Liquids. ISBN: 0521454654.
Kroto, H. W. Molecular Rotation Spectra. New York: Dover, 1992.
