Spectra of diatomic molecules
Theory of molecular spectra

Because of the different orders of magnitude between the ranges of rotational, vibrational and electronic energy levels, molecular transitions occur in three distinct regions of the spectrum.

A pure rotational spectrum, corresponding to transitions between rotational energy levels of the same vibrational and electronic state, is found in the microwave region of the spectrum, i.e. from 0.1 mm to 30 cm.

A vibration-rotation spectrum, corresponding to transitions between different vibrational levels of the same electronic state, with a fine structure composed of rotational transitions, is found in the 100 to 1 µm infrared region.

As with atomic transitions, transitions between the different electronic levels take place in the near-infrared, visible and ultraviolet regions.