The detailed knowledge of atomic and molecular structure has been obtained from spectroscopic studies of the emission, absorption and scattering of electromagnetic radiation accompanying transitions among atomic or molecular energy levels. Whereas atomic spectra involve only electronic transitions, the spectroscopy of molecules is more intricate because vibrational and rotational degrees of freedom come into play as well. Early observations of absorption or emission by molecules were characterized as band spectra in contrast to the line spectra exhibited by atoms. It is now understood that...