Quadrupole spectra, angular dependence

The angular dependence of the intensities of a quadrupole-split spectrum from a single crystal absorber allows one to determine the line associated with the 3/2- l/2 transition from the other line, and hence the sign of the princi-... 

The electronic states in a molecule, analogs of the atomic states and characterized by total electronic angular momentum and spin, are generally separated in energy by about the same order of magnitude as for isolated atoms, usually several electron volts (eV). Thus the transitions from molecular electronic states, which also correspond to different potentials, are best observed in the ultraviolet (UV) region. Excitation depends on the presence of UV radiation, and electronic transitions are usually seen in absorption, as in the state of H2. The strength of the transition depends on the dipole (heteronuclear molecules and ions) or the quadrupole (homonuclear) moments. Vibrational states dominate the optical and infrared, and rotational states are best observed in the millimeter and centimeter portions of the spectrum. 

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