Two related types of high intensity molecular spectra are discussed. These are designated as charge-resonance spectra and charge-transfer spectra. In the molecular orbital approximation, both these types usually involve transfer of one electron from a bonding to the corresponding antibonding molecular orbital. In the atomic orbital approximation, charge-resonance spectra involve a transition from a symmetrical to a corresponding antisymmetrical state, the wave function of each of these two states being a linear combination of two wave functions representing two hypothetical states of a molecule that are equivalent or nearly so but different in charge distribution. Charge-transfer spectra in the atomic orbital approximation involve a transition from a state having a non-ionic to one having an ionic wave function (N→V transition). Theoretical equations for dipole strengths and f values for both these types are given, in each case calculated according to each of the two approximations. The equations are very simple, the dipole strength being proportional to the distance across which the charge resonates or is transferred. Examples of charge-resonance spectra in H2+, O2+, and NO are cited, and N→V (charge-transfer) transitions in the spectra of the molecules H2, O2, C2H4, C6H6, and the halogens are discussed. Although experimental data are scanty, they appear to be in harmony with the theory. For the O2 Schumann-Runge bands, where f is accurately known experimentally, its value is intermediate between the values calculated theoretically according to the two approximations. A suggestion is made to explain Carr and Stücklen's result of progressive broadening toward longer wave-lengths in the N→V spectrum as radicals are substituted for H atoms in C2H4. The 1A1g→1E1u transition in C6H6 calculated by Sklar and identified by him with bands near and below λ2000 is here concluded to be of the N→V class. In connection with the theory of light absorption in organic compounds, it will be shown in later papers that intense transitions in the visible and ordinary ultraviolet probably can be accounted for in a large proportion of cases (including dyes) as charge-transfer (N→V) spectra, or, in part, as charge-resonance spectra. The existence of spectra of the charge-resonance type in certain kinds of dyes has already been recognized and very briefly discussed by Pauling in the new edition of Gilman's Organic Chemistry.

Mulliken, R. S. (1939). Intensities of Electronic Transitions in Molecular Spectra II. Charge-Transfer Spectra. The Journal of Chemical Physics, 7(1), 20–34.