A Theory of Intensity Distribution in Band Systems

A theory of the relative intensity of the various bands in a system of electronic bands is developed by an extension of an idea used by Franck in discussing the dissociation of molecules by light absorption. The theory predicts the existence of two especially favored values of the change in the vibrational quantum numbers, in accord with the empirical facts as discussed by Birge.

A means of calculating the intensity distribution from the known constants of the molecule is presented and shown to be in semi-quantitative agreement with the facts in the case of the following band systems: SiN, AlO, CO (fourth positive group of carbon), ${\mathrm{I}}_2$ (absorption), ${\mathrm{O}}_2$ (Schumann-Runge system), CN (violet system), CO (first negative group of carbon), ${\mathrm{N}}_2$ (second positive group of nitrogen), and ${\mathrm{N}}_2$ (first negative group of nitrogen).

In the case of ${\mathrm{I}}_2$ there is a discrepancy, if Loomis' assignment of $n^{\prime \prime }$ values is used, which does not appear if Mecke's original assignment is used. It is suggested that at least some of the lower levels postulated by Mecke are real but that absorption from them always results in dissociation of the molecule and so they are not represented in the quantized absorption spectrum.