Intensity Distribution in the Forbidden 1Σ—3IIr Transition of SiO

The intensity distribution in the 1Σ—3II transition of SiO, observed by Verma and Mulliken, is discussed. The appearance of the subband 1Σ—3II2, which violates the selection rule ΔΩ=0, ±1 and the appearance of the Q branch of the 1Σ—3II0 subband, which violates the selection rule that ΔJ=0 is forbidden for Ω=0→Ω=0 transitions, are explained by the mixing of the 1Σ state with a 1II state as a result of the rotation of the molecule. The effect of spin-orbit interaction is considered only insofar as it mixes singlet functions with the multiplet components of 3IIr. Some indication is given as to why this explanation is inapplicable to the a 3II—X 1Σ+ transition of AlCl where the selection rule for Ω is apparently rigorously obeyed.