Interpretation of the Methyl Iodide Absorption Bands Nearλ2000

Abstract

The theory of the types and structures of bands in an electronic transition in an axially symmetrical molecule of symmetry $C_{3v}$ is discussed. This is applied to the methyl iodide $B$ band system near $\lambda 2000$, and it is shown that the two types of bands observed (strong and weak bands of pseudo-parallel type, and weak bands looking like perpendicular bands but of abnormally wide spacing) can both be explained. The pseudo-parallel bands are really electronic-allowed perpendicular bands, the narrowness of their structure resulting from an electronic angular momentum ${\zeta }_e$ in the upper state which we find from the data to be equal to 1.0. This ${\zeta }_e$ is explained by the electronic structure of the upper state, which behaves nearly like a II state of a linear molecule. The weak widely-spaced bands are another kind of perpendicular bands made allowed by an interaction of the degenerate electronic state with a degenerate ($e$-type) vibration; they are 1←0 and 0←1 bands of such a vibration, and of a second similar vibration. Their spacing is approximately $1+{\zeta }_e+{\zeta }_v$ (${\zeta }_v$ is the magnitude of the angular momentum of the degenerate vibration which is excited), as contrasted with a spacing $1-{\zeta }_e$ for the pseudoparallel bands (which correspond to the excitation, at most, only of totally-symmetrical vibrational states). Several molecular constants for the ground and excited electronic states are determined from the observational data (cf. Table I). The moment of inertia for rotations around the symmetry axis is found to be about 6 percent greater in the excited state than in the ground state. The theory of the coupling of spin, orbital angular momentum, and molecular vibration for various strengths of spin-orbit coupling is discussed. The actual nearly $J$, $j$-like coupling causes the 0←1 band of the $e$ vibration to appear with a relatively low intensity compared with the 1←0 band of the same vibration.