An unusual example of angular momentum coupling: the dependence of the hyperfine constants of the A state of ICI on internuclear separation

Abstract

Hyperfine Constants for both the iodine and chlorine atoms have been measured for 17 vibrational levels ranging fromv=7 tov=34 (32 for the chlorine atom) of theA3II1 electronic excited state of the ICI molecule. This provides sufficient data to allow the vibrational dependence of the constants to be inverted to give the dependence on internuclear separation. The constants for the iodine atom have an especially strong dependence on internuclear separation, but they change in a fairly continuous fashion. In contrast, the chlorine atom magnetic hyperfine constant drops abruptly from −50 MHz to −20 MHz atr ≈ 3.5 A. In general, the hyperfine data shows that theA state electronic configuration is quite different from theB state, even though in molecular orbital theory these are just two spin orbit states of the same electronic configuration. To probe the nature of the electronic wave function in more detail, a separated atom model is used to fit the data. One property that can be extracted from the data using this model is the orientation of the bonding orbital on each atom as a function of internuclear distance. The iodine orbital orientation varies from 50° atr=2.6 A to 70° atr=5.0 A. The chlorine orbital orientation values fall mostly in the range 46° ± 4°. The LCAO-MO model for theA state predicts that each of the orbitals would be oriented at 45° independent ofr. Although the values for the orbital orientation forr < 3.5 A need to be used with caution, the data provides an usually detailed example of how the electronic wave function depends on internuclear distance.