Variation of the electronic wave function with internuclear separation: High-resolution spectroscopy of the A 3Π(1) state of I 35Cl near the dissociation limit

Fluorescence excitation spectra of the A 3Π(1)−X 1Σ+ electronic transition of I 35Cl were acquired using a single-frequency dye laser and a well collimated molecular beam. Data were collected for eleven vibrational levels in the range v′=11–34. Of these, ten were well enough resolved for the hyperfine structure of both nuclei to be observed. For the highest vibrational level studied, v′=34, only the splitting due to the iodine nucleus could be resolved. The measured hyperfine constants for the A state were found to vary significantly with vibrational energy, and were found to be inconsistent with an analysis based on a single electronic configuration [linear combination atomic orbital/molecular orbital (LCAO/MO)]. Instead, it is found that the unpaired electron on the iodine atom is primarily oriented perpendicular to the bond axis for large internuclear separations and only achieves the orientation predicted by the LCAO/MO description for r<3.0 Å. The chlorine orientation is roughly in accord with the LCAO/MO prediction. These conclusions were reached by analyzing the hyperfine constants in a separated atom basis set. Additionally, a perturbation was found in the spectra for the v′=27 level. Evidence is presented that the cause of this perturbation is an interaction with the weakly bound a 3Π1 electronic state of ICl.