Probing Excited Electronic States Using Vibrationally Mediated Photolysis:  Application to Hydrogen Iodide

We measure branching ratios and anisotropy parameters for the photolysis of HI(v=2,J=0) → H + I(2P°1/2)/I(2P°3/2) over the wavelength range 297−350 nm. HI is prepared in the v = 2, J = 0 rovibrational level using direct IR absorption, and the H-atom photofragments are probed with resonance enhanced multiphoton ionization coupled with core-extraction time-of-flight mass spectrometry. The new branching ratio measurements disagree with predictions obtained from ab initio calculations and from the results of an empirical analysis based on experimental values of the HI/DI(v=0) absorption cross sections and branching ratios. Thus, we combine our new data with all existing absorption coefficient and branching fraction data for HI/DI in a global analysis that provides a new empirical determination of the final-state potential curves and transition moment functions for the four excited electronic states contributing to the A-band UV absorption continuum of HI. This analysis yields two models for the radial dependence of the excited state potential energy curves and transition dipole moment functions. The existing data cannot differentiate these models, but this work identifies a range of experiments that would do so. More generally, we demonstrate that photodissociation of vibrationally excited molecules is a sensitive probe of the repulsive excited electronic states encountered in bound-free transitions.