Energy partitioning and assignment of excited states in the ultraviolet photolysis of HI and DI

Abstract

Photofragment spectroscopy has been applied to the photolysis of hydrogen iodide and deuterium iodide at 37 550 cm⁻¹ (266.2 nm). The extent of excited I(²P_1/2) atom production and the symmetry of the excited molecular states leading to dissociation are derived by measuring the translational energy and angular distributions of the recoiling H and D atoms. It is found that for HI, 36%±5% of the I atoms are formed in their excited state, while for DI the fraction is 26%±3%. In both cases, the transition leading to excited atoms is parallel, agreeing with Mulliken’s spectral interpretation. These results, together with measurements reported by others at different wavelengths, are used to analyze the first absorption continuum of HI and DI. This continuum is found to be consistent with overlapping transitions to ³Π₁, ³Π_0⁺, and ¹Π excited states, as predicted by Mulliken. A nonlinear curve fitting technique is applied to the available data to separate each state’s contribution to the total continuum absorption and to provide estimates of the shape of each excited state potential energy curve.