The UV photodissociation of HI revisited: REMPI measurements of I(2P) atom spin–orbit branching fractions

The UV photodissociation dynamics of HI has been studied at 5 photolysis wavelengths, λ=222, 248, 258, 266 and 280 nm, by measuring branching fractions for formation of spin–orbit excited I(2P1/2) atoms using (2+1) resonance enhanced multi-photon ionization (REMPI) detection of I(2P3/2) and I(2P1/2). Branching fractions are derived from experimental REMPI signals using an independently established calibration factor, and the results are compared with previous determinations based on different H-atom detection schemes. Discrepancies between these previous measurements for photolysis wavelengths longer than 220 nm are resolved by comparison with the I-atom REMPI results and by detailed modelling of the H-atom detection experiments previously performed in this laboratory. The branching fraction for I(2P1/2) peaks at a value of 0.50±0.05 for wavelengths around 250 nm.