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

The UV photodissociation dynamics of HI has been studied at 5 photolysis wa velengths, lambda = 222, 248, 258, 266 and 280 nm, by measuring branching f ractions for formation of spin-orbit excited I(P-2(1/2)) atoms using (2 + 1 ) resonance enhanced multi-photon ionization (REMPI) detection of I(P-2(3/2 )) and I(P-2(1/2)). Branching fractions are derived from experimental REMPI signals using an independently established calibration factor, and the res ults are compared with previous determinations based on different H-atom de tection schemes. Discrepancies between these previous measurements for phot olysis wavelengths longer than 220 nm are resolved by comparison with the I -atom REMPI results and by detailed modelling of the H-atom detection exper iments previously performed in this laboratory. The branching fraction for I(P-2(1/2)) peaks at a value of 0.50 +/- 0.05 for wavelengths around 250 nm . (C) 1999 Elsevier Science B.V. All rights reserved.