ON THE UV PHOTODISSOCIATION DYNAMICS OF HYDROGEN IODIDE

The near ultraviolet photodissociation of hydrogen iodide has been inv estigated in detail using the technique of H Rydberg atom photofragmen t translational spectroscopy. Branching ratios for forming ground I(P- 2(3/2)) and spin-orbit excited I(P-2(1/2)) atoms have been determined at 24 different wavelengths in the range 201.0-303.0 nn, whilst detail ed angular distributions for both fragmentation channels have been mea sured at five wavelengths within this range. The branching ratio measu rements confirm previous measures of this ratio at excitation waveleng ths lambda < 250 nm, but at longer wavelengths show a significantly la rger relative yield of spin-orbit excited iodine (I) atoms than hithe rto reported. The ground state H + I products are confirmed as arising as a result of a perpendicular (i.e., Delta Omega = 1) electronic exc itation but, again in contradiction to the conventional wisdom, the an gular distribution measurements show the I atoms to arise almost excl usively (ca. 93%) via a parallel (i.e., Delta Omega = 0) photoexcitati on pathway. The observations are discussed in the light of available i nformation regarding the number and nature of the various repulsive ex cited states contributing to the near UV absorption spectrum of HI. (C ) 1998 Elsevier Science B.V. All rights reserved.