On the UV photodissociation dynamics of hydrogen iodide Original Research Article

The near ultraviolet photodissociation of hydrogen iodide has been investigated in detail using the technique of H Rydberg atom photofragment translational spectroscopy. Branching ratios for forming ground I(2P3/2) and spin-orbit excited I(2P1/2) atoms have been determined at 24 different wavelengths in the range 201.0–303.0 nm, whilst detailed angular distributions for both fragmentation channels have been measured at five wavelengths within this range. The branching ratio measurements confirm previous measures of this ratio at excitation wavelengths λ<250 nm, but at longer wavelengths show a significantly larger relative yield of spin-orbit excited iodine (I*) atoms than hitherto reported. The ground state H+I products are confirmed as arising as a result of a perpendicular (i.e., ΔΩ=1) electronic excitation but, again in contradiction to the conventional wisdom, the angular distribution measurements show the I* atoms to arise almost exclusively (ca. 93%) via a parallel (i.e., ΔΩ=0) photoexcitation pathway. The observations are discussed in the light of available information regarding the number and nature of the various repulsive excited states contributing to the near UV absorption spectrum of HI.