Near-ultraviolet photodissociation dynamics of HBr and HI revealed by H (Rydberg) atom photofragment translational spectroscopy

摘要

The near UV photodissociation of hydrogen bromide and hydrogen iodide has been investigated using the technique of H Rydberg atom photofragment translational spectroscopy. Branching ratios for ground and spin-orbit excited halogen atom formation have been measured as a function of excitation wavelength, as have the angular distributions for both fragmentation channels in both hydrogen halides. In the case of HI, the branching ratio measurements - which show a marked excitation wavelength dependence - reinforce previous findings at excitation wavelengths $lambda $LS 250 nm, but at longer wavelengths show larger relative yields of spin- orbit excited iodine (I*) atoms than reported hitherto. The perpendicular nature of the electronic transition yielding ground state H $PLU I products is confirmed but, again in contradiction to previous results, the angular distribution measurements show that the I* atoms arise almost entirely via a parallel photoexcitation pathway. HBr photolysis in the wavelength range 200 $LS $lambda $LS 253 nm yields Br*/Br spin-orbit branching ratios consistently approximately 0.22; as in HI, all ground state products formed in this wavelength range derive from a perpendicular photoexcitation process, but the angular anisotropy of the fragmentation yielding excited Br* atoms shows a marked wavelength dependence. The observations are reviewed in the light of current knowledge of the repulsive excited states contributing to the near UV absorption spectra of the hydrogen halides.