Isotope effects in the quenching of electronically excited atoms

Spin-orbit relaxation of I(52Psub(1/2)) by the molecules H2O, D2O, HCl, DCl, HBr, DBr, HI and DI has been studied using time resolved atomic absorption spectrophotometry (lambda = 206nm; T = 293 K). It is concluded that quenching by the two most efficient molecules, H2O and HBr, proceeds via resonant energy transfer channels (electronic → vibration + rotation) probably involving quadrupole-dipole coupling. However, the small isotope effects and relatively large cross-sections observed for the other molecules suggest that an efficient curve crossing mechanism dominates for HCl, DCl, DBr, HI and DI. This is consistent with other recent work involving Br(42)Psub(1/2)) and the hydrogen halides. (author)