A quadrupole—quadrupole mechanism for various collision induced transitions within the ground state multiples of I, Tl, Te, Hg and Pb atoms with H 2, HD and D 2 as partners is investigated using accurate vibrational wavefunctions. While this long range mechanism accounts satisfactorily for the rates of those vibronic processes that are nearly resonant, the calculated isotopic rate ratios, e.g. k H 2 / k HD, are often more than two orders of magnitude away from observed values. Other effects that can enhance vibronic quenching are discussed, including the long range convergence of the energy surfaces associated with ground and excited state atoms. The most likely additional effect, however, seems to be a short range coupling and the necessary magnitude of the off-diagonal matrix element is shown to be reasonable. There remains a small group of systems for which energy matching is very poor but which have quenching rates only two or three orders of magnitude smaller than the value expected for perfect energy matching; an entirely different mechanism may operate for these systems, perhaps involving a long lived collision complex.
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Jan 1, 1989
Frank Pacard 
