Radiationless transitions in a new light

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Abstract
Radiationless transitions in polyatomic molecules prove to be quite amendable to a semiclassical treatment both below and above crossings between the potential surfaces involved in the transition. Below such crossings, tunneling integrals are easily performed which give good estimates of the dependence of the nonradiative rate on the energy gap and excess energy in the electronic state. Above the surface crossing, the transitions become classically allowed and a Tully–Preston surface hopping model suffices. We find that a nonlinear dependence of ln(knr) vs E plots is the rule rather than the exception. The ln(knr) vs E plots tend to flatten out with increasing energy. This effect can occur below surface crossings, but is most dramatic when a surface crossing is reached. The recent beam results of Smalley and co‐workers on pyrazine and pyrimidine are seen to be a possible case of this simple behavior.