Phonon-kick mechanism for defect reactions enhanced by electronic excitation

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Abstract
Excitation of a localised electron coupled strongly with phonons is followed by vibrational relaxation within the excited electronic state. Phonons emitted during it greatly enhance various reactions of the localised centre such as its migration or destruction, ejection or capture of an electron, and non-radiative de-excitation or transfer of the excitation itself. The driving force of the enhancement, called the phonon kick, is mediated by normal-mode components contained commonly between the energy-accepting coordinate Qp of phonons coupled with the electronic excitation and the reaction coordinate QR. The enhancement can be described by a reduction, during vibrational relaxation, of the thermal activation energy of the reaction from its thermal-equilibrium value. The amount of the reduction depends not only on the total phonon energy deposited in QP upon electronic excitation but also on the direction cosine g between QP and QR in the phonon-coordinate space. The more mod g mod approaches unity the larger the reduction. This describes the selectivity of the phonon kick, compared with no selectivity in the enhancement obtained at elevated temperatures.
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