Initial processes of self-trapping of holes and electron-hole pairs in KI and RbI crystals have been investigated by means of a femtosecond pump-probe spectroscopy. We have found that a new short-lived intermediate is formed as a precursor of the self-trapped hole in the form of the halogen molecular ion with a D2h symmetry. This state is identified to be a one-center type self-trapped hole on the basis of the quantum mechanical cluster calculations. A one-center self-trapped state is also created as a precursor for self-trapped excitons.
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