Ultrafast lattice relaxation dynamics of photo-excited states in halogen-bridged mixed valence metal complexes

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Ultrafast relaxation dynamics of quasi-one-dimensional halogen-bridged mixed-valence complexes (MX-compounds) has been investigated by the femto-second transient absorption measurements. In Pt-compounds, generation of vibrational excited states of self-traped excitons (STE's) and those of polarons and their relaxation processes to the meta-stable satates have been observed. On the other hand, it was found that the neutral solitons are generated within the time resolution (150fs). It is considered to be due to the dissipative nature of the potential surfaces. It was found that the time constants of the relaxation to the meta-stable states of STE's and the decay of times meta-stable STE's become shorter as the halogen ions were replaced in the order Cl, Br, and I in Pt-complexes. The generation efficiency of polarons has bee found to increases in the same order. These results are considered to be due to the increment of transfer energy and to the decrement of electron-phonon interaction. The generation efficiency of vibrational excited states of polarons was found to increase when the platinum ions were replaced by palladium ions. This is considered to be due to the increment of electron correlation.Vibrations of wave packet motion of photo-generated polarons and those of STE's in the potential surfaces around the meta-stable states have been found in a Pt-I-compound.Intense two-photon absorption caused by charge transfer excitons with odd parity has been found in MX-compounds. The response time was found to be within the time resolution.