Photo-induced ultrafast responses of a polycrystalline Pt thin-film immersed in aqueous solutions were investigated by transient reflectivity measurements. Solvent dependence was observed to relate to the affinities solute anions had towards the metal surface. Numerical analysis showed a previously unknown relaxation component having a relaxation time of 20 fs. The experimental results were discussed in terms of an interface reaction between adsorbed solute anions and photoexcited electrons in the Pt film.
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