Beyond vibrationally mediated electron transfer: coherent phenomena in a sub-10-femtosecond reaction regime

This study investigates the systems alizarin as well as coumarin 343 coupled onto TiO₂ nanoparticles with a femtosecond pump/probe setup. The investigation applies sub 20 fs pump pulses, provided by a noncollinear optical parametric amplifier (NOPA) and a supercontinuum generated in CaF₂ for a ultrabroadband detection covering a spectral range from 300-960 nm. This work also studies coherent wavepacket propagation succeeding electron transfer at the two systems alizarin/coumarin on TiO₂. Due to the superfast time scale of the investigated electron transfer (ET) reactions of about 4-7 fs the systems show fundamental differences to conventional molecular ET systems. The ET process is no longer mediated by molecular vibrations and therefore classical molecular ET theories lose their applicability. Results show the real-time coherent excitation of molecular vibrational eigenmodes directly by the ET reaction as well as the generation of phonons during polaron formation in the TiO₂ lattice. The presented investigations in combination with the classification of the appearing mechanisms could be the basis for an extension of molecular ET theories to superfast systems typically found at molecule/solid state interfaces.