Beyond Vibrationally Mediated Electron Transfer: Coherent Phenomena Induced by Ultrafast Charge Separation

TL;DR

This study investigates ultrafast electron transfer from alizarin dye to TiO2, revealing non-vibrational mediated mechanisms and real-time phonon generation, challenging classical electron transfer theories.

Contribution

It demonstrates that ultrafast ET induces vibrational wave packets and phonons, extending molecular ET theories to ultrafast, non-vibrationally mediated processes.

Findings

Electron transfer occurs in about 6 fs, faster than vibrational timescales.

ET process generates vibrational wave packets without laser excitation.

Real-time observation of phonon generation during polaron formation.

Abstract

Wave packet propagation succeeding electron transfer (ET) from alizarin dye molecules into the nanocrystalline TiO2 semiconductor has been studied by ultrafast transient absorption spectroscopy. Due to the ultrafast time scale of the ET reaction of about 6 fs the system shows substantial differences to molecular ET systems. We show that the ET process is not mediated by molecular vibrations and therefore classical ET theories lose their applicability. Here the ET reaction itself prepares a vibrational wave packet and not the electromagnetic excitation by the laser pulse. Furthermore, the generation of phonons during polaron formation in the TiO2 lattice is observed in real time for this system. The presented investigations enable an unambiguous assignment of the involved photoinduced mechanisms and can contribute to a corresponding extension of molecular ET theories to ultrafast ET systems like alizarin/TiO2.