1. Department of Physics, Uppsala University, Box 530, Uppsala University, Box 532, 75121 Uppsala, Sweden
2. Department of Physical Chemistry, Uppsala University, Box 532, 75121 Uppsala, Sweden
3. Swiss Light Source, Paul-Scherrer-Institut, 5232 Villigen-PSI, Switzerland
4. MAX-Lab, University of Lund, Box 118, 22100 Lund, Sweden

Correspondence to: Paul A. Brühwiler¹ Correspondence and requests for materials should be addressed to P.A.B. (e-mail: Email: paul.bruhwiler@fysik.uu.se).

The ultrafast timescale of electron transfer processes is crucial to their role in many biological systems and technological devices. In dye-sensitized solar cells^{1, 2, 3, 4}, the electron transfer from photo-excited dye molecules to nanostructured semiconductor substrates needs to be sufficiently fast to compete effectively against loss processes and thus achieve high solar energy conversion efficiencies⁴. Time-resolved laser techniques indicate an upper limit of 20 to 100 femtoseconds^{5, 6, 7, 8, 9} for the time needed to inject an electron from a dye into a semiconductor, which corresponds to the timescale on which competing processes such as charge redistribution^{10, 11} and intramolecular thermalization of excited states^{12, 13, 14} occur. Here we use resonant photoemission spectroscopy, which has previously been used to monitor electron transfer in simple systems with an order-of-magnitude improvement in time resolution^{15, 16}, to show that electron transfer from an aromatic adsorbate to a TiO₂ semiconductor surface can occur in less than 3 fs. These results directly confirm that electronic coupling of the aromatic molecule to its substrate is sufficiently strong to suppress competing processes¹⁷.