Electron transfer/transport at metal-molecule interfaces probed by femtosecond time-resolved two-photon photoemission: Heptane and fullerene on Au(111)

Electron transfer and transport at metal‐molecule interfaces has been a central problem in a number of disciplines. There is renewed interest in this problem from the rapidly growing field of molecule‐based electronics, where the so‐called “contact problem” is usually probed in transport measurements. Here, we probe interfacial electron transfer/transport using time‐resolved two‐photon photo‐emission spectroscopy. Such an approach allows us to quantitatively measure the energetic alignment of occupied and unoccupied molecular orbitals to the metal Fermi level, the electronic coupling strength (or spectral density) between the molecular orbital and the metal substrate, and electronic‐nuclear coupling leading to dynamic localization. These three types of information are in fact key ingredients in quantitative theories for electron transfer and transport at molecule‐metal surfaces. Recent examples from our laboratory are used to illustrate the point. They include femtosecond dynamics of image potential resonances on heptane‐covered Au(111) and exciton relaxation dynamics in C60 epitaxial films on Au(111). On heptane/Au(111), the lifetime of the n = 1 image potential resonance increases from 52 fs at one monolayer (ML) coverage to 700 fs at 2 ML and 2.8 ps at 3 ML. On C60/Au(111), the Frenkel exciton (involving the LUMO+1 level) couples over long distance to the metal substrate, with lifetime of 350 fs at 30 ML to 80 fs at 2 ML coverage. The film‐thickness‐dependent decay dynamics in both cases can be attributed to interfacial electron transfer. Fitting the distance‐dependent electron transfer rates to a simple exponential function gives the characteristic distance parameter of ß = 0.6 ± 0.1 and 0.023 ± 0.005 Å−1 for heptane‐ and C60‐covered Au(111), respectively. The difference in the ß parameters illustrates the essential role of the electronic structure of molecules in mediating interfacial electron transfer/transport.