Distance-dependent electronic coupling at molecule-metal interfaces:: C60/Cu(111)

We probe electronic interaction at molecular- solid/metal interfaces in the model system of C-60/CU(111) using femtosecond two-photon photoemission (2PPE) spectroscopy. The second lowest unoccupied molecular orbital (LUMO+ 1) and the LUMO+2 levels in C-60 are transiently populated via the creation of electronic excitons, with lifetimes in the 10(-14)-10(-13) s region, likely due to self-trapping and/or decay into lower-lying exciton states. These lifetimes decrease as film thickness decreases. The effect is seen for films as thick as 50 Angstrom and is attributed to quenching via charge transfer between the Cu substrate and electronic bands in C-60. The rates of quenching are found to depend exponentially on film thickness, with a beta value of 0.11 +/- 0.02 Angstrom(-1) for both LUMO+1 and the LUMO+2 levels.