Quantum beats at the metal/organic interface

Time resolved two-photon photoemission (TPPE) is used to probe the unoccupied electronic structure of monolayer films of dicarbonitrile-quaterphenyl (NC-Ph4-CN) on Ag(1 1 1) and cobalt phthalocyanine (CoPc) on Ag(1 0 0). For both samples, photoelectron spectra show a well-formed series of electronic states near the vacuum level. These are assigned as the 1 ≤ n ≤ 4 image-potential states (IPS's) based on the scaling of their binding energies and lifetimes. Time domain measurements at energies near the vacuum level exhibit intensity oscillations (quantum beats) which are due to excitation of an electronic wave packet of the n ≥ 4 IPS's. The wave packets remain coherent until population decay renders them unobservable. These measurements clearly demonstrate that the classical image-potential state structure is retained to high order (n ∼ 6) in the presence of aromatic organic adlayers. This represents the first definitive observation via TPPE of quantum beats of electronic origin at the metal/organic interface.