Electronic structure and femtosecond electron transfer dynamics at noble metal/tris-(8-hydroxyquinoline) aluminum interfaces

The electronic structures of tris-(8-hydroxyquinoline) aluminum (Alq₃) on Cu(111) and Au(111) surfaces are studied by using ultraviolet photoelectron spectroscopy and two-photon photoelectron (2PPE) spectroscopy. The work function decreases with increase of the coverage due to surface dipole of 5.1 D along the surface normal. The ionization potential from the highest occupied state 6.38 eV does not depend on the metal substrates used in this study. The anion states of Alq₃ adsorbed is created by photoinduced electron transfer from the metal substrates and are located at 2.85 and 3.71 eV above the Fermi level on Cu(111) and Au(111) surfaces, respectively. The full width at half maximum of the anion states is 0.2 eV on both the surfaces. Time-resolved 2PPE measurements show that the anion state created by electron transfer from the metal decays with the lifetime of 31±2 fs on Cu(111) and about three times shorter on Au(111). The angle-resolved 2PPE and the coverage dependence of the lifetime of the anion state indicate that the electron transferred from the metal surface is localized at a molecule in the first layer. Thus, the ultrafast electron back transfer from the anion state of Alq₃ in the first layer dominates over the electron hopping to the second layer.