Femtosecond Dynamics of Electrons Photoinjected into Organic Semiconductors at Aromatic-Metal Interfaces

The layer dependent evolution of the unoccupied electronic structure and electron dynamics at the naphthalene/Ag(111) and the anthracene/Ag(111) interfaces have been investigated with femtosecond time and angle resolved two photon photoemission. With the exception of the peaks observed for the naphthalene monolayer, all excitations in the two photon photoemission spectra fit a hydrogenic progression, substantiating their assignment as image potential states. The monolayer excitations for naphthalene cannot be assigned as either image potential states or electron affinity (EA) levels, but rather as hybridized EA/image potential states. The binding energies and lifetimes of the image potential states for naphthalene and anthracene exhibit two significant differences that demonstrate the tremendous variation in the coupling between the image potential and the EA levels of naphthalene and those of anthracene. First, the binding energies at the naphthalene/Ag(111) interface exceed those of the anthracene/Ag(111) interface, even though anthracene has a larger EA than naphthalene. Second, the 1.1 ps lifetime for the n = 1 image potential state for a bilayer of anthracene exceeds the n = 1 lifetime for a bilayer of naphthalene by a factor of 30. Theoretical calculations demonstrate that the transition from a near resonant to a nonresonant interaction between the image potential and the adsorbate EA levels causes these significant variations in binding energies and lifetimes.