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

Abstract

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.

The role of the van der Waals interactions in the adsorption of anthracene and pentacene on the Ag(111) surface

“…The large reduction in the work function is due to the push-back effect arising from the Pauli repulsion between the electron density of the molecule and the surface electrons, which is dominant in physisorbed systems. We notice that our PBE and PBE+sc-vdW surf results for the change in the work function (−0.81 and −1.04 eV, respectively) are significantly larger than experimental values obtained by measuring the onset current into the sample, −0.5 ± 0.1 eV 5 , and by determining the photoemission kinetic energy width, −0.68±0.02 eV 44 . It should be mentioned, however, that the experimental data available were obtained for multilayer anthracene adsorption and/or for lower molecular density.…”

Two-photon Photoemission of Organic Semiconductor Molecules on Ag(111)

“…The binding energies of the image states are 0.69 ± 0.10 eV and 0.19 ± 0.10 eV for the n = 1 and n = 2 states, respectively, which corresponds to a quantum defect of 0.11. This value of the quantum defect is similar to other planar aromatic hydrocarbons [12]. The low kinetic energy shoulder has a binding energy of 0.98 eV, and the small feature present in both uv- Referenced to the Fermi level E F using:…”

“…The n = 2 IPS is theoretically at a greater distance from the surface than n = 1, and thus harder to populate from the metal if thicker coverages have been applied. The shift in vacuum level is similar to vacuum level shifts for naphthalene [12] and benzene [10].…”

“…Gaffney et al [12] reported hybridized molecular/IPS states of naphthalene on Ag(111) and presented a systematic study of binding energy and lifetime trends with increasing coverages of linear acenes [10,12]. A localized state in a benzene multilayer was attributed to structural disorder.…”

Temperature and Coverage Dependent Quasi-reversible Two-photon Photoemission of 1-phenyl-1-propyne on Cu(111)

“…Two-photon photoemission spectroscopy (2PPE) has been widely employed to investigate the interfacial energy levels and the electronic couplings at molecules/metal, however, the metal-UMO resonance has been debated because the resonance is absent in most cases. 1,2,[7][8][9] Motivated by this, we further delve into this issue by investigating the 1-phenyl-1-propyne/Cu(111) system by means of 2PPE, ultraviolet photoelectron spectroscopy (UPS) and temperature-programmed desorption (TPD) spectroscopy.…”