Interfacial quantum well states of Xe and Kr adsorbed on Ag(111)

Femtosecond Trapping of Free Electrons in Ultrathin Films of NaCl on Ag(100)

We report the excited-state electron dynamics for ultrathin films of NaCl on Ag(100). The first three image potential states (IPSs) were initially observed following excitation. The electrons in the spatially delocalized n = 1 IPS decayed on the ultrafast time scale into multiple spatially localized states lower in energy. The localized electronic states are proposed to correspond to electrons trapped at defects in the NaCl islands. Coverage and temperature dependence of the localized states support the assignment to surface trap states existing at the NaCl/vacuum interface. These results highlight the importance of electron trapping in ultrathin insulating layers.

Probing of an adsorbate-specific excited state on an organic insulating surface by two-photon photoemission spectroscopy

In this study, we investigate the photoexcited electronic states of ferrocene (Fc) molecules adsorbed on an organic insulating surface by two-photon photoemission spectroscopy. This insulating layer, composed of a decanethiolate self-assembled monolayer formed on an Au(111) substrate, enables us to probe the electronically excited states localized at the adsorbed Fc molecules. The adsorbate-specific state is resonantly excited by photons at 4.57 eV, which is 0.5 eV smaller than the energy of the first molecular Rydberg state of free Fc in the gas phase. This result indicates that the electrons are bound to both the excited hole formed in the adsorbate and the positive image charge induced in the substrate. The hybridized electronic characteristics of the adsorbate-specific state are responsible for the strong transition selectivity and short lifetime of the excited state.

Noble-gas resonant radiation effects on electron emission in plasma devices

Experimental investigation results for photoemission affected by vacuum ultraviolet radiation of xenon and krypton atoms from a solid in vacuum and the target surface in contact with plasma (gas) are presented. It is demonstrated that, for adsorption (or implantation) of gas atoms into the target, the photoemission response considerably (to an order of magnitude) increases. This is caused by a change in the mechanism of photoemission from a solid surface in contact with plasma (gas), as compared to vacuum. This phenomenon can be characterized by the term adsorption- (implantation-) induced resonant photoemission. The inclusion of this phenomenon has largely transformed our view of gas discharge ignition and glowing, in addition to operating a variety of plasmic and photoelectron devices. A different class of gas discharge instruments can be realized on this basis as well.

Dispersion and localization of electronic states at a ferrocene/Cu(111) interface

Low-temperature scanning tunneling microscopy and spectroscopy combined with first-principles simulations reveal a nondissociative physisorption of ferrocene molecules on a Cu(111) surface, giving rise to ordered molecular layers. At the interface, a 2D-like electronic band is found, which shows an identical dispersion as the Cu(111) Shockley surface-state band. Subsequent deposition of Cu atoms forms charged organometallic compounds that localize interface-state electrons.

Delocalized electron resonance at the alkanethiolate self-assembled monolayer∕Au(111) interface

We probe the electronic structure of alkanethiolate self-assembled monolayers (SAMs) on Au(111) using two-photon photoemission spectroscopy. We observe a dispersive unoccupied resonance close to the vacuum level with a lifetime shorter than 30 fs. The short lifetime and the insensitivity of the energy level and dispersion to molecular length (and thus layer thickness) suggest that the probability density of the electron wave function is concentrated inside the molecular layer close to the SAM/Au interface. Such an interfacial resonance results from the image like potential at the SAM/Au interface.

Using image resonances to probe molecular conduction at the n-heptane∕Au(111) interface

The binding energies and lifetimes of the n=1 image resonance on Au(111) are measured as a function of n-heptane layer thickness by femtosecond time-resolved two-photon photoemission (TR-2PPE) spectroscopy. The lifetime of the image resonance dramatically increases from approximately 4 fs on clean Au(111) to 1.6 ps with three layers of n-heptane. Because the image resonance is above the L1 band edge of Au, this increase in lifetime is attributed to the tunneling barrier presented by the sigma-sigma* band gap of the n-heptane film. We use the one-dimensional dielectric continuum model (DCM) to approximate the surface potential and to determine the binding energies and the lifetimes of the image resonances. The exact solution of the DCM potential is determined in two ways: the first by wave-packet propagation and the second by using a tight-binding Green's function approach. The first approach allows band-edge effects to be treated. The latter approach is particularly useful in illustrating the similarity between TR-2PPE and conductance measurements.

Electron transmission through organized organic thin films

Low-energy electron photoemission spectroscopy (LEPS) allows the study of the electronic properties of organized organic thin films (OOTF) adsorbed on conducting surfaces by monitoring the energy and angular distribution of electrons emitted from the substrate and transmitted through the film. The transmission properties are explained by electronic band structure in the organic film. This band is an example of an electron resonance that is delocalized in the layer. It results from the two-dimensional nature of the layer. Other resonances in the transmission spectra are also discussed, as well as their experimental manifestation. The temperature dependence of the electron transmission efficiency is explained in terms of dependence of the transmission probability on the initial momentum of the electron and on the relative orientation of the electron velocity and the molecules in the film. Hence, despite the fact that the molecules in the OOTF are weakly interacting, when not charged, the electron transmission through the film is governed by cooperative effects. These effects must be taken into account when considering electronic properties of adsorbed layers.

Effect of an atomically thin dielectric film on the surface electron dynamics: image-potential states in the Ar/Cu(100) system

The effect of an atomically thin Ar layer on the image-potential states on Cu(100) surfaces is studied in a joint experimental-theoretical study, allowing a detailed analysis of the interaction between a surface electron and a thin insulator layer. A microscopic theoretical description of the Ar layer is developed based on mutually polarizing Ar atoms. Account of the 3D Ar layer structure allows one to predict energies and lifetimes of the image states in excellent agreement with the observations. The Ar layer, even as thin as one monolayer, is efficiently insulating the state from the metal.

Electron transfer at molecule-metal interfzces: a two-photon photoemission study

Electron transfer between a molecular resonance and a metal surface is a ubiquitous process in many chemical disciplines, ranging from molecular electronics to surface photochemistry. This problem has been probed recently by two-photon photoemission spectroscopy. The first photon excites an electron from an occupied metal state to an unoccupied molecular resonance. Subsequent evolution of the excited electronic wavefunction is probed in energy, momentum, and time domains by the absorption of a second photon, which ionizes the electron for detection. These experiments reveal the important roles of molecule-metal wavefunction mixing, intermolecular band formation, polarization, and localization in interfacial electron transfer.

Femtosecond studies of electron dynamics at interfaces

A delocalized electron at a metal-dielectric interface interacts with the adlayer and spatially localizes or self-traps on the femtosecond time scale into what is termed a small polaron. The dynamics can be studied by two-photon photoemission. Theoretical and experimental analyses reveal the interaction energy and the lattice vibrational mode that mediates electron localization. These results contribute to a fundamental understanding of electron behavior in weakly bonded solids and can lead to a better understanding of carrier dynamics in many different systems, including organic light-emitting diodes.

FEMTOSECOND DYNAMICS OF ELECTRONS ON SURFACES AND AT INTERFACES

Two-photon photoemission is a promising new technique that has been developed for the study of electron dynamics at interfaces. A femtosecond laser is used to both create an excited electronic distribution at the surface and eject the distribution for subsequent energy analysis. Time- and momentum-resolved two-photon photoemission spectra as a function of layer thickness fully determine the conduction band dynamics at the interface. Earlier clean surface studies showed how excited electron lifetimes are affected by the crystal band structure and vacuum image potential. Recent studies of various insulator/metal interfaces show that the dynamics of excess electrons are largely determined by the electron affinity of the adsorbate. In general, electron dynamics at the interface are influenced by the substrate and adlayer band structures, dielectric screening, and polaron formation in the two-dimensional overlayer lattice.