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We study the role of electronic interface states on the electron transfer dynamics between layers of the organic semiconductor 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) and the (111) and (100) surfaces of silver. For this purpose, we investigate the change of the decay dynamics of the first (n = 1) image-potential state on these surfaces upon adsorption of an ordered monolayer of PTCDA by means of time-resolved two-photon photoemission (2PPE). We find that the already short lifetime of the (n = 1)-state on Ag(111) is only slightly further reduced by PTCDA adsorption, whereas a much stronger reduction by a factor of three is observed for adsorption on Ag(100) resulting in similar lifetimes for both orientations. We show by model calculations on the basis of an analytical one-dimensional pseudo-potential that the enhanced decay for PTCDA/Ag(100) can be attributed to the opening of an additional channel for electron-electron scattering by the formation of an interface state which is derived from the Shockley-type surface resonance of Ag(100). [ABSTRACT FROM AUTHOR]

We report on the direct mapping of electron transfer in the momentum space of bulk MoS2 by means of time- and angle-resolved two-photon photoemission with a high-harmonic probe. For this purpose, we have combined a high-repetition rate high-harmonic source with tunable femtosecond pump pulses and a 3D (kx, ky, E) electron spectrometer. We show that optical excitation slightly above the A exciton resonance results in an immediate occupation of the conduction band at ... followed by an ultrafast transfer (<50 fs) to the conduction band minimum at ... Both signals, at ... and ..., do not vanish over the observed period of 400 fs. The technique described here enables direct access to the charge transfer dynamics in k-space and allows the study of decay times and decay channels in various systems with dependence on the excess energy or helicity of the excitation. [ABSTRACT FROM AUTHOR]

Buried interface states in Ar/Cu(100) were studied by means of one- and two-photon photoemission experiments. With increasing Ar overlayer thickness, a transition from broad electron scattering resonances in the Ar conduction band into a hydrogen-like series of quasi-bound states at the Ar/Cu interface was observed. The thickness dependence of energies and lifetimes is compared to theoretical resonance positions and linewidths derived from a parameterized one-dimensional potential. [ABSTRACT FROM AUTHOR]

We describe an interferometric time-resolved photoemission technique that makes it possible to simultaneously observe the decay of optical induced polarizations and populations at surfaces in a two-color excitation scheme. In this scheme initially unoccupied electronic surface states are coherently excited by the interaction of laser pulses with frequency ωa and the two-photon polarization which is induced by laser pulses with frequency ωa/2. Interference is observed by changing the delay between both laser pulses using an actively stabilized two-color Mach–Zehnder interferometer. We demonstrate this technique for excitation of the n=1 image-potential state on a Cu(100) surface. [ABSTRACT FROM AUTHOR]

Abstract: Image-potential states have been investigated widely by time-resolved two-photon photoemission (2PPE) as a model system to study electron dynamics at metal surfaces. Rare-gas layers, which represent a prototype for dielectric overlayers, can modify the coupling of these states to the bulk metal in a controlled way. It will be shown that binding energies and lifetimes for inelastic decay depend in a straightforward way on the electron affinity of the rare gas and the layer thickness. Many of the experimental results can be accounted for in a simple one-dimensional description with a potential that only varies perpendicular to the surface. For rare-gas layers of sufficient thickness new electronic states arise from the screened image-potential of the metal within the adlayer. These states have similar properties as the image-potential states on clean surfaces, but are spatially located at the dielectric/metal interface and can have energies above the vacuum level. We discuss the origin and the basic properties of these previously unexplored interface states for the system Ar/Cu(100). We will show how time-resolved 2PPE can be used to study their decay by resonant tunneling through the layer into vacuum and by electron–hole-pair decay into the metal. [Copyright &y& Elsevier]

Image-potential states in front of a clean Cu (100) surface were investigated by time- and angle-resolved two-photon photo-emission (2PPE). We observe a previously unknown quasi-elastic relaxation channel, which efficiently couples states with different quantum numbers, n, and parallel momenta, k[sub ||] . This process of resonant interband scattering (RIS) is independent of sample temperature and shows a close relationship to the pure dephasing of image-potential states. [ABSTRACT FROM AUTHOR]

Abstract. Ultrafast electronic coherence phenomena of metallic image-potential states and of silicon dangling-bond states are discussed. The image states were investigated on a Cu(100) surface by means of time-resolved two-photon photoemission (2PPE). Spectroscopical and dynamical information about states with high quantum numbers (n Is greater than or equal to 4) were obtained by the coherent excitation of several eigenstates of the Rydberg series and detection of the resulting quantum beats. Electron wave packets that describe the quasi-classical periodic motion of weakly bound electrons at metal surfaces could be created by the superposition of several eigen-functions around n = 7. A surface-sensitive purely optical technique is introduced for the investigation of rapid dephasing processes of excited dangling-bond states of Si(111)7 x 7. The measurement of the diffracted second-harmonic signal from a transient population grating yielded decoherence times of about 5 fs. [ABSTRACT FROM AUTHOR]

Abstract. The dynamics of electrons in image-potential states has been studied by means of time- and energy-resolved two-photon photoemission. The loss of coherence in quantum-beat spectroscopy shows directly the influence of pure dephasing which can be determined also from the difference between the results of linewidth and decay rate measurements. We present several examples for the correlation of decay and dephasing with surface defects and disorder. [ABSTRACT FROM AUTHOR]

Image-potential states have been studied by two-photon photoemission for the surface of Al(100) where the whole series is energetically degenerate with free-electron-like bulk states. In contrast with expectations, the series of resonances is not smeared out to one broad structure as a result of a strong coupling to the bulk continuum. Instead, the first resonance (n = 1) is found to be suppressed, and the resonances with quantum numbers n = 2, 3, 4, 5 are resolved as individual peaks in the time-resolved spectra. Both effects are suggested to be a consequence of resonance trapping. [ABSTRACT FROM AUTHOR]

Time-resolved two-photon photoemission experiments performed on the Cu(1 0 0) surface with one or several layers of argon reveal image state lifetimes unusually long compared with those obtained for the clean surface. The interaction of the image state electron with the metal surface is thus dramatically altered by the adsorbed layers, which induce a decrease in the overlap between the image state wave function and the bulk states. We report a calculation of the n=1, 2 image-potential state lifetimes in the Ar/Cu(1 0 0) system, obtained within the GW approximation of many-body theory. Image state wave functions are calculated from a one-dimensional model potential which describes the surface band structure and incorporates the modified interaction in the adlayer region. Results obtained from this model are in fairly good agreement with the experimental data. [Copyright &y& Elsevier]

Decoupling of image-potential states from bulk Cu(1 0 0) by Ar spacer layers has been investigated by time-resolved two-photon photoemission. Ordered films with thicknesses of up to five monolayers have been prepared that lead to a lifetime increase of almost three orders of magnitude. It is also demonstrated that continuum models fail to give a satisfactory description of the adlayer electronic structure. [Copyright &y& Elsevier]

Frontier orbitals determine fundamental molecular properties such as chemical reactivities. Although electron distributions of occupied orbitals can be imaged in momentum space by photoemission tomography, it has so far been impossible to follow the momentum-space dynamics of a molecular orbital in time, for example, through an excitation or a chemical reaction. Here, we combined time-resolved photoemission using high laser harmonics and a momentum microscope to establish a tomographic, femtosecond pump-probe experiment of unoccupied molecular orbitals. We measured the full momentum-space distribution of transiently excited electrons, connecting their excited-state dynamics to real-space excitation pathways. Because in molecules this distribution is closely linked to orbital shapes, our experiment may, in the future, offer the possibility of observing ultrafast electron motion in time and space. [ABSTRACT FROM AUTHOR]

We describe the design and operation of a time-of-flight electron spectrometer which is capable of simultaneously acquiring the energy and momentum distribution of low-energy photoelectrons in two dimensions parallel to the surface. We discuss its capabilities and limitations in particular for time- and angle-resolved two-photon photoemission (2PPE) with pulsed lasers. The performance of the spectrometer is demonstrated by presenting 2PPE data on the momentum-dependent electron dynamics of the first ( n = 1) image-potential state on Cu(0 0 1). The data reveal a weak but systematic dependence of the decay dynamics on sample azimuth with one-fold symmetry which we attribute to a small residual step density on the nominal flat surface. [ABSTRACT FROM AUTHOR]

Time- and angle-resolved two-photon photoemission (2PPE) was used to study the electronic structure and ultrafast electron dynamics of the p-doped topological insulator Sb2Te3 and its derivative Sb2Te2S. Our 2PPE experiments directly reveal that the massless Dirac-cone like energy dispersion of topological surface states is realized above the Fermi energy in both materials. The observed bulk conduction bands of Sb2Te2S are found to be shifted to higher energies as compared to Sb2Te3. This shift has, however, surprisingly almost no influence on the electron dynamics in the topological surface state, which proceed on a picosecond time scale. [ABSTRACT FROM AUTHOR]

The unoccupied electronic states of epitaxially grown graphene on Ru(0001) have been explored by time- and angle-resolved two-photon photoemission. We identify a Ru derived resonance and a Ru/ graphene interface state at 0.91 and 2.58 eV above the Fermi level, as well as three image-potential derived states close to the vacuum level. The most strongly bound, short-lived, and least dispersing image-potential state is suggested to have some quantum-well character with a large amplitude below the graphene hills. The two other image-potential states are attributed to a series of slightly decoupled states. Their lifetimes and dispersions are indicative of electrons moving almost freely above the valley areas of the moire superstructure of graphene. [ABSTRACT FROM AUTHOR]

The dynamics of electrons in image-potential resonances on the Ag(111) surface, i.e., image-potential states that are resonant with bulk bands, have been studied by time-resolved two-photon photoemission in combination with quantum-beat spectroscopy. Energies and lifetimes of these resonances were determined up to a quantum number n = 7. Both quantities show a hydrogen-like scaling with quantum number n. The measured decay time of the first image-potential state (n = 1), which is still located in the projected band gap, is 31.5(1.5) fs. The decay time decreases to 23(2) fs for the resonance n = 2 and then increases up to 1 ps for n = 7. It is concluded that the elastic decay of the resonances due to elastic electron transfer into the bulk is nearly one order of magnitude faster than the inelastic electron-hole pair decay. Nevertheless, the elastic decay is found to be slower than theoretically predicted. The pure dephasing times of the resonances n = 3-7 are longer than the decay times. This suggests that electron-phonon scattering is weak despite a large bulk penetration of the resonance wave functions. [ABSTRACT FROM AUTHOR]

The properties of Shockley-type interface states between ir-conjugated organic molecular layers and metal surfaces are investigated by time-resolved two-photon photoemission experiments and density functional theory. For perylene- and naphthalene-tetracarboxylic acid dianhydride (PTCDA and NTCDA) adsorbed on Ag(111), a common mechanism of formation of the interface state from the partly occupied surface state of the bare Ag(111) is revealed. The energy position is found to be strongly dependent on the distance of the molecular carbon rings from the metal and their surface density. Bending of the carboxyl groups enhances the molecular overlap of the interface state. [ABSTRACT FROM AUTHOR]

Recent experimental work has reached an accurate determination of surface-state linewidth by scanning tunneling spectroscopy, photoemission and directly in the time domain by two-photon photoemission. The concomitant progress in the theoretical description of surface-state decay has led to an excellent agreement between theory and experiment for low-index noble-metal surfaces. The understanding of the behavior of s–p-like surface states and image-potential states on these surfaces constitutes the basis for the application to more complicated systems. [Copyright &y& Elsevier]

The theoretical modeling of metal-organic interfaces represents a formidable challenge, especially considering the delicate balance of various interaction mechanisms and the large size of the involved molecular species. In the present study, the energies of interface states, which are known to display a high sensitivity to the adsorption geometry and electronic structure of the deposited molecular species, have been used to test the suitability and reliability of current theoretical approaches. Two well-ordered overlayer structures (relaxed and compressed monolayers) of 1,4,5,8-naphthalene-tetracarboxylic acid dianhydride (NTCDA) on Ag(111) have been investigated using two-photon photoemission to derive precise interface-state energies for these closely related systems. The experimental values are reproduced by our density-functional theory (DFT) calculations with two approaches to treat dispersion interactions (semi-empirical correction DFT-D3 and parametrized functional optB88) and basis set approaches (localized numerical atomic orbitals, plane waves) with remarkable accuracy. Our results underline the trustworthiness and some of the limitations of current DFT-based methods regarding the description of geometric and electronic properties of metal-organic interfaces. [ABSTRACT FROM AUTHOR]

The dependence of the inelastic lifetime of electrons in the first n = 1 image-potential state of clean and rare-gas covered Ag(111), Cu(111), and Cu(100) surfaces on their momentum parallel to the surface has been studied experimentally by means of time- and angle-resolved two-photon photoemission spectroscopy (2PPE) and theoretically by calculations based on the many-body theory within the self-energy formalism. Similar to the previously studied clean Cu(100) surface, the theoretical results are in excellent agreement with the experiment findings for Cu(111). For Ag(111), the theory overestimates the decay rate and its momentum dependence, which is attributed to the neglect of surface plasmon excitations. With increasing parallel momentum, the n = 1 state shifts out of the projected bulk band gap on both surfaces and turns into an image-potential resonance. This opens an additional decay channel by resonant electron transfer into the bulk, which is theoretically treated by the application of the wave packet propagation approach. The expected stronger increase of the decay rate upon crossing the edge of the band gap, however, is not observed in the experiment. The decoupling of the image-potential states from the metal surface upon adsorption of rare-gas layers results in a decrease of the decay rate as well as of its momentum dependence by a similar factor, which can be successfully explained by the change of interband and intraband contributions to the total decay rate. [ABSTRACT FROM AUTHOR]