Electrons In Image-potential States Research Articles

Spectroscopy and Dynamics of a Two-Dimensional Electron Gas on Ultrathin Helium Films on Cu(111).

Electrons in image-potential states on the surface of bulk helium represent a unique model system of a two-dimensional electron gas. Here, we investigate their properties in the extreme case of reduced film thickness: a monolayer of helium physisorbed on a single-crystalline (111)-oriented Cu surface. For this purpose we have utilized a customized setup for time-resolved two-photon photoemission at very low temperatures under ultrahigh vacuum conditions. We demonstrate that the highly polarizable metal substrate increases the binding energy of the first (n=1) image-potential state by more than 2 orders of magnitude as compared to the surface of liquid helium. An electron in this state is still strongly decoupled from the metal surface due to the large negative electron affinity of helium and we find that even 1 monolayer of helium increases its lifetime by 1 order of magnitude compared to the bare Cu(111) surface.

Quantum Confinement of Hot Image-Potential State Electrons

Discrete image-potential state (IS) resonances at Co nanoislands on Au(111) are probed using scanning tunneling microscopy and spectroscopy. We observe particle-in-box-type standing wave patterns, which is surprising in view of the high energy of the IS electrons when compared to the confining potential imposed by the island edges. The weak confining potential experienced by the IS electrons results in electronic interaction effects between closely spaced islands. Probing high-energy ISs hence provides a novel route to investigate electronic coupling between nanoislands on surfaces.

Light tuning of the image potential state electron-electron interactions

We present the experimental evidence of the interplay between the variation of the linewidth and effective mass of electrons in image potential states on a Cu(1 1 1) surface induced by a selective alteration of the hot-electron gas density generated by ultra-short laser pulse excitations.

Scattering of electrons in image-potential states by steps

Image-potential states on several stepped copper surfaces were studied by time, energy, and angle-resolved two-photon photoelectron spectroscopy. For different step densities binding energies and effective masses were determined in the energy-resolved and decay rates in the time-resolved mode of two-photon photoemission. The asymmetry with respect to the electron motion up or down the steps of the total decay rates is enhanced for less-ordered step arrangements. Elastic interband scattering strongly depends on details of the backfolding of the bands by the periodic step structure. Elastic intraband scattering is of similar importance as inelastic decay may contribute significantly to the asymmetry of the decay rates.

Scattering of electrons on the Cu(001) surface by Co adatoms

Excited electrons at surfaces can be scattered by adsorbate atoms or defects, which changes the energy or momentum. Such scattering processes can be studied by energy, time and angle-resolved two-photon photoelectron spectroscopy. In this article the influence of statistically distributed Co adatoms on a Cu(001) surface on the dynamics of electrons in image-potential states is investigated. Different scattering mechanisms, such as interband, intraband, and bulk scattering are identified and analyzed quantitatively. Cobalt adatoms cause mainly quasielastic scattering of electrons in image-potential states. Inelastic processes are due to interactions with electrons in the substrate and are not significantly increased by Co adatoms. The results are compared to previous experimental and theoretical work on Cu adatoms.

Lifetimes of Stark-Shifted Image States

The inelastic lifetimes of electrons in image-potential states at Cu(100) that are Stark shifted by the electrostatic tip-sample interaction in the scanning tunneling microscope are calculated using the many-body GW approximation. The results demonstrate that in typical tunneling conditions the image state lifetimes are significantly reduced from their field-free values. The Stark shift to higher energies increases the number of inelastic scattering channels that are available for decay, with field-induced changes in the image state wave function increasing the efficiency of the inelastic scattering through greater overlap with final state wave functions.

Carrier dynamics on surfaces studied by two-photon photoemission

The lifetime of electronic excitations at surfaces play an important role for desorption processes induced by electronic transitions. Two-photon photoemission with time, energy, and angular resolution allows to study the scattering processes of excited electrons in detail. For electronic surface states various scattering channels can be identified, such as elastic vs. inelastic or intraband vs. interband. As examples the scattering of electrons in image-potential states by copper adatoms on Cu(0 0 1) and the decay and exciton formation in the dangling-bond surface conduction band at the Si(1 0 0) c(4 × 2) surface are presented. These studies of the carrier dynamics at surfaces imply that photochemical or electronic desorption processes are influenced by surface defects and are more efficient at semiconductor than on metal surfaces.

Athermal electron distribution probed by femtosecond multiphoton photoemission from image potential states

Image potential states are populated through indirect, scattering-mediated multiphoton absorption induced by femtosecond laser pulses and revealed by single-photon photoemission. The measured effective mass is significantly different from that obtained with direct, resonant population. These features reveal a strong coupling of the electrons residing in the image potential state, outside the solid, with the underlying hot electron population created by the laser pulse. The coupling is mediated by a many-body scattering interaction between the image potential state electrons and bulk electrons in highly excited states.

Scattering of hot electrons by adatoms at metal surfaces.

The scattering of electrons in image-potential states by Cu adatoms on Cu(001) surfaces has been investigated by means of time- and angle-resolved two-photon photoemission. Several interband and intraband-scattering mechanisms have been identified and their contributions to the total decay of the states determined quantitatively. The adsorbates mainly cause quasielastic scattering processes. Inelastic processes in contrast are due to interactions with electrons in the substrate and are not significantly increased by Cu adatoms. Quasielastic scattering into bulk bands contributes significantly to the depopulation of surface states.

Scattering by Cu adatoms between image-potential bands on Cu(001)

With the increasing resolution and sensitivity of photoelectron spectroscopy, the influence of defects is becoming more and more obvious. Scattering processes induced by adsorbate atoms can be studied by time- and angle-resolved two-photon photoemission. We have examined the dynamics of electrons in image-potential states on the Cu(001) surface for statistically distributed Cu adatoms and have identified different scattering mechanisms. Scattering of electrons from the second (n=2) to the bottom of the first (n=1) image-potential band is observed, which we attribute to inelastic interband scattering with electrons in the bulk. At energies above the bottom of the n=2 band, resonant interband scattering from the n=2 to the n=1 image-potential band is found. The rate for these processes can be determined by modeling the time-resolved measurements via optical Bloch equations of a four-level system. Comparison of the transition and decay rates reveals that the decay rate of the n=2 electrons is almost exclusively changed by additional resonant interband-scattering processes upon adsorption.

Decay and dephasing of image-potential states due to surface defects and disorder

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 cor- relation of decay and dephasing with surface defects and disorder.

Ultrafast dynamics of electrons in image-potential states on clean and Xe-covered Cu(111).

Lifetimes of electrons in the $n=1$ and $n=2$ image states on Cu(111) are studied with femtosecond time-resolved photoemission. Adsorption of one monolayer of Xe results in a pronounced increase of the image-state lifetime, which for the $n=1$ state changes from 18\ifmmode\pm\else\textpm\fi{}5 fs at clean Cu(111) to 75\ifmmode\pm\else\textpm\fi{}15 fs at the Xe-covered surface. The slower relaxation rate induced by the Xe layer is attributed to a reduced overlap of the image-state wave function with bulk states. A density-matrix calculation reveals the importance of dephasing in the excitation process.