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. PACS 73.20.At; 79.60.Ht; 68.49.Jk     

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. PACS 73.20.At; 68.49.Jk; 79.60.Ht     

Scattering of image-potential-state electrons by steps on Cu(001)

Scanning tunneling spectroscopy (STS) reveals a distinct asymmetry in the scattering properties of an isolated step for the n=1 image-potential state on Cu(001). The elastic scattering probability for an electron traveling downstairs is determined from the strength of density oscillations in front of a step edge and is found to be approximately two times higher than for the opposite upstairs direction. A one-dimensional scattering model is extended to the case of asymmetric transmission and reflection coefficients. The calculations using the asymmetry measured by STS explain the dispersion and the decay rate of the n=1 band on Cu(119) measured by two-photon photoemission. In particular, the asymmetry of the decay rate can be described quantitatively with a minimum of adjustable parameters. While the results can also be transferred successfully to the Cu(1115) surface, the limit of applicability is reached for Cu(117) with a step separation of 3.5 nearest-neighbor distances. PACS 73.20.At; 79.60.Bm; 68.37.Ef; 72.10.Fk     

Electron scattering at steps: image-potential states on Cu(119)

The dynamics of image-potential states on Cu(119) have been studied with two-photon photoemission. Direction-dependent quasielastic scattering processes with large momentum transfer are attributed to the finite terrace-width distribution on the stepped surface. This effectively couples image-potential states via interband scattering and leads to an asymmetry of the decay rate. Electrons in the first image-potential state live apparently longer when running upstairs.     

Spin-dependent electron dynamics in front of a ferromagnetic surface

Exchange splitting and dynamics of image-potential states in front of a 3 monolayer iron film on Cu(100) have been studied with time-, energy-, and spin-resolved bichromatic two-photon photoemission. For the first image-potential state n=1 we observe an exchange splitting of 56 +/- 10 meV and spin-dependent lifetimes of 16 +/- 2 fs for majority-spin and of 11 +/- 2 fs for minority-spin electrons, respectively. The time-resolved studies of both the population and the linewidth of image-potential states manifest that at the magnetic surface not only inelastic but also quasielastic scattering processes are spin dependent.     

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.     

Image-potential states on stepped Cu (001) surfaces

Image-potential states on Cu (117) and Cu (119) surfaces were studied by means of two-photon photoelectron spectroscopy. The regular array of steps generates a lateral potential on the vicinal surfaces, which modifies the surface-electronic structure. Compared to Cu (001), the band bottom of the n=1 image-potential states shifts by 40 meV to lower binding energy. The periodicity of the step-induced superlattice manifests itself as back-folding of the n=1 and 2 dispersion bands. At the surface Brillouin zone boundary a mini-gap opens with a width of 135 meV for the first image-potential state on Cu (117). On the vicinal surfaces the lifetime of the image-potential states is reduced by a factor of three as compared to Cu (001). This is attributed to a narrowing of the surface-projected bulk-band gap when projected along the [11n] direction. While the dephasing rate of the first image-potential state is close to the decay rate, higher members of the Rydberg-like series show negligible dephasing. Received: 16 October 2001 / Revised version: 9 April 2002 / Published online: 6 June 2002     

Phase and population decay of image-potential states – the influence of defects

Quantum-phase and population decay of image-potential states have been investigated by two-photon photoemission with femtosecond time resolution. The influence of steps and defects on quasielastic and inelastic scattering processes is illustrated for a vicinal Cu(119) surface and diluted adsorbate layers of CO and Cu on Cu(001). Received: 19 April 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Ultrafast optical spin injection into image-potential states of Cu(001)

We report the observation of a net spin polarization in the n=1 image-potential state at the Cu(001) surface. The spin polarization is achieved by spin-selective multiphoton excitation of electrons from the spin-orbit split Cu d bands to the image-potential state using circularly polarized ultrafast light pulses. We show that by tuning the exciting photon energy, we can adjust the resonant coupling of the image-potential state to d bands of different double-group symmetry. This allows us to tune the spin polarization injected into the image-potential state.     

Time-resolved two-color interferometric photoemission of image-potential states on Cu(100)

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 ω_a and the two-photon polarization which is induced by laser pulses with frequency ω_a/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. PACS 78.47.+p; 79.60.Bm; 73.20.-r; 82.53.Kp; 42.50.Md     

Lifetimes of image-potential states on the Pt(111) surface probed by time-resolved two-photon photoemission spectroscopy

We have investigated the population dynamics of image-potential states on the clean Pt(111) surface. The first two image-potential states have been resolved exhibiting lifetimes of 26±7 fs and 62±7 fs. Those lifetimes are in contrast to the (111) surfaces of Ag and Cu, where the n=2 state is degenerate with bulk states leading to lifetimes shorter than 20 fs. Received: 30 March 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Electronic structure and electron dynamics at Si(100)

The electronic structure and electron dynamics at a Si(100) surface is studied by two-photon photoemission (2PPE). At 90 K the occupied D_up dangling-bond state is located 150±50 meV below the valence-band maximum (VBM) at the center of the surface Brillouin zone ̄ and exhibits an effective hole mass of (0.5±0.15)m_e. The unoccupied D_down band has a local minimum at ̄ at 650±50 meV above the VBM and shows strong dispersion along the dimer rows of the c(4×2) reconstructed surface. At 300 K the D_down position shifts comparable to the Si conduction-band minimum by 40 meV to lower energies but the dispersion of the dangling-bond states is independent of temperature. The surface band bending for p-doped silicon is less than 30 meV, while acceptor-type defects cause significant and preparation-dependent band bending on n-doped samples. 2PPE spectra of Si(100) are dominated by interband transitions between the occupied and unoccupied surface states and emission out of transiently and permanently charged surface defects. Including electron–hole interaction in many-body calculations of the quasi-particle band structure leads us to assign a dangling-bond split-off state to a quasi-one-dimensional surface exciton with a binding energy of 130 meV. Electrons resonantly excited to the unoccupied D_down dangling-bond band with an excess energy of about 350 meV need 1.5±0.2 ps to scatter via phonon emission to the band bottom at ̄ and relax within 5 ps with an excited hole in the occupied surface band to form an exciton living for nanoseconds. PACS 73.20.At; 79.60.Bm; 79.60.Dp; 79.60.Ht     

Photoelectron spectroscopy of Ar/Cu(100) interface states

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. PACS 73.20.-r; 73.40.Ns; 79.60.-i; 78.47.+p     

Time-resolved two-photon photoemission of buried interface states in Ar/Cu(100)

We demonstrate the existence of buried image-potential states at the interface between thick Ar films and a Cu(100) substrate. The electron dynamics of these solid-solid interface states, energetically located above the vacuum level in the band gaps of both materials, could be investigated with time-resolved two-photon photoemission for an Ar layer thickness up to 200 A. Relaxation on time scales between 40 and 200 fs occurs via two distinct channels, resonant tunneling through the insulating layer into the vacuum and electron-hole pair decay in the metal.     

Resonant interband scattering of image-potential states

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_∥. 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. Received: 1 October 2001 / Revised version: 24 October 2001 / Published online: 23 November 2001     

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. PACS 73.20.At; 79.60.Bm; 72.10.Fk     

Momentum-resolved lifetimes of image-potential States on cu(100)

The dependence of the inelastic lifetime of electrons in the image-potential states of Cu(100) on their momentum parallel to the surface has been studied experimentally by means of time- and angle-resolved two-photon photoemission and theoretically by evaluating the electron self-energy within the GW approximation. The pronounced decrease of the n = 1 lifetime from 40 fs at normal emission (k(parallel) = 0) to 20 fs for k(parallel) = 0.33 A(-1) cannot be accounted for by interband decay processes to bulk states. We show that intraband transitions within the image-state band give a contribution to this decrease comparable in magnitude with the interband channel.     

Modelling of interferometric multiphoton photoemission

A new interferometric multi-photon photoemission scheme has been recently introduced by Güdde et al. [1] and applied to a study of the n=1 image state on Cu(100). We report on the results of a theoretical modelling of this photoemission scheme based on a wave function representation. It incorporates both resonant transitions and two-photon non-resonant transitions. In the case of the n=1 image state on Cu(100), the initial state of the photoemission belongs to a continuum. Summation of the contributions to the photoemission yield from the various initial states is shown to drastically blur the interference structure, acting as an efficient dephasing process. The sensitivity of the new scheme and of the one-colour TR-2PPE scheme to the system dynamical properties is discussed. PACS 73.20.-r; 78.47.+p; 79.60.-i; 82.53.Kp; 42.50.Md     

Spin-orbit hybridization points in the face-centered-cubic cobalt band structure

Linear magnetic dichroism is observed in spin-, time-, and energy-resolved two-photon photoemission from valence bands of epitaxial fcc cobalt on Cu(001). With image-potential states as spectator states we identify initial bulk and surface states with minority spin character as the source for dichroic intensities and apparent dichroic lifetimes. Excellent agreement with ab initio fully relativistic calculations of the cobalt fcc band structure allows us to precisely determine spin-orbit hybridization points close to the Fermi level. These spin hot spots enhance spin-flip scattering by several orders of magnitude and are therefore assumed to be crucial in ultrafast demagnetization.     

Quantum coherence of image-potential states

The quantum dynamics of the two-dimensional image-potential states in front of the Cu(100) surface is measured by scanning tunneling microscopy and spectroscopy. The dispersion relation and the momentum resolved phase-relaxation time of the first image-potential state are determined from the quantum interference patterns in the local density of states at step edges. It is demonstrated that the tip-induced Stark shift does not affect the motion of the electrons parallel to the surface.