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     

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.     

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.     

Spectroscopy of image-potential states by two-photon photoemission

Unoccupied electronic states in solids and at solid surfaces are usually studied by inverse photoemission. An alternative method is two-photon photoemission. It is superior in resolution but limited to states of sufficiently long lifetime below the vacuum level. So far this method has mainly been applied to image-potential states on metal surfaces. On Ag(111) and Cu(111) a narrow surface state below the Fermi level serves as the initial state, which results in a pronounced resonance in the two-photon photoemission. Ni(111) shows similar results. In the resonance the image-potential state is so highly populated that electron-electron interaction leads to an Auger-type process. Nevertheless, the system is not so greatly disturbed as to show deviations from the one-photon photoemission results concerning the occupied states. Ag(100) and Cu(100) have a smooth continuum of initial states. Consequently, no resonance occurs. The binding energy does not depend on the material but changes with surface orientation: it is about 0.80 eV at the (111) surfaces and about 0.55 eV at the (100) surfaces. The effective mass is free electron like except on Ag(111), where it is 30% heavier. The lifetime on Ag(100) is about 20 fs. The agreement with theory is excellent in some cases and only fair in others.     

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.     

Time-resolved measurements of electron transfer processes at the PTCDA/Ag(111) interface

The electron transfer processes at the interface between 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) and Ag(111) have been studied using time- and angle-resolved two-photon photo-emission (2PPE). For this system a dispersing unoccupied interface state can be identified that is located 0.6 eV above the Fermi level with an effective electron mass of 0.39 m_e at the [`(G)]\overline{\Gamma}-point. The lifetime of 54 fs for the interface state is relatively short indicating a large penetration of the wavefunction into the metal. Supported by model calculations this interface state is interpreted as predominantly arising from an upshift of the occupied Shockley surface state of the clean metal substrate due to the interaction with the PTCDA overlayer. Coverage dependent measurements show a second long-lived component in the time-resolved measurements for higher PTCDA coverages that can be associated to charge transfer processes from the PTCDA multilayers into the metal substrate. Additionally the influence of the PTCDA adlayers on the image-potential states is studied indicating that the n = 1 image-potential state is localized in the first two monolayers of the PTCDA film.     

Time-resolved two-photon photoemission of unoccupied electronic states of periodically rippled graphene on Ru(0001)

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 moiré superstructure of graphene.     

高激发态钠原子的量子拍实验的某些分析

本文推导了一个量子拍实验中观察量的解析表式,与实验对比可得n²D态钠原子的精细裂距,其它特征也与实验符合。同时还讨论了上述高激发态钠原子在弱电场中的能态变化特性,指出了观察此特性的可能实验方法。 关键词：     

Influence of Ar,Kr, and Xe layers on the energies and lifetimes of image-potential states on Cu(100)

The influence of well-ordered adlayers of Ar, Kr, and Xe on the energetic and dynamical properties of image-potential states on Cu(100) has been investigated in a comprehensive study using time-resolved two-photon photoemission (2PPE). The effect of these insulating films varies systematically with the electron affinity EA of the condensed rare gases and with the film thickness. For the electron-repulsive Ar layers (EA=-0.25 eV), a strong lifetime increase of the n=1 state from 40 fs on clean Cu(100) to as much as 10 ps at a coverage of 5 monolayers is observed. For Kr and Xe layers (EA=+0.3 and +0.5 eV, respectively), decoupling from the metal is less efficient. These layers exhibit quantum-well-like resonances of the n=2 state as a function of layer thickness. The energies of the series of states depend characteristically on the affinity level and the dielectric constant of the films. A microscopic model is developed that includes the discrete atomic structure of the adsorbate layers. It is capable of describing the experimental results to a high degree of quantitative agreement. PACS 78.47.+p; 73.20.At; 77.55.+f     

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     

Femtosecond electron and spin dynamics in Ni/W(110) films

The collapse of the magnetic exchange splitting in 7 monolayer thick epitaxial Ni/W(110) films following a femtosecond laser pulse was measured using time-resolved photoemission spectroscopy. Ultrafast demagnetization during the laser induced hot electron cascade proceeds via spin-flip excitations with a relaxation time constant of 300+/-70 fs. At longer times the electronic system cools down and the magnetization is finally reestablished with a time constant of 3.2+/-0.2 ps.     

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.     

Observation of spin-orbit splitting in lambda single-particle states

The spin-orbit splitting of Lambda single-particle states in (13)(Lambda)C was measured. The 13C(K-,pi(-))(13)(Lambda)C reaction was used to excite both the 1/2(-) and 3/2(-) states simultaneously, which have predominantly 12C(0(+)) x p(Lambda) configuration. gamma rays from the states to the ground state were measured in coincidence with the pi(-)'s, by which ls splitting was found to be 152+/-54(stat)+/-36(syst) keV. The value is 20-30 times smaller than exhibited by the ls splitting in the nuclear shell model. This value gives us new insight into the YN interaction.     

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     

Image-potential-induced surface state at Si(100)

We have identified a surface state on Si(100) (2×1) at a binding energy of 0.69±0.05 eV with respect to the vacuum level. Band-structure calculations within the GW method reveal that almost 80% of the probability density of the resonance is located in front of the surface. We therefore assign the surface resonance to an image-potential state. It has a lifetime of about 10 fs and contributes significantly to two-photon photoemission from Si(100). PACS 73.20.At; 79.60.Bm; 79.60.Dp; 79.60.Ht     

Spin-orbit coupling and anisotropy of spin splitting in quantum dots

In lateral quantum dots, the combined effect of both Dresselhaus and Bychkov-Rashba spin-orbit coupling is equivalent to an effective magnetic field +/- B(SO) which has the opposite sign for s(z)= +/- 1/2 spin electrons. When the external magnetic field is perpendicular to the planar structure, the field B(SO) generates an additional splitting for electron states as compared to the spin splitting in the in-plane field orientation. The anisotropy of spin splitting has been measured and then analyzed in terms of spin-orbit coupling in several AlGaAs/GaAs quantum dots by means of resonant tunneling spectroscopy. From the measured values and sign of the anisotropy we are able to determine the dominating spin-orbit coupling mechanism.     

Zero-field satellites of a zero-bias anomaly

Spin-orbit (SO) splitting, +/-omega(SO), of the electron Fermi surface in two-dimensional systems manifests itself in the interaction-induced corrections to the tunneling density of states, nu(epsilon). Namely, in the case of a smooth disorder, it gives rise to the satellites of a zero-bias anomaly at energies epsilon = +/-2 omega(SO). Zeeman splitting, +/-omega(Z), in a weak parallel magnetic field causes a narrow plateau of a width delta epsilon = 2 omega(Z) at the top of each sharp satellite peak. As omega(Z) exceeds omega(SO), the SO satellites cross over to the conventional narrow maxima at epsilon = +/-2 omega(Z) with SO-induced plateaus delta epsilon = 2 omega(SO) at the tops.     

Nanoscale depth-resolved coherent femtosecond motion in laser-excited bismuth

We employ grazing-incidence femtosecond x-ray diffraction to characterize the coherent, femtosecond laser-induced lattice motion of a bismuth crystal as a function of depth from the surface with a temporal resolution of 193+/-8 fs. The data show direct consequences on the lattice motion from carrier diffusion and electron-hole interaction, allowing us to estimate an effective diffusion rate of D=2.3+/-0.3 cm(2)/s for the highly excited carriers and an electron-hole interaction time of 260+/-20 fs.     

Unoccupied surface states and surface barrier in metals

Recent progress in the spectroscopy of empty electronic states at metal surfaces allows for measuring the energy vs. momentum dispersion of both crystal-induced and image-potential surface states with high precision. This allows for deriving the effective barrier potential for an electron near a metal surface with considerable accuracy by comparing the experimental data with corresponding calculations based on the one-step model of inverse photoemission. The method is demonstrated for Cu(100) where four empty surface states are known experimentally.     

Few-cycle attosecond pulses via periodic resonance interaction with hydrogenlike atoms

We show that it is possible to produce nearly bandwidth-limited few-cycle attosecond pulses based on periodic resonance interaction of a quasi-monochromatic radiation with the bound states of hydrogenlike atoms. A periodic resonance is provided by a far-off-resonant laser field with intensity much below the atomic ionization threshold via periodic tunnel ionization from the excited states and adiabatic Stark splitting of the excited energy levels. Without external synchronization of the spectral components, it is possible to produce 135 as pulses at 13.5 nm in Li2?-plasma controlled by radiation of a mode-locked Nd:YAG laser, as well as 1.25 fs pulses at 122 nm in atomic hydrogen controlled by radiation of a CO? laser.