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±5 fs at clean Cu(111) to 75±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

Direct and indirect excitation mechanisms in two-photon photoemission spectroscopy of Cu(111) and CO/Cu(111)

It is demonstrated that the dependence of the two-photon photoemission (2PPE) yield on the polarization of the exciting laser light provides detailed information about the excitation mechanism and the orientation of transition dipole moments in the 2PPE process. In particular, it is possible to distinguish between a direct two-photon excitation process, where both electronic transitions are induced by the electric fields at the surface, and an indirect mechanism, where the first excitation step occurs in the substrate. In the latter process the intermediate state in 2PPE is populated by scattering of photoexcited hot electrons from the substrate, which are subsequently photoemitted by the second laser pulse. The analysis is applied to 2PPE from clean and CO covered Cu(111). Furthermore, we have derived analytical expressions for the 2PPE signal based on the optical Bloch equations for a three-level system excited with continuous light beams. They allow us to calculate 2PPE spectra of surface states for a variety of cases

Paradox in Wave-Particle Duality

We report on the simultaneous determination of complementary wave and particle aspects of light in a double-slit type "welcher-weg" experiment beyond the limitations set by Bohr's Principle of Complementarity. Applying classical logic, we verify the presence of sharp interference in the single photon regime, while reliably maintaining the information about the particular pinhole through which each individual photon had passed. This experiment poses interesting questions on the validity of Complementarity in cases where measurements techniques that avoid Heisenberg's uncertainty principle and quantum entanglement are employed. We further argue that the application of classical concepts of waves and particles as embodied in Complementarity leads to a logical inconsistency in the interpretation of this experiment

Ultrafast Electron Dynamics at Cu(111): Response of an Electron Gas to Optical Excitation

Time-resolved two-photon photoemission is used to directly investigate the electron dynamics at a Cu(111) surface with 60 fs laser pulses. We find that the time evolution of the photoexcited electron population in the first image state can be described only by solving the optical Bloch equations to properly account for coherence in the excitation process. Our experiments also provide evidence that the dynamics of photoexcited bulk electrons is strongly influenced by hot electron cascades and that the initial relaxation rates are in agreement with Fermi liquid theory

Femtosecond time-resolved photoemission of electron dynamics in surface Rydberg states

Femtosecond time-resolved photoelectron spectroscopy provides a unique tool to study the dynamics of optically excited electrons at surfaces directly in the time domain. We present a new model for two-photon photoelectron spectroscopy from surface and image potential (or Rydberg) states which is based on density matrix theory. The formalism accounts for the influence of both energy and phase relaxation on experimental spectra and thus permits the study of the nature of inelastic and elastic scattering processes at surfaces in more detail. The analysis of experimental data employing the proposed model reveals a new mechanism for optical excitation of electrons to normally unoccupied states at surfaces which is feasible due to the influence of electronic dephasing. We discuss the nature of different relaxation channels with respect to our studies of image state dynamics on the bare and Xe or Kr covered Cu(111) surfaces

Dynamics of Electron-Induced Manipulation of Individual CO Molecules on Cu(111)

Electrons tunneling from a scanning tunneling microscope tip to individual CO molecules on Cu(111) can cause their hopping from the surface to the tip if the bias exceeds a threshold of 2.4 V. Polarization- and time-resolved two-photon photoemission identifies the underlying elementary process as intermediate population of a CO 2π* -derived level, which exhibits an ultrashort lifetime of 0.8–5 fs. From an isotope effect of 2.7 - 0.5 + 0.3 it can be calculated that ≈ 0.05 % of the tunneling current transiently occupies this level while a desorption of the excited molecule occurs only in 5 × 10 - 9 of the cases

Dynamics of Excited Electrons in Copper and Ferromagnetic Transition Metals: Theory and Experiment

Both theoretical and experimental results for the dynamics of photoexcited electrons at surfaces of Cu and the ferromagnetic transition metals Fe, Co, and Ni are presented. A model for the dynamics of excited electrons is developed, which is based on the Boltzmann equation and includes effects of photoexcitation, electron-electron scattering, secondary electrons (cascade and Auger electrons), and transport of excited carriers out of the detection region. From this we determine the time-resolved two-photon photoemission (TR-2PPE). Thus a direct comparison of calculated relaxation times with experimental results by means of TR-2PPE becomes possible. The comparison indicates that the magnitudes of the spin-averaged relaxation time \tau and of the ratio \tau_\uparrow/\tau_\downarrow of majority and minority relaxation times for the different ferromagnetic transition metals result not only from density-of-states effects, but also from different Coulomb matrix elements M. Taking M_Fe > M_Cu > M_Ni = M_Co we get reasonable agreement with experiments.Comment: 23 pages, 11 figures, added a figure and an appendix, updated reference

Measurement of hot electron momentum relaxation times in metals by femtosecond ellipsometry

Copyright © 2005 The American Physical SocietyTime-resolved ellipsometric measurements were made upon Au, Cu, Ag, Ni, Pd, Ti, Zr, and Hf thin films. Using an elliptically polarized pump beam, the decay of the optically induced polarization of the sample was observed. Characteristic relaxation times are extracted and interpreted in terms of scattering of linear and angular momentum of hot electrons. A systematic variation is observed between different metals that reflects their underlying band structure

Dynamics of Excited Electrons in Copper: Role of Auger Electrons

Within a theoretical model based on the Boltzmann equation, we analyze in detail the structure of the unusual peak recently observed in the relaxation time in Cu. In particular, we discuss the role of Auger electrons in the electron dynamics and its dependence on the d-hole lifetime, the optical transition matrix elements and the laser pulse duration. We find that the Auger contribution to the distribution is very sensitive to both the d-hole lifetime tau_h and the laser pulse duration tau_l and can be expressed as a monotonic function of tau_l/tau_h. We have found that for a given tau_h, the Auger contribution is significantly smaller for a short pulse duration than for a longer one. We show that the relaxation time at the peak depends linearly on the d-hole lifetime, but interestingly not on the amount of Auger electrons generated. We provide a simple expression for the relaxation time of excited electrons which shows that its shape can be understood by a phase space argument and its amplitude is governed by the d-hole lifetime. We also find that the height of the peak depends on both the ratio of the optical transition matrix elements R=|M_{d \to sp}|^2/|M_{sp \to sp}|^2 and the laser pulse duration. Assuming a reasonable value for the ratio, namely R = 2, and a d-hole lifetime of tau_h=35 fs, we obtain for the calculated height of the peak Delta tau_{th}=14 fs, in fair agreement with Delta tau_{exp} \approx 17 fs measured for polycrystalline Cu.Comment: 6 pages, 6 figure

Variable Electron-Phonon Coupling in Isolated Metallic Carbon Nanotubes Observed by Raman Scattering

We report the existence of broad and weakly asymmetric features in the high-energy (G) Raman modes of freely suspended metallic carbon nanotubes of defined chiral index. A significant variation in peak width (from 12 cm-1 to 110 cm-1) is observed as a function of the nanotube's chiral structure. When the nanotubes are electrostatically gated, the peak widths decrease. The broadness of the Raman features is understood as the consequence of coupling of the phonon to electron-hole pairs, the strength of which varies with the nanotube chiral index and the position of the Fermi energy.Comment: 13 pages; submitted to Phys. Rev. Let