Time-Resolved Coherent Photoelectron Spectroscopy of Quantized Electronic States on Metal Surfaces

Time-resolved two-photon photoemission in combination with the coherent excitation of several quantum states was used to study the ultrafast electron dynamics of imagepotential states on metal surfaces. For a (100) surface of copper, the spectroscopy of quantum beats made previously unresolved high-order states (quantum number n Ն 4) experimentally accessible. By exciting electrons close to the vacuum level, electron wave packets could be created and detected that described the quasi-classical periodic motion of weakly bound electrons. They traveled more than 200 Å away from the surface and oscillated back and forth with a period of 800 femtoseconds. Photoelectron spectroscopy has developed into one of the most versatile and successful tools for surface studies. Particularly attractive features of this technique are the high surface sensitivity associated with the low escape depth of the photoelectrons and the capability of angle-resolved photoemission to completely characterize electronic states in energy and momentum space (1). Recently, these features have been combined with ultrafast laser excitation for direct time-domain investigations of electron dynamics at surfaces (2). Here, we demonstrate another facet of this powerful technique, the investigation of coherence phenomena in real time. In contrast to experimental methods that rely merely on intensities, coherent spectroscopies offer the unique capability of accessing not only the amplitudes but also the phases of the wave functions of interest (3). This technique dramatically increases the amount of information that one is able to obtain about the temporal evolution of fast processes. In this report, we discuss the dynamics of image-potential states, that is, the quantized excited states of electrons that exist in front of many metal surfaces (4, 5). Using femtosecond time-resolved two-photon photoemission (2PPE), we observed the interference between the wave functions of neighboring eigenstates and the quasi-classical motion of electron wave packets created by the coherent superposition of several quantum states. Recently, the imaging of the static charge density of related surface electronic (ground) states in real space with the scanning tunneling microscope has attracted considerable interest (6); the present results reveal the dynamical evolution of excited electrons in real time. Image-potential states are conceptually rather simple. An electron at a distance z in front of a conducting metal surface experiences an attractive force F(z) ϭ Ϫe 2 /(2z) 2 identical to that produced by a positive (mirror image) charge at a distance z inside the metal converging toward the vacuum energy, where the influence of the surface potential on the binding energy E B ϭ ϪE n is approximated by a quantum defect 0 Յ a Յ 0.5. Experimentally, image-potential states have been studied with 2PPE on many metal surfaces including surfaces covered with adsorbates and metallic overlayers (5, 7-11). One photon with energy ប a (ប is Planck's constant h divided by 2 and is the photon frequency times 2) excites an electron out of an occupied state below the Fermi energy E F into the image-potential state n. A second photon with energy ប b excites the electron to an energy above E vac The experimental setup consisted of a 80-MHz Ti:sapphire laser system that generated infrared (IR) pulses of 70-fs duration. Frequency-tripled 95-fs ultraviolet (UV) pulses from this laser were used for the excitation step (ប a ϭ 4.7 eV). The photoelectrons were emitted by the fundamental IR pulses (ប b ϭ 1.57 eV) and were detected in a hemispherical analyzer with an energy resolution of 30 meV and an angular acceptance of Ϯ0.6°about the surface normal. The preparation of the Cu(111) and Cu(100) samples and details of the ultrahigh-vacuum chamber have been described elsewhere (5). The samples were kept at room temperature. Typical energy-resolved 2PPE spectra of C

Self-energy of image states on copper surfaces

We report extensive calculations of the imaginary part of the electron self-energy in the vicinity of the (100) and (111) surfaces of Cu. The quasiparticle self-energy is computed by going beyond a free-electron description of the metal surface, either within the GW approximation of many-body theory or with inclusion, within the GW$\Gamma$ approximation, of short-range exchange-correlation effects. Calculations of the decay rate of the first three image states on Cu(100) and the first image state on Cu(111) are also reported, and the impact of both band structure and many-body effects on the electron relaxation process is discussed.Comment: 8 pages, 5 figures, to appear in Phys. Rev.

Epigenetics and the power of art

This review presents an epigenetic view on complex factors leading to development and perception of “genius.” There is increasing evidence which indicates that artistic creativity is influenced by epigenetic processes that act both as targets and mediators of neurotransmitters as well as steroid hormones. Thus, perception and production of art appear to be closely associated with epigenetic contributions to physical and mental health

Lifetimes of unoccupied surface states on Pd(111)

Using time-resolved two-photon photoemission we have measured the linewidths and lifetimes of the unoccupied Shockley surface state and first image-potential state on Pd(111). Calculated values are in good agreement with the experimental results.This work was supported by the Departamento de Educacion del Gobierno Vasco and by Acciones Integradas Hispano-Alemanas.Peer reviewe

A systematic review of beliefs involved in the use of complementary and alternative medicine.

People might be attracted to and use complementary and alternative medicines (CAM) because they hold beliefs that are congruent with CAM. This article collates, examines and synthesizes the evidence surrounding this hypothesis. Most studies are cross-sectional and focus on a limited number of beliefs. Multivariate studies suggest that beliefs related to control and participation, perceptions of illness, holism and natural treatments, and general philosophies of life predict CAM use when controlling for demographic and clinical factors. Further research should examine the robustness of these relationships in different illness groups and the prospective relationships among beliefs and CAM use over time