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Image-Potential States at Metal Surfaces

Image-potential states are quantized electronic states that exist at metal surfaces with a band gap near the vacuum level. The Coulomb-like attractive image force, experienced by any charged particle in front of a conductive material, and the repulsive surface then form a potential well for weakly bound electrons. The energy levels in this potential are described by a Rydberg series similar to electrons in the hydrogen atom

En = -0.85 eV/(n+a)2, n = 1,2,...

Part A of the figure to the right shows a corresponding potential energy diagram for an electron in front of a Cu(100) surface. For this surface the sp-band gap in the direction perpendicular to the surface is particularly large and the image states are located almost in the middle of the band gap. The squares of the wave functions of the lowest three image states are also shown in this figure.
Experimentally, image-potential states are investigated with two-photon photoemission (2PPE). One photon excites an electron out of an occupied state below the Fermi energy into the image potential state n. A second photon excites the electron to an energy above the vacuum level. The electron leaves the surface and its kinetic energy is measured.

Image-potential states - because of their simplicity - provide an extremely useful model to study the relaxation behaviour of excited surface electronic states due to their interaction with the underlying substrate. This coupling in turn governs the cross section and branching ratios of practically all electronically induced adsorbate reactions at metal surfaces.

The figure below shows the results of direct time-domain experiments using 95 fs pump and 70 fs probe pulses for the 2PPE measurements.tspec02c

Coherent excitation of image states

impot97When sub-100-fs UV pulses are used for the excitation of image states their bandwidth becomes comparable or larger than the energy separation of the states of Rydberg series with quantum numbers n>4. The coherent excitation of several states leads to the observation of quantum beats in the 2PPE intensity. The coherent states resulting from the excitation of four or more image states form electron wavepackets that describe the quasiclassical periodic motion of weakly bound electrons in front of metal surfaces.

For more information click on the figure to the right or see:
U. Höfer et al., Science 277, 1480-82 (1997).


Literature

P. M. Echenique, J. B. Pendry,
The existence and detection of Rydberg states at surfaces,
J. Phys. C: Solid State Physics 11, 2065-2075 (1978).

Th. Fauster, W. Steinmann,
Two-photon photoemission spectroscopy of image states,
in Photonic Probes of Surfaces, P. Halevi, Ed. (Elsevier, Amsterdam, 1995), pp. 347-411.

I. L. Shumay, U. Höfer, Ch. Reuß, U. Thomann, W. Wallauer, Th. Fauster,
Lifetimes of image-potential states on Cu(100) and Ag(100) measured by femtosecond time-resolved two-photon photoemission,
Phys. Rev. B 58, 13974-13981 (1998).

P. M. Echenique, R. Berndt, E. V. Chulkov, Th. Fauster, A. Goldmann, U. Höfer,
Decay of electronic excitations at metal surfaces,
Surface Science Reports 52, 219-317 (2004).

Thomas Fauster, Martin Weinelt, Ulrich Höfer,
Quasi-elastic scattering of electrons in image-potential states,
Progr. Surf. Sci. 82, 224-243 (2007).

Zuletzt aktualisiert: 27.05.2015 · armbrusn

 
 
 
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