Direkt zum Inhalt
Surface Physics
Sie sind hier:» Universität » Physik » Research Groups » Surface Physics » Surface Dynamics » Two-Photon Photoemission

Time-Resolved Two-Photon Photoemission

contact: Andreas Damm, Dr. Jens Güdde, Prof. Ulrich Höfer

2ppestatTwo-photon photoemission (2PPE) is a variant of photoelectron spectroscopy which allows to investigate unoccupied electronic states located between the Fermi level and the vacuum level of a metal or semiconductor. A first (pump) laser pulse with photon energy ℏωa populates the unoccupied state |i>, a second (probe) laser pulse with photon energy ℏωb photoemits the electron. The measurement of its kinetic energy  by

Ekin = ℏωb - Ei

arpesprovides a straightforward means to determine the binding energy Ei  of the unoccupied state with respect to the vacuum level. 
Dynamical information can be obtained with time-resolved 2PPE. The intensity recorded from a given intermediate state as a function of the variable delay between the sufficiently short pump and the probe pulses is determined by the lifetime of the intermediate state. 
Both,  spectroscopic and time-resolved 2PPE from surfaces benefit from angle-resolved detection of the photoelectrons. The photoemission process preserves the parallel momentum of the electron. It is simply given by

k=√2mEkin sinθ

Since electronic states of well-ordered surfaces are fully characterized by their energy and parallel momentum, time- and angle-resolved 2PPE are able to provide extremely detailed information about the time-evolution of electronic excitations from two-dimensional systems.
Further expansion of the capabilities of 2PPE is possible by detecting the spin of the photoelectrons and by exploiting coherence phenomena.


2PPE Experiment


An experimental apparatus for time- and angle-resolved 2PPE is shown schematically on the right hand side in the figure above. It consists of an ultra-high-vacuum (UHV) chamber equipped with instrumentation for surface characterization and a femtosecond laser system. The laser source is a Ti:sapphire amplifier system (Coherent RegA 9050) with selectable repetition rates between 10 and 250 kHz pumped by two Nd:YVO4 solid state lasers (Coherent Verdi V5/V10). The output of the amplifier feeds an optical-parametric amplifier (Coherent OPA 9450), which generates visible pulses between 470 and 700 nm with a pulse duration of about 50 fs and a pulse energy up to 400 nJ at 100 kHz repetition rate. Part of the visible light is doubled in thin BBO crystals and can be used for exciting unoccupied states as far as 5 eV above the Fermi level. The photoelectrons emitted by the visible pulses are detected by an hemispherical analyzer with an angle-resolved lens mode equipped with a two-dimensional image-type detector. The energy and angle resolution of this experiment reaches 30 meV and 0.42°, respectively (For further information klick here). A two-dimensional spectrum recorded with this analyzer is shown on the left hand side in the figure above. To see two-dimensional spectra at variing delays between the exciting and photoemitting laser pulses together with the pump-probe traces of the first three image-potential states click on the on the link provided here or on the figure above.
We employ this apparatus for the investigation of image-potential states, interface states and scattering processes as well as for studies of electron dynamics of semiconductor surfaces



Review Articles

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.

R. Haight,
Electron dynamics at surfaces,
Surface Science Reports 21, 275-325 (1995).

H. Petek, S. Ogawa,
Femtosecond time-resolved two-photon photoemission studies of electron dynamics in metals,
Progr. Surf. Sci. 56, 239-310 (1997).

M. Weinelt,
Time-resolved two-photon photoemission from metal surfaces,
J. Phys.: Condens. Matter 14, R1099-R1141 (2002).

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).


2PPE from Solid Surfaces - Worldwide Links

Martin Aeschlimann University of Kaiserslautern, Germany
Michael Bauer Christian-Albrechts-Universität Kiel, Germany
Hermann A. Dürr BESSY Berlin, Germany
Thomas Fauster Universität Erlangen-Nürnberg, Germany
Charles B. Harris University of California, Berkeley, USA
Ulrich Höfer Philipps-Universität Marburg, Germany
Toshiaki Munakata Osaka University, Japan
Richard Osgood Columbia University, New York, USA
Hrvoje Petek University of Pittsburg, USA
Martin Weinelt Max-Born-Institut Berlin, Germany
Martin Wolf Fritz-Haber-Institut der MPG, Berlin, Germany
Helmut Zacharias Universität Münster, Germany

Zuletzt aktualisiert: 02.03.2015 · armbrusn

Fb. 13 - Physik

Surface Physics, Renthof 5, D-35032 Marburg
Tel. +49 6421/28-24216, Fax +49 6421/28-24218, E-Mail: Irene.Dippel@physik.uni-marburg.de

URL dieser Seite: https://www.uni-marburg.de/fb13/researchgroups/of/dynamics/2ppe.html

Impressum | Datenschutz