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Electron Dynamics

Electron Dynamics

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Prof. Dr. Martin Wolf
Abt. Physikalische Chemie
Fritz-Haber Institut der MPG
Faradayweg 4-6
14195 Berlin, Germany
phone:+49 30 8413-5111
Fax: +49 30 8413-5106

hitherto (2010):
Department of Physics
Free University Berlin
Arnimallee 14
D - 14195 Berlin

Electron dynamics and surface photochemistry

The dynamics of electronic excitations at surfaces, including charge transfer between bulk solids and molecules adsorbed on the surface, proceeds on a femtosecond timescale and is essential to a large number of photostimulated reactions at metal and semiconductor surfaces. In the primary step of surface photochemistry, irradiation of the substrate with visible or UV-photons leads to the formation of electron-hole pairs. The photoexcited electrons will subsequently relax on a femtosecond timescale in scattering processes by generation of secondary electrons. This leads to the formation of so called secondary or hot electron cascades. Inelastic (resonant) scattering events between hot electrons and molecules adsorbed on the surface may eventually stimulate chemical reactions (see diagram below).

The dynamics of hot electrons in the bulk and the lifetimes of electrons which are resonantly captured by an adsorbate are crucial for photoinduced surface reaction cross sections. The carrier dynamics can be studied with femtosecond time-resolved two-photon photoemission (2PPE), where a second photon is used to probe the energetics and lifetimes of intermedidate excited states.

Hot electrons in metals

Here we show time-resolved two-photon photoemission spectra of the copper(111) surface as a function of the time delay between pump and probe laser pulses. These spectra show 'snapshots' of the temporal evolution of the photoexcited electron distribution. We observe that the relaxation dynamics of the hot (or secondary) electrons is slower for energies closer to the Fermi level. This qualitative behavior is expected for a nearly free electron gas according to Fermi Liquid theory.

Surface states and adsorbates

The electron dynamics at surfaces can be strongly influenced by electronic states which are localized at the surface. As a model system we have studied the dynamics of image potential states, and their interaction with various physisorbed adsorbate overlayers. This allows modification of the electron dynamics in a controlled way and elucidation of the interaction of image potential states with negative ion resonances of adsorbed molecules.

2-Photon-Photoemission | Image Potential States | Adsorbates | Magnetism

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