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Time-resolved coherent photoelectron spectroscopy

Contact: Jens Güdde

Whereas most experimental methods record intensities and thus measure amplitudes of wave functions, coherent spectroscopies offer the unique capability of accessing also phase information. On ordered surfaces, the combination of time- and angle-resolved two-photon photoemission (2PPE) with coherent excitation allows one to completely determine the temporal evolution of the wave function of electronic states.


Electron wave packets

By exciting electrons into high-order image-potential states close to the vacuum level we have recently succeeded in creating electron wave packets at the surface of a metal. The calculated spatial and temporal evolution of such a wave packet is shown in the inset of the figure. It is a coherent superposition of several hydrogen-like wave functions in the vicinity of the quantum number n=7.


The dynamics of the wave packet resembles the classical motion of an electron oscillating in the Coulomb-like image potential with a period of 800 fs. The maximum distance from the surface is ~200 A.impot97
The perodic motion of the wave packet is detected by time-delayed probe pulse that photoemits the electron. The photoemission signal is clearly correlated with the probability of finding the electron close to the surface. Far away from the surface the electron is nearly free and cannot absorb a photon because both energy and momentum cannot be conserved in such a process. Near the surface the rapid gradient in the 1/z-potential provides such a source of momentum.
The figure to the right is schematic quasi-classical picture of the experiment. A pump pulse excites an electron from below the Fermi level of the metal into a region with no allowed states in the bulk. The electron moves away from the surface but its energy is not high enough to escape into the vacuum. The restoring image forced makes it move back to the metal where it is reflected from the repulsive surface barrier and continues its oscillatory motion.

Quantum beat spectroscopy

tpp264fgAlso in cases without a dramatic spatial dynamics the exploitation of coherence effects in two-photon photoemission can provide very valuable information. An example is the resolution of closely spaced energy levels with quantum beat spectroscopy.

With increasing quantum number n the binding energy of image-potential states

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

rapidly converges towards the vacuum level. Even with the best electron analysers it is difficult to resolve states with quantum numbers n>4. After coherent excitations the level spacing is clearly observable as oscillations (quantum beats) of the 2PPE signal as a function of pump-probe delay. In the example shown in the figure, the 95 fs pump pulse predominantly excites electrons into the n=4 and n=5 states. The beating period observed for short delay is 230 fs, corresponding to an energy difference E(5)-E(4) = 17.8 meV. After 2 ps most of the population in the n=4 state has decayed. The oscillations now reflect the interference of the amplitudes in the n=5 and the weakly excited n=6 state that persists at these delays. The beating period of 430 fs corresponds to a level spacing of 9.6 meV.

See also inelastic and quasielastic scattering processes and generation and investigation of ultrashort electrical currents.


U. Höfer, I. L. Shumay, Ch. Reuß, U. Thomann, W. Wallauer, Th. Fauster,
Time-resolved coherent photoelectron spectroscopy of quantized electronic states on metal surfaces,
Science 277, 1480-82 (1997).

E. W. Plummer,
More than skin deep,
Science 277, 1447-48 (1997).

Ch. Reuß, I. L. Shumay, U. Thomann, M. Kutschera, M. Weinelt, Th. Fauster, U. Höfer,
Control of the Dephasing of Image-Potential States by CO Adsorption on Cu(100),
Phys. Rev. Letters 82, 153-156 (1999).

U. Höfer,
Time-resolved coherent spectroscopy of surface states,
Appl. Phys. B 68, 383-392 (1999).

U. Höfer,
Dynamik von Bildpotentialzuständen,
in 31. IFF-Ferienkurs - Dynamik in kondensierter Materie (Forschungszentrum Jülich, 2000) F4.1-15.

Zuletzt aktualisiert: 27.05.2015 · armbrusn

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