Book of Abstracts Book of Abstracts - Universität Konstanz

Fast Electronic Relaxation in Metal Clusters via Excitation of
Coherent Shape Deformations: Slipping Through a Bottleneck
V. V. Kresin 1 , Yu. N. Ovchinnikov 2 , V. Z. Kresin 3
1 Dept. of Physics, University of Southern California, Los Angeles, California 90089-0484, USA
2 Landau Institute for Theoretical Physics, Russian Academy of Sciences, Moscow 117332, Russia
3 Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720, USA
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We introduce and analyze a channel of electronic relaxation in metallic clusters which
enables large energy transfer to the ionic subsystem and, correspondingly, fast electron
relaxation dynamics. The discreteness of electronic levels in finite size-quantized systems such
as metal nanoparticles, quantum dots, etc., represents a challenge for understanding the
relaxation behavior of excited electrons. As is known, cases when the spacing between
electronic energy levels exceeds the vibrational frequencies raise the prospect of so-called
relaxation bottleneck phenomena. That is to say, if in order to drop to a lower-lying level an
excited electron needs to emit multiple simultaneous vibrational quanta, the probability of such
a transition should be strongly suppressed: multiphonon processes normally occur only as high
order interaction corrections.
The proposed relaxation mechanism which overcomes the bottleneck involves the special ability
of free clusters to deform. A cluster with an electron in a previously unoccupied orbital (e.g.,
excited by light or externally injected) will proceed to deform from its original shape.
Microscopically, this deformation invalidates the selection rule which in the lowest order allows
only single-phonon processes: whereas the rule assumes that the ionic equilibrium position is
fixed during the electronic transition, a shift in the cluster’s ionic framework can produce a
coherent multiphonon transition without any additional smallness. We have developed a method
which allows us to describe the evolution of the deformation parameter up to the level crossing
point, followed by electronic intershell transitions. The process is analogous to internal
conversion in polyatomic molecules, but generally involves more than a single pair of electronic
orbitals. It is found that the corresponding relaxation time indeed can be quite short.
As an application, we have performed a model calculation for aluminum clusters. This was
motivated by recent experimental work [1] which used time-resolved two-photon photoemission
of Aln¯ (n=6-15) to demonstrate that the intermediate electronic state decays within a few
hundred femtoseconds in all of the above cluster sizes, even in the closed-shell Al13¯. Our
results are in agreement with the observed lifetime.
This work was supported by the NSF (V.V.K.), a NATO Collaborative Linkage Grant (V.V.K.
and Yu.N.O.), and DARPA (V.Z.K.).
References
[1] P. Gerhardt, M. Niemietz, Y. D. Kim, G. Ganteför, Chem. Phys. Lett. 382, 454 (2003).
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