Complete report - Donostia International Physics Center - Euskal ...

2011 DIPC Highlight
Quantum plasmon tsunami
on a Fermi sea
Lucas, G. Benedek, M. Sunjic, and P.M. Echenique
New Journal of Physics 13, 013034 (2011)
A highly charged ion flying-by a fullerene molecule violently strips away one of its valence electrons
leaving behind a plasmon quantum tsunami, consisting in a coherent state of excitations
of the charge density. The electron transfer confers an energy gain to the ion minus the energy
required to excite the plasmon coherent states. By explaining a 15-years-old conundrum raised
by state-of-the-art scattering experiments, a team of researchers from DIPC and the Universities
of Namur, Milano-Bicocca and Zagreb suggest a new tool to investigate energetic collective excitations
in nanostructures.
A beam of energetic, highly charged Ar q+ ions is sent through a dilute C 60 fullerene vapor. An ion, which
happens to pass by a neutral molecule, strongly polarizes it to the point where it may capture one or several
electrons which land onto the ion discrete Rydberg states (Figure). The electronic potential energy released
by the charge transfer causes an increase of the ion kinetic energy: Some understanding of the
basis of the mechanism was already pointed out some years ago however, the highly unexpected shape
of the measured energy gain distributions could not find a straightforward explanation. The gain curves
extend smoothly to very high energies and are topped, from beginning to end, by series of small oscillations
of 6 eV period (see Figure inset (a)), which evidently bear very little relation to the underlying set of
discrete ion Rydberg levels. The work summarized here demonstrates that the discrete Rydberg spectrum
is the due to the occurrence of a plasmon tsunami on the molecule. This consists of a violent rearrangement
of valence electrons around the positive hole left behind on C 60 by the fast electron transfer. Charge
density waves rush to screen the naked positive hole as it suddenly appears. This causes multiple plasmon
excitations whose energy quanta are provided by the ion kinetic energy. The electrons always land on Rydberg
levels but the energy liberated is apportioned between the ion kinetic energy increase and creations
of plasmons. Thus the intensity of the kinetic energy gain peak, expected to appear at the nominal position
of a Rydberg level, is in fact redistributed over a Poissonian series of plasmon peaks (hence the oscillations)
extending on the loss side of the level.
The theory developed in this work convincingly accounts for the observed spectra for all momenta q, as
shown in the Figure inset (b). The phenomenon is a real electrodynamical quantum tsunami on the molecule
which rings like a bell whenever one or several of its electrons are stripped away by the ion. The experimental
method of highly charged ion energy gain spectroscopy, supported by the new understanding
of the present theory, offers a novel opportunity to investigate collective excitations in a variety of nanostructures
such as large molecules, atomic clusters or solid surfaces.
A highly-charged argon ion, e.g., Ar 15+ , when passing through C 60 gas can fly by a molecule so as to violently
strip away and capture one of its valence electrons (e − ). Charge density waves rush to screen the positive hole
left behind on C 60 thus raising a tsunami of coherent plasmon excitations (orange waves) whose energy quanta
are provided by the ion kinetic energy. The ion gains the energy released by the electron in dropping into a
Rydberg state of the ion, decrease however by the energy required to excite a coherent distribution of C 60 plasmons.
The gain spectra for one and two electron transfers measured by Selberg et al (inset (a)) are well reproduced
by theory (inset (b)) once both and plasmons of C 60 are considered.
A highly charged ion strips away a
valence electron from a molecule leaving
behind a plasmon quantum tsunami.
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