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HREELS
High Resolution Electron Energy Loss Spectroscopy
(Dr. D. Farías, Dr. P. Schilbe)
Research Fields
- Adsorbate Vibrations and Structure
- Surface Phonons
The Method
High resolution electron energy loss spectroscopy is a highly surface sensitive
vibrational spectroscopy method capable of determining adsorbate vibrations
and surface phonons. Its wide energy range
and high parallel momentum transfer. As a local method it does not require
long range order to determine adsorption sites. The wide energy range and
high momentum
transfer allow determination of the complete phonon dispersions relation.
The Equipment
Our HREEL spectrometer
is a Leybold-Heraeus ELS22
with two double-127°-deflectors.
The analysing system is complemented by a LEED system capable of retarding field Auger
spectroscopy, a quadrupole mass spectrometer, and a Kelvin probe.
The Experiments
So far the focus has been on adsorption of hydrogen, oxygen and carbon monoxid
on high index
metal surfaces [Ni(311) and Rh(311)] as well as on the dispersion of surface
phonons in graphite and its intercalation compounds.
Future efforts will concern vibrations in fullerens and fullerite films.
Adsorption on Metals
On Ni(311) the adsorption of CO, H2 and O2 was investigated.
The adsorption of CO on Ni(311) proved to be very similar to the adsorption of CO on Ni(110).
This shows that the crucial structural features that govern the adsorption behavior of this
system are the close packed rows.
For hydrogen adsorption on these fcc(311) surfaces the interesting question is whether
it first adsorbs in the threefold or in fourfold site since it is known that hydrogen
prefers highly coordinated sites and the fcc(311) offers both. On both surfaces it is
seen that the hydrogen first occupies the threefold site.
HREELS is the only method available today that is capable of measuring the phonon
dispersion of graphite over the whole energy range and throughout the whole Brillouin zone.
We measured the graphite phonons in gammaK-direction. We also investigated the phonons
for the graphite intercalation compound with Lanthanum. As one might expect the optical
phonons soften as compared to pure graphite but interestingly the acoustical modes stiffen.
The adsorption of CO on Ni(311)
proved to be very similar to the adsorption of CO on Ni(110).
This shows that the crucial structural features that govern the adsorption behavior of this
system are the close packed rows.
For hydrogen adsorption on these fcc(311) surfaces
the interesting question is whether it
first adsorbs in the threefold or in fourfold site since it is known that hydrogen prefers
highly coordinated sites and the fcc(311) offers both. On both surfaces it is seen that the
hydrogen first occupies the threefold site.
Phonons
HREELS is the only method available today that is capable of measuring the phonon dispersion
of graphite over the whole energy range and throughout the whole Brillouin zone.
We measured the graphite phonons
in gammaK-direction.
We also investigated the
phonons
for the graphite intercalation compound with Lanthanum.
As one might expect the optical
phonons soften as compared to pure graphite but interestingly the acoustical modes stiffen
which is not fully understood yet. We also perform Born-von Karman calculations to model the
phonon dispersions.
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