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HREELS

HREELS


High Resolution Electron Energy Loss Spectroscopy

(Dr. D. Farías, Dr. P. Schilbe)

Research Fields

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

Schematic Drawing of ELS22 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

[Schematic Drawing of the Apparatus] 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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