Institut des Sciences Moléculaires d'Orsay




vendredi 26 mai

Mise à jour
jeudi 11 mai

Accueil du site > Équipes scientifiques > Surfaces, Interfaces : Réactivité et Nanostructuration (SIREN) > Interactions atomes-surfaces

Interactions atomes-surfaces

Probing crystal surfaces with fast atoms and ions at grazing incidences

Probing surface cristallographic structure

  • By observing the diffraction profile (quantum regime) we are able to measure, in few seconds, and with pm accuracy the exact shape of the surface electronic density, e.g. the \beta_2 reconstruction of the GaAs(001) surface at 600°C [more]
  • When diffraction is not observed or of poor quality, we try to characterize the surface structure by atom triangulation [more]

Probing surface defects

  • Watching the extension of the diffraction spots, we are sensitive to extremly small amount of defect probably around one out of 10000 surface atoms [more]
  • The twist and tilt surface mosaicity can be tracked online with high accuracy [more]

Probing dynamics of surface atoms and electrons, a surface can also be investigated by how it reacts, what sort of excitations can we trigger ?

  • How the surface atoms are moving ? [more]
  • What electronic excitations are possible ? [more]
  • How can a given projectile capture or give an electron ? [more]

Probing surface growth

When exposed to molecular or atomic vapor, successive layers start to build up, this is particularly well controlled inside Molecular Beam Epitaxy vessel. Our diffraction technique is able to count the number of deposited layer as well as their quality [more].

How we do it ?
- we shoot a well defined beam of fast atoms or ions at grazing incidence so that they cannot penetrate below the surface and we observe the scattered particles on a position sensitive detector[] placed downstream. Here we call fast all projectiles above 100 eV and we use such energies to be able to detect them with high efficiency with microchannelplates, the kind of device used to in high end night vision systems.

- what for ? one simple answer is to investigated fondamental processes. For instance it was previously unclear why, upon impact of atoms or ions, most insulators emit more electrons than metal whereas they are known to bind more strongly their electrons ??? [more] Here, the answer was that part of this large energy needed to extract an electron can be bypassed when the projectile goes negative before loosing its electron. this was demonstrated by using measurement where emitted electron were detected in coincidence with the scattered projectiles. more generally, when large effects are understood, smaller effects become visible etc. This is exactly what happened, after electron capture and loss we investigated discrete electronic excitations then quasi continuous excitation of electron hole pair, then the excitation of phonons until we discovered elastic diffraction, i.e. conditions were no energy is exchanged with the surface !!


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