Download Adsorbed Monolayers on Solid Surfaces by G.A. Somorjai, M.A. van Hove PDF

By G.A. Somorjai, M.A. van Hove

ISBN-10: 3540095829

ISBN-13: 9783540095828

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4. 02 eV have X = 1 )k and can readily diffract from surfaces. A beam of atoms is chopped with a variable frequency chopper before striking the surface. This way, an alternating intensity beam signal is generated at the mass spectrometer detector, that is readily separated from the "noise" due to helium atoms in the background. There are three processes observed during the scattering of atomic beams 18) of helium that are displayed in Fig. 5. , the helium atoms scatter at an angle that is equal to the angle of incidence) (curve a).

3 SPI (Surface Penning Ionization) A metastable helium atom incident (with thermal kinetic energy) onto a surface allows a surface electron to tunnel to the unoccupied low helium level, enabling the excited helium electron to be emitted and detected 24). This occurs with energy and angular distributions characteristic of the electronic structure and therefore geometry of the surface, in a way similar to photoemission (see the heading: UPS and XPS). Whereas in photoemission the excitation occurs via a dipolar photon-electron interaction, in SPI it is an IF - r-~l-1 electron-electron interaction that takes place, so that the selection rules are different.

5y ' ' ' ' ' X . 5 I-- N Z tlJ o ttJ rr Ld os c) 03 I O, 30 60 ANGLE FROM SURFACE NORMAL 90 Fig. 7. Normalized scattered intensity vs. angle from surface normal for an azimuthal angle o f 4~= 24 ° with a fixed angle of incidence o f 45 ° on a stepped Pt(553) surface for classical scattering for helium atoms. At grazing angles of incidence, the surface appears less spatially rough and thus allows a more intense elastic scattering contribution. At more normal angles of incidence, the elastic scattering distribution is less intense due to an increase in spatial roughening of the surface as seen by the helium atom.

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