Surface vibration helium atom scattering

Graham, A.P., Toennies, J.P. "Determination of the Lateral Potential Energy Surface of Single Adsorbed Atoms and Molecules on Single Ciystal Surfaces Using Helium Atom Scattering", in Int. Conf. on Vibrations at Surfaces. Tokyo, Japan, 1998. 

The translational vibration frequencies of n-C H2 +2 (n 6,8,10) against Cu(110) surfaces at temperatures near 150 K have been measured by helium atom scattering (139). 

Measurements have been made by helium atom scattering of the vibration of benzene against the Cu(110) surface near 150 K (139) and of the frus-trated-translation mode of benzene parallel to the surface on Rh(lll) at 160 K (327). 

D. D. Koleske, Studies of surface vibrations and structure using molecular dynamics simulations, lattice dynamics calculations, and helium atom scattering, 1992)... 

HIGH-RESOLUTION HELIUM ATOM SCATTERING AS A PROBE OF SURFACE VIBRATIONS... 

It is intended for this chapter to be a brief introduction to surface lattice dynamics and to some of the kinds of information that one is able to obtain about the electronic and vibrational properties of surfaces through high-resolution helium atom scattering. Such information is critical to the understanding of many aspects of surface chemistry and to the design of novel materials with specifically desired properties. 

The next phase for the theorists in connection with this work lies in predictions of helium atom scattering intensities associated with surface phonon creation and annihilation for each variety of vibrational motion. In trying to understand why certain vibrational modes in these similar materials appear so much more prominently in some salts than others, one is always led back to the guiding principle that the vibrational motion has to perturb the surface electronic structure so that the static atom-surface potential is modulated by the vibration. Although the polarizabilities of the ions may contribute far less to the overall binding energies of alkali halide crystals than the Coulombic forces do, they seem to play a critical role in the vibrational dynamics of these materials. 

The ionic insulators discussed in some detail in the previous section have closed shell electronic configurations similar to the noble gases and electronic distributions which are localized around the electronic core. The principal interactions are Coulombic, although their polarizabilities appear to influence greatly the response of the electronic distribution to surface lattice vibrations. For other materials, particularly metals and some layered compounds, the conduction and valence electrons are best thought of as somewhat delocalized if not entirely free. These electrons are what the helium atoms scatter from, and their states of motion are significantly modulated by the vibrations of the atomic cores. Thus, for these materials HAS is very... 

In the fields of solid-state and molecular physics, there have been many different experimental techniques introduced for vibrational spectroscopy. However, the investigation of the vibrational properties of bare solid surfaces is dominated by the two techniques of helium atom scattering (HAS) and high-resolution electron energy loss spectroscopy (HREELS). Both have been used to map surface phonon dispersions. In addition, that is, optical techniques such as infrared absorption spectroscopy (IRAS) (Chapter 3.4.1) and recentiy also inelastic tunneling... 

High-resolution electron energy loss (HREELS = high-resolution electron energy loss spectroscopy) experiments and inelastic scattering of helium atoms allow a determination of the surface vibration modes and of the phonon dispersion curves. The same cannot be achieved by inelastic neutron scattering, due to the high penetration depth of the neutrons... 

Section II is devoted to setting up elementary concepts associated with collective vibrational motion of atoms for reference later in the discussion of experimental results. Section III deals with atom-surface scattering theory so that the reader unfamiliar with this kind of spectroscopy can relate to the quantities which are measured directly. A brief description of the helium scattering instmment then follows in Section IV. 

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