Experimental Results: Vibrations of Adsorbates and Thin Films

Abstract

This chapter reviews some exemplary high resolution HAS experiments on surface phonon dispersion curves of adsorbate atoms and molecules either well dispersed or in layers on single crystal surfaces. Here also HAS provides unique information on the low energy vibrations with respect to the surface as well as within the layers. HAS experiments have also been used to follow the evolution of the phonon dynamics as the coverage is increased to produce one or more monolayers. These experiments are first illustrated for rare gases and simple molecules. The alkali metals show a number of new phenomena, including so-called organ-pipe modes which result from the confinement provided by the interface at the substrate. In the classical macroscopic realm the organ-pipe modes are known as Sezawa modes. The HAS dispersion curve measurements of thin films of the superconductor Pb on Cu(111) show a surprising plethora of modes. Recent DFPT and EAM calculations demonstrate that the dynamics throughout the entire film can only be understood by taking account of the strong electron-phonon coupling, which is discussed in detail in Sect. 8.4. Thus modes deep within the film and even modes localized at the interface, known as Stonely waves in the classical limit, make themselves noticed at the surface by a mechanism which is called the quantum sonar effect. HAS has also been used to study thin films of polyatomic organic molecules physisorbed on metals and also self-assembled chemisorbed monolayers. Interesting changes accompany the temperature induced transition from physisorption to chemisorption. The complex phonon dispersion curves of methyl radicals chemisorbed on silicon have recently been understood with the aid of DFPT calculations. The chapter closes with a brief review of HAS prospects for studying one-dimensional chains. Each section is meant as an introduction to the specific phenomena and issues characteristic to each of these different surface types, with special consideration to the questions solved and the new theoretical issues raised. This chapter is complemented by appendices which contain a comprehensive list (up to 2016) of the HAS experimental data on adsorbates on insulators and semiconductors (Appendix A.5), atomic adsorbates on metals (Appendix A.6), diatomic molecules on metals (Appendix A.7) and polyatomic molecules on metals (Appendix A.8).

It is true that we have really in Flatland

a Third unrecognized Dimension called ‘height’.

(Edwin A. Abbott)