Surface Dynamics, Phonons, Adsorbate Vibrations and Diffusion

Abstract

The following papers represent about one-half (twelve out oftwenty-five) of the presentations at the Surphon 11 Workshop heldat Meramec, Missouri in October 2003. Papers of the preceding(2001) workshop were published as a special issue of Journalof Physics: Condensed Matter (2002 volume 14, issue 24). Theannounced scope of these workshops, Surface Dynamics,Phonons, Adsorbate Vibrations, and Diffusion, is so broad thatone may question whether there is an underlying unity to themeetings. The answer is yes, and the discussions and comments onthe contributions, while not reported here, met the goal offostering constructive interactions. In the following, I attemptto illustrate this.One general feature of the work is common to research in most ofmodern condensed matter physics and surface science in particular. The systems underdiscussion are tailored materials or otherwise controlled at the(sub)nanometer length scale. There is an immense variety for thechoice of substrate, face exposed by the substrate, and theadsorbate. Some effort is in effect a service, helping tocharacterize a system for others who will build applications on itor incorporate it in a more complex composite. Other effort aimsto improve understanding at the atomic and molecular scale,especially when the phenomena are `simple enough' that there is agood prospect for a quantitative account using first principlestheory. Characteristically, a wide range of experimental andtheoretical techniques are synthesized to meet these goals. Forinstance, the papers in this volume include experiments withhelium atom scattering (HAS, QHAS) [1], low-energyelectron diffraction (LEED) [2,3], scanning tunnelling microscopy (STM) [3], x-raydiffraction [4], and high-resolution electron energyloss spectroscopy (HREELS) [5]. In closely relatedexperiments, thermal desorption (TDS) [6],ellipsometry, [7] and neutron scattering[8] are applied. The balance has evolved over theseries of workshops and in the near future one may anticipate alarger component of inelastic and quasi-elastic neutron scattering(INS, QENS) as new facilities are completed.There is a continuing attempt to use atomic scattering inqualitatively new ways. The last proceedings had areview [9] of efforts to greatly improve the energyresolution of helium beams using spin-echo methods for 3He. Thepresent one has a review [10] of developments of fieldionization detectors for He beams that have the prospect ofimproved detection efficiency and higher spatial resolution foratom interferometers. Since the detection efficiency for mostthermal helium beam instruments is only 1/105, there appears tobe much room for improvement, but progress has been slow. Anothercontinuing effort is the implementation and interpretation ofsurface scattering with metastable He beams to probe substrateelectronic structures, but the experimental work has had its fullshare of controversy [11-13].The focus of these workshops has been on dynamics, but the settingis defined by the spatial structures and surprisingly complexgrowth scenarios occur for thin films, especially for molecularfilms. These have been clarified with multi-techniqueexperiments [4]. Even for the adsorption of inert gases,it has been a long process to determine the adsorption sites ofminimum energy [2, 14]. In thissub-field, first principles theory now is at the stage ofquantitative comparison withexperiments [15, 16].Two examples using ideas of quantum chemistry were presented atthe workshop but were publishedelsewhere [17, 18].There is work with new materials and more highly controlledmaterials [19]. In one example, the structure of aquasicrystal was determined by LEED and scanning tunnellingmicroscopy and the substrate was then used as an adsorbingsurface [3]. Even in the Henry's law regime, there aresurprises, and a quantitative analysis demonstrates how open theeffective surface is. Another example is the use of vicinalsurfaces to introduce a controlled population of step defects andthen to characterize the excitations of molecules adsorbed at thesteps [5].About half of the papers are primarily theoretical. Further, thestandard for LEED experiments in this area is to includequantitative analysis of intensities [2] andexperiments on adsorption in the Henry's law regime usuallyinclude model building of the adsorption potentialenergy [3]. For inelastic atomic scattering thesituation is decidedly less routine. The contributions hereinclude a review [20] of the development of a mixedquantum-classical theory of inelastic molecular scattering bysurfaces and an extension [21] of analytical resultsfor quasi-elastic helium atom scattering with application to lowenergy and diffusive motions.One of the theory papers [22] relates todissipative processes in friction. At first sight this involves arather different time scale than those for the dynamics emphasizedhere. However, with increased energy resolution, the surfacedynamics experiments get to time scales of several picosecondswhile some experiments on sliding friction get down to time scalesof a few nanoseconds [23]. Thus, there remains aconsiderable gap to bridge before the two areas overlap in auseful fashion. Still, this is one likely direction for progressand illustrates both the changing cast of characters in the fieldand the extensive base of knowledge that is needed forquantitative analysis of both the adsorbate damping and slidingfriction. The Surphon workshops appear to have a long life ahead. L W Bruch Department of Physics, University of Wisconsin-Madison,Madison, Wisconsin 53706, USA (e-mail: lwbruch@wisc.edu)AcknowledgmentThis work has been partially supported by NSF-DMR0104300.