Preface

Abstract

As is well known, the physical properties of a solid are inevitablyconnected with its geometric structure. As the same holds for itssurface, surface physics at its beginnings was largely dominatedby efforts related to surface structure determination and, todate, itcontinues to need the structural input. Not surprisinglytherefore, a number of methods of retrieving the surface structurehave been developed in the past. However, only a few of them, inparticular diffractive methods using electons or photons, are ableto retrieve the full structure of the surface in thecrystallographic sense, and this situation continues to persistalmost two decades since the arrival of the scanning tunnellingmicroscope.However, although the diffractive methods mentioned have proved toapproach the picometer accuracy level for atomic coordinates innumerous cases, they suffer from the phase problem inherentto them. With direct methods to invert the measuredintensity data lacking, trial-and-error based procedures must beused. However, the latter work only if the correct type ofstructural model is known, intuitively guessed or found by tediousexclusion of all other reasonable models. Only then can the numericalvalues of structural model parameters be determined by asearch procedure, which is usually controlled by the quantitativecomparison of computed and measured data applying reliabilityfactors. If the scientist's knowledge or fantasy are limited tothe extent that the correct type of model cannot be found, itsparameter values also remain hidden.Therefore, an idea introduced by Szoke in 1986 to circumventthis problem attracted much attention. He proposed to creatediffraction patterns in a special way, so that they can beinterpreted holographically. This is by creation of inner sourcesthrough external excitation, i.e. by causing certain atoms (ornuclei) within the surface to emit electrons or photons whichreach an external detector both directly (reference wave)and after scattering by neighboured atoms (object wave). So,the detector scanning all angles of emission records ahologram-type interference pattern without the need for anexternal beam splitting device. Also, with path length differencesof only interatomic order involved, the limited coherence of theemitted radiation is no problem. Interference patterns taken formany energies of the particles emitted can be computationallyevaluated, i.e. holographically reconstructed to directlyproduce real space images of the atoms involved and so givevaluable information of the applying type of the structure.Not suprisingly, Szoke's proposal has triggered numerous andintense activities. The corresponding field of research rapidlyspreaded according to the various types of usable radiation andemitting processes as, e.g., photo-, Auger-, LEED- andKikuchi-electrons or x-rays and γ-rays. Diversity holdsalso for different techniques of data recording, including theinterchange of the role of the detector and radiation source, aswell as for the way the holographic reconstruction is carried out.Also, related direct methods emerged adapting e.g. the Pattersontransform method to the electron multiple-cattering case or tosolve the phase problem in surface x-ray diffraction using relatedholographic concepts. This special issue reviews thecorresponding field (A further article will be published in a forthcoming issue.) of these Holographic and other directmethods for surface structures using electrons and photons. K HeinzLehrstuhl fur Festkorperphysik, UniversitatErlangen-Nurnberg,Staudtstrasse 7, D-91058 Erlangen, GermanyE-mail: kheinz@fkp.physik.uni-erlangen.de