The structure of the ͑3 3 1͒ reconstructions of the Si(111) and Ge(111) surfaces induced by adsorption of alkali metals has been determined on the basis of surface x-ray diffraction and low-energy electron diffraction measurements and density functional theory. The ͑3 3 1͒ surface results primarily from the substrate reconstruction and shows a new bonding configuration consisting of consecutive fivefold and sixfold Si (Ge) rings in ͗110͘ projection separated by channels containing the alkali metal atoms. [S0031-9007(98)05973-0] PACS numbers: 61.10.Eq, 61.14. Hg, 68.35.Bs, 71.15.Mb Over the last decade there has been a large effort to understand the structure and properties of reconstructions on elemental semiconductor surfaces. The main driving force behind these reconstructions is the reduction of the number of dangling bonds without introducing too much strain in the surface region. Three structural elements meeting this principle have emerged so far. Particularly important are adatoms which saturate three dangling bonds on (111) surfaces while creating only one unsaturated bond. Adatoms stabilize the clean Ge(111)-c͑2 3 8͒ surface [1] and are also a major stabilizing factor in the dimer-adatom-stacking-fault model of the clean Si͑111͒-͑7 3 7͒ surface [2]. The second structural element that effectively reduces the number of dangling bonds is the dimer frequently found on the (001) surfaces [3]. The third structural element is the p-bonded chain which was first proposed for the clean Si͑111͒-͑2 3 1͒ reconstruction [4]. Yet this simple principle has been of no utility in predicting surface structures as demonstrated by the metal induced ͑3 3 1͒ reconstructions on the (111) surfaces of Si and Ge. Despite the small unit cell, the atomic geometry is still unknown and has been heavily debated over the last ten years [5][6][7][8][9][10][11][12][13]. The mere observation of the symmetry-breaking ͑3 3 1͒ unit cell calls for a unidirectional structural motif and it was, therefore, appealing to introduce p-bonded chains to explain the 3 3 1 periodicity. At present, there are two promising models for the ͑3 3 1͒ reconstruction that have been proposed on the basis of scanning tunneling microscopy (STM) [8], electronic properties [9], and total-energy calculations [10]. The Seiwatz model [see Fig. 1(a)] [7,11,12] consists of parallel p-bonded chains formed by fivefold rings of Si (Ge) atoms in ͗110͘ projection, separated by empty channels, with a top-site adsorbate saturating the surface dangling bonds. The second model is the extended Pandey model [10,13] [see Fig. 1(b)] which consists of a sevenfold ring carrying the p-bonded chain alternating with a five and six-member ring of Si. It is intuitive to describe the ring sequences from these models with the notation 567567 (extended Pandey) and 500500 (Seiwatz model). Unfortunately, neither of these structures is able to explain our surface x-ray diffraction data (SXRD) or low-energy electron diffraction (LEED) data.We determined the structure by using a multipletechnique ...

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The properties of palladium clusters, generated with the electrochemical scanning tunneling microscope, have been investigated both by experiments and by computer simulations. The clusters are found to be larger and more stable if the tip is moved further towards the electrode surface in the generation process. The simulations suggest that the larger clusters consist of a palladium–gold mixture, which is more stable than pure palladium. Dissolution of the clusters occurs from the edges rather than layer by layer.

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The complex flow features that arise from the impact of a shock wave on a concave cavity are determined by means of high-speed video photography. Besides additional information on features that have previously been encountered in specific studies, such as those relating to shock wave reflection from a cylindrical wall and those associated with shock wave focusing, a number of new features become apparent when the interaction is studied over longer times using time-resolved imaging. The most notable of these new features occurs when two strong shear layers meet that have been generated earlier in the motion. Two jets can be formed, one facing forward and the other backward, with the first one folding back on itself. The shear layers themselves develop a Kelvin–Helmholtz instability which can be triggered by interaction with weak shear layers developed earlier in the motion. Movies are available with the online version of the paper.