406 Acc. zyxwvutsr Chem. Res. 1980, zyxwvu 13, zyxwv 406-412 Wildman, and others for their contributions to the work described above. Helpful correspondence andlor conversations with R. zyxwvut J. Abraham, W. Adcock, N. L. Allinger, A. A. Bothner-By, par- ticularly R. E. D. McCEung, B. Pettitt, and R. Wasylishen, are hereby acknowledged. W e are grateful to referees for cogent criticism. This research was supported by Grant A1296 from the Natural Sciences and Engineering Research Council of Canada. Surface Structure Determinations with Ion Beams NICHOLAS WINOGRAD* and BARBARA J. GARRISON Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 Received March 4, 1980 During the last decade, the research objectives of most surface chemists have turned from an interest in macroscopic aspects of interfacial chemical reactions to the development of atomic descriptions of the surface chemical bond. This more advanced understanding now seems feasible, since many spectroscopic methods have entered the scene that can provide the same type of information which has been available on bulk phase systems since the 1930s. Most of the present effort is directed at determining either atomic positions of atoms or small molecules adsorbed on metal single-crystal surfaces or the detailed nature of the molecular orbitals which participate in the bond. The question of atomic structure seems most crucial since correct nuclear coordinates facilitate electronic structure calculations. The battery of techniques presently available may well be adequate to solve the structure problem, although uncertainties in how to interpret the spectroscopic results have restricted most work to fairly simple, model-type systems. For example, the location of a sulfur atom on a Ni(001) surface has just recently been determined to be 1.3 A above the surface Ni plane and presumably in a fourfold coordi- nation site. The same value has been obtained using low-energy electron diffraction (LEED),l photoelectron diffraction (PhD) ,2 and other photoemission tech- nique~.~ The bond distance is not found to be much different than that obtained for bulk NiS. A few other isolated surface bond distances have been determined using a surface EXAFS method4” and an ion back- scattering technique.4b As far as we know, however, the NiS case is the only example where the same result has been obtained by three different methods. And, al- though many LEED structure determinations have been published: in the absence of supporting data from other methods the reliability of the results is still usually open to discussion. Ion Bombardment Methods Here we wish to focus on the question of the atomic structure of surfaces utilizing ion beams of sufficient Nicholas Winograd was born in New London, CN, in 1945. He received his Ph.D. in Chemistry from Case Western Reserve University in 1970 and im- mediateiy joined the facuity of Purdue University. He moved to The Pennsytvania State University in 1979 as Professor of Chemistry. Winograd has been an Alfred P. Sloan Fellow and a John Simon Guggenheim Fellow. Barbara J. Garrison was born in Big Rapids, zyxwvutsrq MI. After she obtained her Ph.D. from the University of California at Berkeley in 1975, she did postdoctoral work at Purdue and held a lectureship at Berkeley. She joined the faculty at The Pennsylvania State University as an Assistant Professor in 1979. She has won a Camille and Henry Dreyfus Award for newly appointed facuity and has been named an Alfred P. Sloan Fellow for 1980. 0001-4842/80/0113-0406$01.00/0 energy to induce nuclear rearrangements which are controlled by the original configuration of atoms. With this approach, the incident ion, usually an inert gas such as Het, Ne+, or Ar+, is accelerated to a kinetic energy of 200-5000 eV and focused onto the sample surface. The momentum exchange between the primary ion and the atoms of the lattice is sufficient to initiate some atomic motion which has a component of momentum moving out into the vacuum. If this component is sufficient to overcome the surface binding forces, then some secondary particles may be found to eject from the A fraction of these particles are ionized as they leave the surface and can, therefore, be detected directly with a mass spectrometer (i.e., as in secondary ion mass spectrometry or SIMS). The SIMS technique has been of considerable recent interest to the surface analyst, since for elements with low ionization potentials or high electron affinities (e.g., H’, Na+, K+, O-., C1-, Br-, F-) the limit of detection can approach g.7 Fur- thermore, the primary ion can be focused to a diameter of 100 nm, allowing high spatial resolution. The ion microprobe has found numerous applications in the fields of geology, biology, semiconductor technology, and metallurgy.8 It is also possible, although generally with a large loss in sensitivity, to utilize some sort of post-ionization of the neutral specie^.^ This approach eliminates the large variations of the ion yield with the surface electronic properties, making the technique more quantitative. To utilize SIMS for examination of surface structure, at least two major problems have to be solved. First, the primary ion beam is known to induce a great deal of damage which can alter the chemical nature of the sample. In 1970, Benninghoven proposed that if the total primary dose (the number of ions/cm2 to strike (1) M. Van Hove and S. Y. Tong, J. Vac. Sci. Technol., 12,230 (1975); S. Anderson, J. B. Pendry, B. Kasemo, and M. Van Hove, Phys. Reu. Lett., 31, 595 (1973). (2) S. D. Kevan, D. H. Rosenblatt, D. R. Denley, B.-C. Lu, and D. A. Shirley, Phys. Reu. B, 20, 4133 (1979). (3) C. H. Li and S. Y. Tong, Phys. Reu. Lett., 40, 46 (1978). (4) (a) P. H. Citrin, P. Eisenberger, and R. C. Hewitt, Surf. Sci., 89, 28 (1979). (b) J. F. Van Der Veen, R. M. Tromp, R. G. Smeenk, and F. W. Saris, Surf. Sci., 82, 468 (1979). (5) M. A. Van Hove, Surf. Sci., 81, 1 (1979). (6) This phenomenon is often referred to as sputtering. Since the dictionary definition of sputter is “To emit saliva from the mouth in small particles . . .”, we have, in general, avoided this type of jargon. (7) A. Benninghoven, Surf. Sci., 28, 541 (1971). (8) An excellent illustration of the general applicability of SIMS is given by the papers of an international meeting held in Palo Alto, CA, Aug 27-31,1979, and published in Springer Ser. Chem. Phys., 9 (1979). (9) H. Oechsner, W. Ruhe, and E. Stumpe, Surf. Sei., 85, 289 (1979). 0 1980 American Chemical Society