Faraday zyxwvutsrqp Discuss., 1996,105,295-315 zyxwvu Adsorption on a polar oxide surface: O2 C2H4 and Na on Cr20,(0001)/Cr(110) Bernd Dillmann, Friedmann Rohr, Oliver Seiferth, Gabor Klivenyi, Michael Bender, Klemens Homann, Ivan N. Y akovkin, Daniela Ehrlich, Marcus Baumer, Helmut Kuhlenbeck and Hans-Joachim Freund* Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg zyxw 4-6, 141 95 Berlin, Germany and Lehrstuhl zyxwvuts f iir Physikalische Chemie I, Ruhr-Universitat Bochum, 44780 Bochum, Germany A polar Cr,0,(0001) surface is prepared as an epitaxial film on a Cr(ll0) substrate. The film is thick enough to represent the bulk surface. Applying a variety of surface sensitive techniques [thermal desorption spectroscopy (TDS), reflection absorption infrared spectroscopy (RAIRS), electron energy loss spectroscopy (EELS) and photoelectron spectroscopy (PES)] we have studied adsorption of molecular oxygen, ethene and sodium. Introduction The physics and chemistry of oxide surfaces have received increasing attention recent- ly.1-38 There are several books and review articles that cover various aspects of the field.'-' As a specific issue, the comparison of non-polar and polar surfaces of insulating oxides is interesting from a chemical point-of-view, because of the higher energy content of polar surfaces, which can be used to drive restructuring processes.8 Although it appears to be very difficult to prepare polar surfaces via cleavage of bulk single crystals, preparation as thin films is straightforward. It has been demonstrated for MgO( 111)26 and NiO(l1 1)39 that surface r e c o n s t r ~ c t i o n ~ ~ * ~ ~ plays a prominent role for the clean surface. If, however, the surface is hydroxy-terminated for NiO(l1 l),' we find a p( 1 zy x 1) LEED pattern, characteristic of the ideal hexagonal surface. The reconstruction of the clean, polar rocksalt surfaces involves rather massive mass transport indicating that the mobility of the surface constituents can be large. In particular, the so-called octopolar reconstr~ction~~~~~ extends throughout the first two layers of the (111) surface in order to compensate the diverging surface potential. For corundum surfaces the reduction of charge in the surface layer has been suggested to occur via removal of half of the number of surface ions, thus formally only taking place in the topmost l a ~ e r . ' > ~ ' - ~ ~ However, we know from a recent LEED I / V - ~ t u d y ~ ~ of a Cr,O,(OOOl) surface that very pronounced relaxations occur between several layers near the surface, such relaxations are in line with theoretical predictions for (0001) surfaces of corundum-type o ~ i d e s . ~ ~ ? ~ ~ As a prototype corundum surface we have investigated, in this study, adsorption and reaction of small molecules such as 0, and ethene on a Cr,O,(OOOl) surface, as well as adsorp- tion of Na (to include a metallic adsorbate) by applying a variety of experimental tech- niques, such as TDS, EELS, PES, LEED and RAIRS. Experimental A chromium oxide film with (0001) orientation was grown on a Cr(ll0) surface via oxidation and thermal treatment c y ~ l e s . ~ ' - ~ ~ The order of the film was checked via 295