Theinteractionofgasmoleculesatatmosphericpressureswith surfaces investigated with surface X-ray diraction H. Isern a , K. Peters a , P. Steadman a , O. Robach a , J. Alvarez a,b, * , E. Lundgren a,c , S. Ferrer a a European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex, France b Dept. Fisica CIII, Universidad Autonoma de Madrid, 28049 Madrid, Spain c Inst. Fur Allegemeine Phys., Wiedner Haupstrasse 8-10, A-1040 TU Vienna, Austria Abstract The suitability of surface X-ray diraction to study the structure of chemisorbed layers of light gas molecules is discussed. The sensitivity of the technique, for well ordered adlayers, is comparable to that of Auger spectroscopy around 10-2 monolayers). Surface crystallography is illustrated for CO/Rh111) under UHV and also for CO/ Au111) near atmospheric pressure. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Carbon monoxide; Catalysis; Solid±gas interfaces; Surface chemical reaction; Surface thermodynamics including phase transitions); X-ray scattering, diraction, and re¯ection 1. Introduction One of the major goals of modern surface sci- ence has been the understanding of the surface chemicalbondanditsroleinchemicalreactionsin heterogeneous catalysis, which have major eco- nomic and technological importance. Since the early days of modern surface science, the chemi- sorption of gas molecules such as CO, O 2 andH 2 on transition metals has been studied extensively. Aftermorethan25yearsofresearch,thestructure of several hundreds of chemisorption systems has been determined with a variety of techniques. Practically all the structural studies have been done under ultra-high-vacuum or high vacuum environments. Since catalytic reactions are per- formed at atmospheric pressures, the question of the ``pressure gap'' has been open for many years: aretheUHVstructurestheonesthatactuallyexist onthesurfacesofthecatalystsunderrealreaction conditions? There are fundamental questions which induce to suspect that things may be very dierent be- tween UHV and the atmosphere. The chemical potential of a gas, in the simplistic ideal gas approximation, is l  kT log p. The 13 orders of magnitude from 10 10 mbar to 1 bar imply a change Dl  13 kT i.e. more than 300 meV at room temperature. This additional chemical en- ergy may play a decisive role in surface chemistry. Looking to the kinetics of surface chemical pro- cesses, suppose that a nonnegligible activation Surface Science 482±485 2001) 101±106 www.elsevier.nl/locate/susc * Corresponding author. Address: Dpto ®sica de la materia condens., Universidad Autonoma de Madrid, 28049 Madrid, Spain. Tel.: +34-913975550. E-mail address: jesus@hobbes.fmc.uam.es J. Alvarez). 0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0039-602800)01017-7