Observation of Interference Oscillations in Helium Scattering from Single Surface Defects

Abstract

Large-angle-diffraction oscillations in the intensity of helium atoms elastically scattered from randomly stepped Pt(lll) are reported. A formalism based on the concept of reflection-symmetry interference explains the observed features and the data analysis yields a value of 4.4-4.6 A for the hard-core repulsive cross section of a single step. The observed oscillations are sensitive to the presence of adsorbates at step sites.PACS numbers: 79.20. Rf, 61.16.Fk, 6L70.Ey, 68.35.Dv There is now considerable evidence that surface defects (steps, kinks, and point defects) provide the active sites for chemical reactions on single-crystal surfaces. 1 LEED provides the present standard technique for determining defect geometries and densities, 2 but to date its sensitivity has only been sufficient to study the statistical distribution of defects and not their individual details. The angle or energy dependence of the specular scattering of He atoms has been shown to provide quantitative information on step heights and average densities, 3 and more recently the attenuation of the specular beam in He atom scattering by steps has been demonstrated. 4 Since the integral total cross sections derived in the latter type of measurement are dominated by long-range attractive forces, they are rather large (= 12 A) (Ref. 4) and consequently do not provide direct details on the physical size or orientation of the steps, nor on changes in the immediate environment of the single steps as may occur during gas adsorption. In this Letter, we report on the first observation of large-angle hard-core diffraction from single randomly spaced monatomic steps on a single crystal surface, in this case Pt(lll). Since the observed diffraction patterns are determined by the interference of atom trajectories probing the repulsive potential of the step and the adjacent undisturbed smooth mirrorlike regions, the actual size and orientation of the single steps could be determined. We also see striking changes in the step diffraction as CO is adsorbed at the steps, demonstrating that valuable chemical information on active sites in heterogeneous catalysis can be obtained by this new technique.The observation of diffraction from single steps has been made possible by the introduction of highresolution He nozzle beams and sensitive time-offlight detection techniques used to study surface phonons. 5 The apparatus is essentially identical to that used in the phonon studies; however, in the present experiment only the elastic peak is of interest. The helium beam energy can be varied between 8 and 80 MeV (/c/ = 4-12.7 A"" 1 ) with a velocity resolution Av/v of 1%. The polished Pt (111) crystal was cleaned in vacuum (base pressure 10"" 10 Torr) by cycles of sputtering, annealing at 1000 K, and heating in oxygen until no impurities were detected in the Auger trace. In most of the experiments the sum of incident 0, and final angles B f is 90°. The parallel momentum transfer, AJ£= B fc/(sin0/--sin0/), is varied by rotation of the crystal in the scattering plane ...

Surface Ordering and Kinetics From Atom Beam Diffraction

“…Soon afterwards, similar diffuse intensity oscillations were discovered in step edge defect scattering [8]. Likewise, these oscillations, shown in Fig.l, extended to large parallel momentum transfers, AKI 1 .…”

Role of quantum vortices in atomic scattering from single adsorbates

“…1 This phenomenon is characterized by an incoherence among particles scattered from different defects, 2 and is well illustrated by He scattering from CO molecules adsorbed on the Pt͑111͒ surface, for which there is a wealth of experimental data and theoretical work. 1,3-5 Lahee et al 1 reported for the first time large-angle diffraction oscillations, explaining them in terms of the so-called reflection symmetry interference. This mechanism arises from the combination of adsorbate/surface double collisions, and direct scattering with the adsorbate.…”

“…This mechanism arises from the combination of adsorbate/surface double collisions, and direct scattering with the adsorbate. 1,4 However, this interpretation, based on a hard-wall model, does not account for rainbow effects, which appear when realistic potentials are considered. 3,5 Despite exhaustive work, some controversy regarding the assignment of the dynamical origin of diffraction peaks has been unavoidable.…”

High Resolution Helium Scattering Studies of Inelastic Interference Structures of the Frustrated Translational Mode of CO on Cu(001)

“…The Fraunhofer contribution to the scattering intensity falls off rather rapidly with Q according to the envelope function 1 jg3, but the two illuminated face contributions typically scatter over a large range of angles and they interfere with each other leading to an oscillatory behavior in the scattering intensity at larger Q. These oscillations are a type of supernumerary rainbow behavior and have been called reflection symmetry oscillations [19,20]. The peaks and troughs of the oscillations fall at the same points as those of the experiment.…”