Azimuthal- and Polar-Angle-Dependent Surface Extended X-Ray—Absorption Fine-Structure Study: (2×1)O on Cu(110)

Abstract

A low-symmetry twofold bridge site of (2x1) oxygen on Cu(110) allows the determination of the azimuthal dependence of the surface extended x-ray-absorption fine structure. For normal incidence and E parallel to the [100] and [110] directions we determine the nearest-and second-nearest-neighbor distances to be 1.84(2) and 2.00(5) A, respectively. We show that the atomic oxygen is occupying the low-symmetry twofold bridge site in the [100] direction 0.35 A above the first Cu layer.PACS numbers: 68.20+t, 78.70.DmThe surface version of the extended xsensitive to long-range periodic structure but only ray-absorption fine-structure (SEXAFS) method to local environment, the present experiments indihas become a powerful technique during the last cate strongly a long bridge site on top of the first Cu five years. 1 " 3 Detailed information on bond layer. lengths and next-neighbor coordinations of ad-The adsorption of oxygen on the Cu (110) surface sorbed atoms on single-crystal substrates can be has been studied by numerous techniques such as gained by employing the polarized synchrotron radiwork-function, low-energy electron-diffraction ation as a "searchlight." Only bonds which have_a (LEED), and Auger electron spectroscopy (AES) sizable component of the electric field vector E measurements, 4 ' 7 low-energy ion scattering, 5 and parallel to the adsorbate-substrate internuclear axis helium diffraction. 6 From LEED it is known that will contribute to the amplitude of the EXAFS osoxygen forms a stable (2x1) overstructure at room dilations. There exist two angular dependences: temperature and exposures of 10 to 100 L oxygen. (i) on the polar angle 9 between the E vector and Whether this (2x1) structure in the LEED pattern the surface normal n (detected in most cases) and is caused only by the array of O atoms or by a (ii) on the azimuthal angle <f> in the surface plane.reconstructed first layer of the Cu substrate atoms The latter dependence has not been reported to is not completely clear. The second open question date, because it disappears for high local symmetry for this adsorbate-substrate system is the local posi-(larger than twofold). This ambiguity is lifted for tion of the oxygen atoms. The present experiments low symmetry as for a twofold bridge site. In the are sensitive to the local site since they measure the present Letter we report this observation for the adsorbate-substrate bond length in a direct way. (2x1) structure of atomic oxygen on Cu(110).The fact that for the first time the azimuthal anguThe different SEXAFS spectra for normal incidence lar dependence of EXAFS is detected, and by virtue and E parallel to the [100] (</ > = 0°) and [110] of this the nearest-and second-nearest-neighbor (0 = 90°) directions enable us to determine uniquedistances are determined independently, strengthly the different bond lengths. ens this result. It was assumed 5 that the O atom sits Secondly, the position of chemisorbed O on between the first and second layers of Cu atoms. Cu(llO) for low (10-L [1 L (langmuir) = 10...

Reconstruction behaviour of fcc(110) transition metal surfaces and their vicinals

“…2b), where every other [001] metal row (i.e., perpendicular to the close-packed rows) is missing [50][51][52][53][54][55][56][57][58][59][60][61][62]. The oxygen atoms are located in asymmetric positions close [63] to the long-bridge sites [60,79,80]. Recent STM investigations on Cu(ll0) demonstrated that the dynamics of the reconstruction process actually proceeds via an "added-row" rather than a "missing-row" mechanism [61,62,81].…”

EXAFS spectroscopy: a new method for structural investigation

“…Молекулы Вг 2 в этом случае лежат в плоскости, параллельной поверх-ности обраациц на расстоянии 2,53 А от нее. Атомы Вг расположены при-близительно над центрами шестиугольных графитовых ячеек с длиной связи Вг-Вг, равной 2,31 А.· Увеличение длины связи Вг-Вг по сравнению со слу-чаем свободной молекулы связано с тем, что расстояние между центрами шестиугольных ячеек, над которыми расположены адсорбированные атомы Вг, составляет 2,46 А. Таким образом, изменение концентрации, а следовательно и взаимодей-ствия адмолекула -адмолекула привели, как показывают полученные дан-ные, к сильной перестройке геометрии адмолекул Элегантным примером использования SEXAFS-методов является не-давно выполненное исследование местоположения кислорода, адсорбирован-ного на поверхности (110) Си 152 . Данные ДМЭ свидетельствуют о наличии (2 X 1)-сверхструктуры в исследуемой области температур и экспозиций в кислородной атмосфере, но не дают ответа, обусловлена ли эта сверхструк-тура только слоем атомов кислорода или, кроме того, происходит реконструк-ция поверхностного слоя атомов меди.…”

Oxidation electronics: bond–band–barrier correlation and its applications

“…The O-Cu(110) surface has also been investigated intensively ever since the pioneer LEED study reported by Ertl in 1967 [2]. LEIS [210,211] and SEXAFS [169,212,213] studies suggest a missing-row type reconstruction where every other [010] Cu row on the surface is absent. In comparison, HEIS [214] and STM [47,215] investigations support a buckled-row (BR) model in which the every other row of Cu atoms is not missing but is shifted outwards instead.…”