Abstract
The interaction of oxalic acid with the Cu(110) surface has been investigated by a combination of scanning tunnelling microscopy (STM), low energy electron diffraction (LEED), soft X-ray photoelectron spectroscopy (SXPS), near-edge X-ray absorption fine structure (NEXAFS) and scanned-energy mode photoelectron diffraction (PhD), and density functional theory (DFT). O 1s SXPS and O K-edge NEXAFS show that at high coverages a singly deprotonated monooxalate is formed with its molecular plane perpendicular to the surface and lying in the [11¯0] azimuth, while at low coverage a doubly-deprotonated dioxalate is formed with its molecular plane parallel to the surface. STM, LEED and SXPS show the dioxalate to form a (3 × 2) ordered phase with a coverage of 1/6 ML. O 1s PhD modulation spectra for the monooxalate phase are found to be simulated by a geometry in which the carboxylate O atoms occupy near-atop sites on nearest-neighbour surface Cu atoms in [11¯0] rows, with a CuO bondlength of 2.00 ± 0.04 Å. STM images of the (3 × 2) phase show some centred molecules attributed to adsorption on second-layer Cu atoms below missing [001] rows of surface Cu atoms, while DFT calculations show adsorption on a (3 × 2) missing row surface (with every third [001] Cu surface row removed) is favoured over adsorption on the unreconstructed surface. O 1s PhD data from dioxalate is best fitted by a structure similar to that found by DFT to have the lowest energy, although there are some significant differences in intramolecular bondlengths.
Highlights
It is well-established that carboxylic acids deprotonate when adsorbed on many metal surfaces, and notably on Cu(110), which has proved to be the model surface used in many of these studies
In the present case photoelectron diffraction (PhD) modulation spectra were obtained by measuring photoelectron energy distribution curves (EDCs) of the O 1s peak(s), at 4 eV steps in photon energy, over the photoelectron kinetic energy range of 50–300 eV, for a range of different polar emission angles in the [110] azimuth
At low coverages a doubly-deprotonated dioxalate species lies with its molecular plane approximately parallel to the surface, while at high coverages the adsorbate is a singly-deprotonated monooxalate with its molecular plane perpendicular to the surface lying in the [110] azimuth
Summary
The PhD technique exploits the coherent interference of the directly emitted component of a photoelectron wavefield from a near-surface atom with those components scattered by atoms in the local environment of the emitter. The resulting modulations in the photoemission intensity in a specific direction, as a function of photon energy, provide structural information on the local environment of the emitter. The 2-D detector fitted to the large acceptance-angle electron spectrometer allowed separate spectra to be extracted at 5° intervals over a 50° polar emission angle range for any specific sample orientation. These spectra were processed following our general PhD methodology
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