Structure determination of disordered organic molecules on surfaces from the Bragg spots of low-energy electron diffraction and total energy calculations

Abstract

We show that an analysis of the intensity versus energy variation of Bragg spots due to low-energy electron diffraction from a disordered overlayer of molecules on a crystal surface allows a much more convenient method of determining the local adsorption geometries of such molecules than previously analyzed weak diffuse diffraction patterns. For the case of methanol on Pd~111!, we show that the geometry determined by this means from experimental diffraction data is in excellent agreement with the predictions of density functional total energy calculations. The adsorption of organic molecules on surfaces is a phe- nomenon at the core of many processes of far-reaching tech- nological importance in fields ranging from the lubrication of automobile engines to processes of heterogeneous catalysis that underpin much of our industrial civilization. The key to a fundamental understanding of these processes is the nature of the bonding of such molecules to surfaces, which in turn requires the determination of the geometrical relation of the molecules to the surfaces. A standard method of determining the adsorption geom- etry of molecules that form ordered overlayers on surfaces is the analysis of intensity versus beam energy ( I/E) variations of ~mainly! superstructure Bragg diffraction spots from low- energy electron diffraction ~LEED!. Unfortunately, many ad- sorbates do not form ordered layers on crystal surfaces. Rather, they tend to take up arrangements of locally identical geometries relative to the substrate, but with limited ~or non- existent! long-range order. The signature of such surfaces is background ~diffuse! LEED scattering from the disordered overlayer, rather than superstructure Bragg reflections. In such cases, alternative methods such as photoelectron diffraction1 or x-ray standing wave measurements2 have been able to determine aspects of the adsorption geometries. How- ever, such techniques require access to synchrotron radiation sources, thus limiting their applicability. Scanning tunneling microscopy ~STM! is also capable of probing the structure of disordered layers but cannot accurately measure bond lengths and angles. It has been shown that diffuse LEED intensities contain enough information to determine the structures of disordered surfaces. 3,4 Although this has proven effective in several cases, 5 experimental difficulties in mea- suring the low background intensities have prevented the widespread use of the technique. We point out in this paper that a much more easily accessible signal for accurately de- termining the structures of such disordered adsorbates is the monitoring of their much stronger effect on the Bragg reflec- tions of the ordered substrate. We demonstrate this idea by determining the adsorption geometry of methanol on Pd~111!. The tilted C-O bond found is confirmed by indepen- dent density functional total energy calculations that also de- termine the positions of the H atoms.