Structural randomness improves results of H₂ scattering calculations

Scattering H₂ molecules from a metal surface is a relatively non-invasive way of interrogating features of the surface that can provide information about the metal’s reactivity. Numerical modelling of H₂ diffraction probabilities also enables prediction of the metal’s reactivity — if the underlying potential energy surface is sufficiently accurate.

In the example of Ru(0001), where the ruthenium surface has a hexagonal close packed (hcp) structure, the calculated diffraction probabilities consistently overshot experimental values. This was despite incorporating Debye-Waller attenuation, but assuming a perfect surface structure. An article in The Journal of Chemical Physics reports a straightforward, single-parameter fix for the mismatch by introducing random disorder to the static surface structure.

The authors observed that calculated probabilities overestimated measured values by greater amounts when incident energy was higher. This relationship indicated a static disorder effect which a random disorder implementation appeared to explain. Adding the single parameter attenuation factor to the numeric treatment of Ru(0001) surfaces removed the discrepancy between calculated and observed diffraction probabilities.

Because the disorder was random and there are myriad causes of imperfections, both inherent and process-dependent, the exact nature of the surface disorder for these measurements is not yet clear. Such diffusive loss from imperfect surfaces is not unique to Ru(0001). The basics of the approach might result in similar improvements to H₂ diffraction on Pt(111) and similar systems, but further research is needed to establish this.

Source: “Possible effect of static surface disorder on diffractive scattering of H₂ from Ru(0001): Comparison between theory and experiment,” by G. J. Kroes, Mark Wijzenbroek, and J. R. Manson, The Journal of Chemical Physics (2017). The article can be accessed at https://doi.org/10.1063/1.5011741 .