Diffractive and reactive scattering of H2 from Ru(0001): experimental and theoretical study

Abstract

We present a combined experimental and theoretical study of the diffraction of H(2) from Ru(0001) in the incident energy range 78-150 meV, and a theoretical study of dissociative chemisorption of H(2) in the same system. Pronounced out-of-plane diffraction was observed in the whole energy range studied. The energy dependence of the elastic diffraction intensities was measured along the two main symmetry directions for a fixed parallel translational energy. The data were compared with quantum dynamics calculations performed by using DFT-based, six-dimensional potential energy surfaces calculated with both the PW91 and RPBE functionals, as well as with a functional obtained from a weighted average of both (the MIX functional, which was earlier shown to perform quite well for H(2) + Cu(111)). Our results show that the PW91 functional describes the H(2) diffraction intensities more accurately than the RPBE and the MIX functionals, although the absolute values of these intensities are overestimated in the calculations. For the reaction probabilities a preference for one or the other functional cannot be given over the entire energy range probed by the sticking experiments. The PW91 functional yields too high reaction probabilities over the entire investigated energy range, but is better than RPBE at low collision energies (<0.1 eV). The RPBE functional gives too low reaction probabilities at low energy and somewhat too high reaction probabilities at high energy, but agrees better with experiment than PW91 for energies >0.1 eV. The results suggest that, in order to get a better description of both H(2) diffraction and dissociative chemisorption for this system, a specific reaction parameter functional for H(2) + Ru(0001) is needed that is a weighted average of functionals other than PW91 and RPBE. We speculate that differences between the H(2) + Ru(0001) system (early and low reaction barrier) and H(2) + Cu(111) (late and high reaction barrier) may well lead to fundamentally different specific reaction parameter functionals, and that including a reasonable accurate description of the van der Waals interaction might be important for H(2) + Ru(0001) which has barriers localised far away from the surface. Based on our results we advocate new, systematic combined theoretical and experimental studies of H(2) interacting with transition metals in early and late barrier systems, with the aim of determining whether specific reaction parameter functionals for these systems might differ in a systematic way.

The effect of the exchange-correlation functional on H2 dissociation on Ru(0001)

“…63 It is noted here that the parameters describing the H 2 molecular beam differ somewhat from the parameters presented earlier, 30 as an error was made in the analysis of the TOF measurements. 63 …”

“…Comparisons have also been made to experimental molecular beam studies on dissociative adsorption 31 as well as diffractive scattering. 30 The results of the comparison with experiments showed that neither functional could properly describe reaction over the entire interval of incidence energy, in the sense that the calculated reaction probability curve as a function of incidence energy was too narrow compared to the experimental curve, suggesting the energetic corrugation of the used potential energy surfaces to be too small. 30,31 The same semi-empirical mixture of these two functionals as the one which worked well for H 2 dissociation on Cu(111) 17,18 was also not able to describe the reaction probability of H 2 on Ru(0001) over the entire range of incidence energies.…”

“…To compare with the experimental diffraction probabilities, 30 first rovibrationally elastic diffraction probabilities were computed by (15) where P nm is the rovibrationally elastic probability for scattering into the diffraction state denoted by the n and m quantum numbers. These probabilities are then degeneracy averaged by…”

“…This discrepancy was attributed to the used XC functionals. 30 It was argued that the van der Waals interaction, which is not taken into account in the usual (semi-)local XC functionals, 32, 33 could be important for an early barrier system such as H 2 dissociation on Ru(0001). To our knowledge, so far no studies exist in which the van der Waals interaction has been taken into account explicitly in dynamics calculations on H 2 dissociation on metal surfaces.…”

“…Previously, PESs were constructed for H 2 dissociation on Ru(0001), 28 and quantum dynamics calculations have been performed 29,30 to compare two DFT XC functionals, PW91 16 and RPBE, 10 with each other. Comparisons have also been made to experimental molecular beam studies on dissociative adsorption 31 as well as diffractive scattering.…”