Host: The Japan Society of Vacuum and Surface Science
Name : Annual Meeting of the Japan Society of Vacuum and Surface Science 2023
Location : [in Japanese]
Date : October 31, 2023 - November 02, 2023
By using multiple scattering calculations, we have reported that RHEED is quite useful for determination of adsorbed hydrogen positions on metal surfaces [1,2]. A hydrogen is the lightest atom and its scattering factor is much smaller than those of the substrate metals. In the case of LEED, relatively large scattering amplitude of H in the forward direction gives the changes in the intensity-energy (I-V) curves which can be used for analysing the hydrogen position. The use of large scattering amplitude of H in the forward direction is the same in the case of RHEED. In addition, in RHEED by rotating the sample, it is easy to choose diffraction conditions sensitive to H positions, where reflected intensity curves are modified largely by adsorbed H position.
In this presentation, by using a multiple scattering calculation, we will study whether we can determine subsurface H positions just below metal surfaces by RHEED. It is important to know the positions of subsurface hydrogens, which are located just one or two monolayers below the substrate. We take H absorbed at Pd(100) as an example, since no definite positions of the subsurface H are reported so far. We assume that the H coverage is 1.5 ML; 1 ML of H are pre-adsorbed at the hollow sites on the surface and 0.5 ML of H are absorbed below the surface. As the absorption sites, two very likely sites are considered. One is the octahedral (Oh) site and the other is the tetrahedral (Td) site. The rocking curves and the azimuthal plots from the surfaces with absorbed H at Oh sites and those at Td sites are compared. In order to see how large the differences are in RHEED intensities from surfaces with absorbed H at two different sites, i.e., Oh and Td sites, figure shows one example of the azimuthal plots from the surfaces with 0.5 H absorbed at the two subsurface sites around the [10]-azimuth. The solid line shows the 00 intensities from the surface with 1 H at hollow sites with 0.11 Å high with 5% relaxation of the topmost Pd layer. The dotted and the dashed lines show the 00 intensities from the same surface but with absorbed 0.5 H at Oh sites and that at Td sites, respectively. The latter two 00-azimuthal plots clearly show different variations, which can be used for identifying the absorbed site. In a similar manner, the rocking (intensity vs. glancing angle) curves from the surface with absorbed H at Oh sites and those at Td sites are different enough to distinguish the two sites by choosing diffraction conditions sensitive to subsurface H. We can also determine the height of absorbed H by choosing diffraction conditions sensitive to subsurface H height. Then, we can conclude that RHEED is quite useful to determine subsurface H position at Pd(100) as well as the adsorbed H position.
References
[1] T. Kawamura, K. Fukutani, Surf. Sci. 688 (2019) 7–13.
[2] T. Kawamura, Y. Fukaya, K. Fukutani, Surf. Sci. 722 (2022) 122098.
