Effect of hydrogen on the atomic structure of Pd(001)

TL;DR

This study uses LEED I/V analysis to examine how hydrogen adsorption affects the atomic structure of Pd(001), revealing that hydrogen causes surface expansion and contamination explains previous discrepancies.

Contribution

First experimental verification of hydrogen-induced surface expansion on Pd(001) aligning with ab initio predictions, clarifying previous conflicting reports.

Findings

Hydrogen adsorption causes a 4.7% expansion of the first interlayer spacing.

Surface contamination by residual hydrogen explains previous observations of expansion.

Interlayer spacing increases monotonically with hydrogen coverage.

Abstract

The atomic structures of clean and hydrogen-adsorbed Pd(001) are investigated by low energy electron diffraction (LEED) I/V analysis. Clean Pd(001) shows little surface relaxation in sharp contrast to previous reports. Adsorbing 1 monolayer of hydrogen on Pd(001), we observe sizable expansion of the interlayer spacing of the first two surface layers, $d_{12}$ by 4.7% of the corresponding one of bulk Pd. Both experimental observations are in excellent agreement with the predictions of recent {\it ab initio} calculations. A series of experiments with varying coverages of hydrogen adsorbed on Pd(001), reveals that $d_{12}$ monotonically increases with the increasing coverage. Such an observation strongly supports the contention that the previous observation of expanded $d_{12}$ in clean Pd(001) results from contamination of the surface by residual hydrogen.