Surface core-level shifts at vicinal tungsten surfaces

Core-level $4f_{7/2}$ photoemission spectra are measured for W(110) and for two stepped-surface planes along the {100} zone: W(540) and W(320). The vicinal-surface spectra are fit using a four-component Doniach-Sunjic–Gaussian function with variable parameters that allow independent variation in the peak locations and intensities based on the different nearest-neighbor coordination of surface atoms. The new experimental results differ significantly from all prior experimental studies of surface core-level shifts associated with vicinal tungsten surfaces. Surface step-atom core-level binding energies obtained from the photoemission spectra are found to be in excellent agreement with simple local-coordination models and initial-state shifts from ab initio calculations of surface core-level shifts and confirm general trends associated with application of coordination models to both low- and high-index tungsten surfaces. No compelling evidence of coordination-dependent final-state effects is observed. Effects on core-level spectra and surface core-level shifts associated with surface hydrogen dose are investigated. Vicinal surface spectra of tungsten are shown to be more sensitive to hydrogen doses than parent low-index surfaces, possibly accounting for prior difficulty in obtaining consistent core-level photoemission spectra for vicinal tungsten surfaces.

Figure 1

(Color online) Atomic structure of (unreconstructed) W(110), W(540), and W(320). Rows of surface atoms having different coordination are identified. $S_E$ are step-edge atoms, and $S_C$ are atoms that lie under $S_E$. Terrace atoms $S_T$ are surface atoms $S_2–S_9$ for W(540) and $S_2–S_5$ for W(320). $S_B$ designates bulk atoms.

Figure 2

(Color online) Photoemission spectra for W(320), W(540), and W(110). The W(110) spectra is fit using a two-line function; W(540) and W(320) spectra are fit using a four-line function (refer to text). The component peaks are the bulk peak, $S_B$; the corner atom peak, $S_C$; the terrace-atom peak, $S_T$; and the step-edge peak, $S_E$.

Figure 3

(Color online) Photoemission spectra for clean and H-dosed W(320). The H-dosed spectra is decomposed to the four lines that fit the data: $S_B$, $S_T$, $S_C$, and $S_E$.

Figure 4

(Color online) Plot of the SCLS for various tungsten surface atoms on both low- and high-index surfaces, as a function of the coordination parameters $C$ [Eq. (1)]. All experimental points (solid circles) (except the result for W(320) $S_1$ by Chauveau et al. (solid pentagon) are believed to be valid (refer to the text). The bars for ab initio calculations represent the range of values including initial-state effects.