Giant Coster-Kronig Transitions and Intrinsic Line Shapes of the Anomalous Pd $M_{45}VV$ Auger Spectrum of $\mathrm{P}\mathrm{d}/\mathrm{A}\mathrm{g}(100)$ Dilute Surface Alloys

The Pd $M_{4}VV$ and $M_{5}VV$ Auger spectra of the 0.1 ML $\mathrm{P}\mathrm{d}/\mathrm{A}\mathrm{g}(100)$ dilute surface alloy have been measured using Auger-photoelectron coincidence spectroscopy. The $M_{4}VV$ spectrum indicates that Pd $3d_{3/2}$ core holes have a Coster-Kronig decay rate that is $\sim 10$ times that of Pd metal. Our calculations show that this giant enhancement arises from the local electronic structure of excited Pd atoms at the surface. Anomalous features in the Auger line shape are similar to those seen in dilute bulk PdAg alloys, and these features in the $M_{5}VV$ and $M_{4}VV$ lines are in good agreement with theoretical predictions.

Figure 1

(a) Photon excited singles Pd $M_{45}VV$ Auger spectra from bulk metallic Pd (dashed curve) and the dilute $\mathrm{P}\mathrm{d}/\mathrm{A}\mathrm{g}(100)$ surface alloy. (b) Pd $M_{5}VV$ Auger spectrum obtained in coincidence with photoelectrons emitted at 107.5 eV (the nominal kinetic energy of the Pd $3d_{5/2}$ photoelectrons). (c) Pd $M_{4}VV$ Auger spectrum obtained in coincidence with photoelectrons emitted at 102.5 eV (the nominal kinetic energy of the Pd $3d_{3/2}$ photoelectrons). In panels (b) and (c), the data points with error bars are the coincidence data, the thin solid curve is a smooth to that data, and the dotted curve is a theoretical prediction for the $M_{5}VV$ and $M_{4}VV$ line shapes, respectively. In panel (c) the thick solid curve is the intrinsic $M_{4}VV$ line shape and the shaded region is an estimate of the Coster-Kronig contribution.

Figure 2

The singles (long-dashed curve) and coincidence (data points with error bars) Pd $3d$ core level spectra from the dilute Pd surface alloy. The thick solid curve is a fit from which one can determine the fractional contribution of Pd $3d_{3/2}$ and Pd $3d_{5/2}$ photoelectrons to the emission at a kinetic energy of 102.5 eV.

Figure 3

Calculated Pd local density of states in the presence of a $3d$ core hole for a Pd atom in Pd bulk (thin solid curve), at the Pd(100) surface (dashed curve), in bulk Ag (dotted curve), and in a $c(2\times 2)$ $\mathrm{P}\mathrm{d}/\mathrm{A}\mathrm{g}(100)$ surface alloy (thick solid curve). Inset: CK transition probabilities for these systems as a function of $\Delta$, the energy separation of the core levels.