Selectivity of Auger Decays to the Local Surface Environment

The line shape of the Auger decay of adatoms is studied by a joint theoretical and experimental effort, the former within a DFT framework, and the latter with synchrotron radiation measurements. We investigate the $K{L}_{2,3}V$ Auger deexcitation of Na on Al(111), a system with different adsorption geometries. In particular, we study the $(\sqrt{3}\times \sqrt{3})R30°$ phase at $1/3$ ML (monolayer) and the more complex $(2\times 2)$ structure at $1/2$ ML coverage. From the comparison between theory and experiment, we unambiguously determine features that allow for the determination of the adsorption environment from the adatom Auger spectrum.

Figure 1

Na $2p$ photoemission spectra taken with a photon energy of 150 eV and overall energy resolution better than 325 meV for the upper spectrum and 450 meV for the other two. RT spectra at $1/2$ ML (saturation coverage) [$(2\times 2)$ phase], RT spectra at $1/3$ ML [$(\sqrt{3}\times \sqrt{3})R30°$ phase], and LT spectra at $1/3$ ML are shown from top to bottom. The arrows identify the DFT energies.

Figure 2

The Na $KLV$ Auger spectra taken from three different Na films. From top to bottom: the $(2\times 2)$ phase for $1/2$ ML of Na deposited at RT; the $(\sqrt{3}\times \sqrt{3})R30°$ phase for $1/3$ ML of Na at RT; the phase for $1/3$ ML of Na at LT. The spectra have been taken keeping the sample at the same temperature of the corresponding deposition. The dashed line is the background from the clean Al(111) surface.

Figure 3

Top panels: the Na $(\sqrt{3}\times \sqrt{3})R30°$ phase ($1/3$ ML at RT); ground state LDOS [panel (a)] and core ionized LDOS [panel (b)] calculated around Na. Bottom panels: the Na $(2\times 2)$ phase ($1/2$ ML at RT); core ionized LDOS’s calculated around the inner Na atom [panel (c)] and the outer one [panel (d)].

Figure 4

Panel (a) the Na $(\sqrt{3}\times \sqrt{3})R30°$ phase ($1/3$ ML at RT). Calculated (solid line) and measured (circles) $K{L}_{2,3}V$ Auger profiles. The calculated curve, normalized to the experimental data, obtained convolving the Auger rate with a Lorentzian with half width $\Gamma =0.42\mathrm{e}\mathrm{V}$ for the core hole lifetime and a Gaussian for nominal instrumental broadening. The measured one obtained after background subtraction. Panel (b) the Na $(2\times 2)$ phase ($1/2$ ML at RT). Calculated (solid line) and measured (circles) $K{L}_{2,3}V$ Auger profiles. The calculated curve as in panel (a) but summing the contributions from the two inequivalent Na atoms (see text).