P. Riccardi

The excitation of collective electronic modes in Al by slow single charged Ne ions

Abstract We have studied electron emission from decay of plasmons excited in the interactions of slow single charged Ne ions with Al surfaces as a function of incident energy ranging from 0.7 to 8 keV. Bulk plasmon excitation occurs above a threshold incident energy of ∼1 keV and initially coexists with potential excitation of surface modes. The excitation of bulk plasmons in Al is determined by fast electrons traveling inside the solid.

MECHANISMS FOR ION-INDUCED PLASMON EXCITATION IN METALS

Abstract We have studied the excitation of plasmons produced by 100 eV He + , Ne + and Ar + and by 5–100 keV H + and He + projectiles in Al and Mg through the observation of electrons from plasmon decay, ejected from clean and cesiated surfaces. At low velocities, plasmon excitation occurs only for ions of high potential energy and is independent of velocity. The effect of Cs adsorption on this potential plasmon-excitation mechanism on Al surfaces suggests that the excited plasmons are not bulk plasmons, as was assumed previously, but short-wavelength surface plasmons. For ions moving faster than a threshold velocity v th ∼1.3 v Fermi predicted by electron gas theories, kinetic plasmon excitation can occur because the valence electrons cannot respond instantaneously to screen the moving charge. We found that, contrary to theoretical expectations, plasmon excitation by H + and He + projectiles occurs below v th . With the aid of a simple model, we suggest that this sub-threshold excitation results from energetic secondary electrons.

Plasmon excitation in Al by keV Ne and Ar ions

Abstract We present experimental studies of plasmon excitation by keV Ne and Ar single charged ions on Al surfaces. Plasmons are identified through the characteristic energy spectrum of electrons from plasmon decay. At 1 keV incident energy, surface plasmons are excited by Ne+ but not by Ar+. For higher energies we observe bulk plasmon excitation resulting, most likely, from excitation by fast electrons, such as Al-2p Auger electrons, traveling inside the solid.

Bulk and surface plasmon excitation in the interaction of He+ with Mg surfaces

Abstract We report on studies of plasmon excitation in Mg under the impact of slow He + ions in the incident energy range 0.1–0.5 keV. Consistent with studies on Al surfaces, a transition from surface to bulk plasmon excitation occurs as the energy of the ion is increased. Previous methods in the literature to obtain information about yields of electron emission from plasmon decay are discussed. A new data analysis procedure is presented, which allows disentangling the contributions to the experimental spectra arising from different emission mechanisms, showing that potential excitation of multipole surface plasmons gives an important contribution to electron emission.

Bulk plasmon excitation in the interaction of Ne+ and Ar+ ions with polycrystalline Al surfaces

Abstract We report experimental studies of electron emission from decay of bulk plasmons excited in the interaction of slow single charged Ne and Ar ion with polycrystalline Al surfaces. To answer the question of whether bulk plasmons can be excited by charged particles moving outside the surface, we performed measurements as a function of emission angle. Our experiments show that bulk plasmon excitation occurs inside the solid for incident ion energies above a threshold of ∼1 keV. Bulk plasmon excitation in Al by keV single charged Ne and Ar ions is caused by fast electrons traveling inside the solid, which are energized in collisions initiated by the projectile.

Sub-threshold plasmon excitation in free-electron metals by helium ions

Abstract In this work, we re-examine sub-threshold plasmon excitation by ions measuring the energy distributions of electrons emitted from Al and Mg under the impact of 0.16–4 keV single charged Helium ions. Electron energy distributions for the case of He+ projectiles contain well distinguished structures that confirm the excitation of multipole surface plasmons, due to the potential energy released by the neutralization of incoming ions. It is shown that electron emission from multipole plasmon decay is more intense than from Auger neutralization (AN). Our experiments indicate that scattering and cascade in the solid of electrons excited by plasmon decay and by AN may give an important contribution to electron emission.

Double 2p electron excitation in low-energy Ne+ single scattering from a Si surface : an energy loss study

Abstract Results of 1950 eV Ne+ single scattering from a Si surface show that all the backscattered Ne+ ions have suffered severe inelastic energy loss and the total inelasticity in the close binary encounter amounts to 45 ± 4eV. This value is well beyond that required for one-electron excitation from the neutralized Ne0, but agrees well with that for the double 2p electron excitation of a neutralized Ne to 2p4(1D)3s2. These findings contrast the re-ionization model proposed by Souda et al. [Phys. Rev. Lett. 75 (1995) 3552; Surf. Sci. 363 (1996) 139], and confirm that the formation of autoionization states which decay far away from the surface is the main mechanism for Ne+ scattered from Si.

Ar L-shell and metal M-shell Auger electron emission for 14 keV Ar+ ion impact on Ca, Sc, Ti, V, Cr, Fe, Co, Ni, and Cu

Abstract Ar projectile L-shell and metal target M-shell Auger electron emission have been studied by 14 keV Ar+ ion bombardment on Ca, Sc, Ti, V, Cr, Fe, Co, Ni, and Cu targets at an incidence angle of 60° with respect to the surface normal. We observe that the width of the Ar Auger peak progressively reduces and its total intensity decreases exponentially with increase of the target atomic number and that the metal M-shell autoionization peaks are completely absent.

Evidence of alkali adatom island formation induced by ion bombardment on a Na covered Pt surface

Abstract We report on adatom island formation induced by Ne + ion bombardment on a Na covered Pt surface. By using the collisionally excited autoionization electron spectroscopy (CEAES) we observe that, for large initial coverages, ion impact causes the Na adatom aggregation creating large surface areas which are relatively Na-rich and Na-depleted. Decay of excited atoms above these alkali islands and above the more or less clean Pt substrate zone with very different work functions results in the splitting of each atomic transition line in two well separated components.

Adsorbate-adsorbate interactions in Na covered Mg surfaces

Abstract We present a study on alkali cluster formation induced by ion bombardment, the information being obtained by Collisionally Excited Autoionization Electron Spectroscopy (CEAES). We show that a uniform alkali film on a metal surface can be easily converted in large patches by low energy ion bombardment. The observed phenomenon is coverage dependent, in fact, for alkali density below a critical value the adlayer is uniformly removed. Beyond this critical value a rearrangement mechanism leads to the formation of extended adatom aggregates revealed by the splitting in two components of both Ne and Na autoionization lines. A Mg atomic structure is also revealed in the spectrum, and, according to our interpretation model, no splitting is observed for this peak due to slow excited atoms.