Dependence of inner-shell vacancy production upon distance in hard Li-Al collisions

We match the predictions of molecular-dynamics simulations of 1.2 keV and 2.0 keV 7Li+ scattered from Al(100) to observed total Li atom spectra measured by time-of-flight spectroscopy. In doing so we determine the relevant parameters in a simple distance of closest approach model for the probability of production of single and double vacancies in the Li 1s shell during hard Li-Al collisions. In the standard Fano-Lichten model of vacancy production, vacancies are produced with unit probability if the collision is hard enough to force the collision partners past some critical distance of closest approach. We find that such an assumption is insufficient to fit our simulations to experimental observations, and that we must allow for a gradual turning on of the vacancy production probability as the distance of closest approach decreases. The resulting model may be useful in modeling atomic excitation effects in simulations of other ion-impact processes

Internal electronic structure of adatoms on Fe(110) and Fe(100) surfaces: A low-energy Li+ scattering study

The neutralization of 400-3000 eV Li-7(+) ions scattered from clean and adsorbate-covered Fe(110) and Fe(100) surfaces was measured with time-of-flight spectroscopy. Li singly scattered from bromine, iodine, and cesium adatoms has a consistently larger neutral fraction than that for scattered from substrate sites. This suggests that the local electrostatic potential directly above these adatoms is reduced from that of the clean substrate. The neutral fraction of Li scattered from halogen adatoms is surprising in that it decreases as the emission angle moves off-normal, yet increases in the usual manner for cesium and silver adatoms. This indicates that the charge distribution associated with a halogen adsorbate is nonuniform, most likely due to internal polarization. A semiquantitative theoretical analysis shows that a nonuniform internal electron density would give rise to the observed behavior. The polarization of halogen adatoms is likely responsible for anomalous work function changes observed previously. Alkali-ion scattering is shown to be an effective tool for detecting the internal electronic structure of an adatom

Detection of quantum confined states in Au nanoclusters by alkali ion scattering

Charge state-resolved time-of-flight spectra were collected for 2.0 keV Na-23(+) scattered from Au nanoclusters deposited on TiO2(110). The neutral fraction of Na scattered from metallic Au is low (similar to3%), but it is surprisingly high (up to 50%) for small clusters. The results demonstrate that alkali ions couple to electronic states specific to the nanoclusters, and that the energy of the states is a function of the nanocluster size. This technique provides a new method for the spectroscopy of nanomaterials

Ionization of Al recoiled and sputtered from Al(100)

The absolute ionization probability of energetic ( > 500 eV) particles recoiled from Al(100) by 2 and 5 keV Xe+ bombardment was measured with time-of-flight spectroscopy. These values were then used to calibrate the energy and angular distributions of low-energy (10-600 eV) sputtered ions collected with an electrostatic analyzer. The independent-particle model of nonadiabatic surface-atom charge exchange, which is typically used to analyze single scattering events, was applied to the ion fractions of the recoiled and sputtered atoms. The model describes all the experimental data from a few eV to the keV range if a different surface electronic temperature is used for recoiling and sputtering. This suggests that the ionization process depends on the instantaneous surface condition at the time of ion emission

Formation of excited Ag atoms in sputtering of silver

A model is presented for the formation of excited Ag* (4d 9 5s 2 ) atoms during sputtering of Ag metal by energetic Ar ϩ ions. The essential part of the formation process is the slow diffusion of 4d holes in the collision cascade from the sites of violent Ag-Ag collisions to the emitted Ag atoms. A computer simulation of Ag cascades and of the 4d-hole transport allows us to quantify the model and to describe all characteristic features of the available experimental data, in particular the fact that the sputtered Ag* atoms exhibit a narrower kinetic energy distribution than those ejected in the electronic ground state

Formation of excited Ag atoms in sputtering of silver

A model is presented for the formation of excited Ag-* (4d(9)5s(2)) atoms during sputtering of Ag metal by energetic Ar+ ions. The essential part of the formation process is the slow diffusion of 4d holes in the collision cascade from the sites of violent Ag-Ag collisions to the emitted Ag atoms. A computer simulation of Ag cascades and of the 4d-hole transport allows us to quantify the model and to describe all characteristic features of the available experimental data, in particular the fact that the sputtered Ag-* atoms exhibit a narrower kinetic energy distribution than those ejected in the electronic ground state