J.A.M. Pereira

Dynamics of secondary ion emission: Novel energy and angular spectrometry

A new spectrometer has been developed based on the combination of standard time-of-flight technique and position sensitive delay line detectors. The basic features of the spectrometer, particularly of the multi-hit capable detector, are described. To demonstrate the performance of this new system, the dynamic emission characteristics, i.e. the threedimensional velocity distribution, of desorbed H þ from Al target by Ar 0 impact (570 keV) is presented. It is found that the desorption yield is maximum for radial and axial emission velocities at 1.2 and 12 km/s respectively, corresponding to 1.5 eV ions emitted at 57 to normal (following the projectile radial direction). The initial energy distribution spreads out over 16 eV. 2002 Elsevier Science B.V. All rights reserved.

PIXE and PDMS Methods Applied to Aerosols Analysis

The aim of this study was to apply the PIXE (Particle Induced X-ray Emission) and PDMS (Plasma Desorption Mass Spectrometry) techniques to characterize airborne dust particles containing metals. Aerosols generated at a mineral-sand processing plant were characterized in this study. The aerosol samples were collected at a plant that processes mineral sands to obtain rutile, ilmenite, zircon, and monazite concentrates. A cascade impactor with six stages was used to collect mineral dust particles with aerodynamic diameters in the range of 0.64 to 19.4 mum. The particles impacted on each stage of the cascade impactor were analyzed by PIXE, which permits the determination of the elemental mass air concentration and the MMAD (Mass Median Aerodynamic Diameter). The chemical compositions of the aerosol samples were identified by PDMS analysis. This study shows that, by using PIXE and PDMS techniques, it is possible to determine the chemical compounds in which the elements are associated in the aerosol particles. Based on the results of the PIXE analysis, the elemental mass concentrations and the MMADs were determined.

Axial energy distributions of Li+ and F− desorbed from LiF surfaces by fast ion impact

Initial axial kinetic energy (Ez) distributions of Li+ and F− secondary ions desorbed from LiF surfaces, bombarded by low current (∼500 particles/s) N3+ beams in the 0.075–7.5 MeV primary ion energy (EPI) range, were measured by using the time-of-flight technique. In this energy range, the electronic energy loss, Se, increases from 25 to 160 eV/A, while the nuclear stopping power, Sn, decreases from 8.0 to 0.25 eV/A. The observed F− initial axial kinetic energy distribution can be described by the linear collision cascade theory and no other contribution was found. The F− total yield decreases proportionally to Sn. The Li+ distribution presents a remarkable deviation from the cascade prediction, indicating the existence of additional mechanisms related to the electronic energy-loss process. For the range of EPI studied, these mechanisms produce a Li+ energy distribution with a Maxwell-Boltzman-like shape, which vanishes above Ez ∼ 10 eV and presents a maximum at Ez ∼ 1.2 eV. The Li+ yield is nonlinear with the electronic energy loss, Se. A simple desorption model, based on the spatial distribution of the energy deposited by the projectile and on the effective energy-loss concept, is presented. This spatial distribution of the deposited energy is due to secondary electron cascades and is connected with a ESD-like mechanism on the surface.

Nonlinear effects in ion emission from LiF induced by N+ and N2+ MeV ion impact

Abstract Beams of N+ and N2+ in the 75–7500 keV/atom energy range have been used to bombard a LiF surface. Desorption yields and energy distributions of positive and negative secondary ions have been measured by the time-of-flight technique. Measurements of the energy distributions revealed the relative contribution to desorption due to collision cascades and electronic excitations in the solid. Experiments have shown that F− ions are desorbed as a result of elastic collision cascades and that the corresponding ion yield depends linearly on the number of constituents of the projectile, i.e. Y(N2+) > 2Y(N+). In contrast, emission of clusters, such as Li2F+, were found to be caused by electronic excitation in the solid with the respective yields revealing a nonlinear dependence: Y(N2+) > 2Y(N+). Both processes were found to contribute to Li+ desorption. Nonlinear effects are discussed in terms of effective stopping power which depends not only on the electronic stopping power of the primary ion, but also on the track size.

Energies and yields of secondary ions ejected from alkali halides by a 0.18–6.0 MeV nitrogen beam

Abstract Nuclear and electronic sputtering was studied for the alkali halides LiF, CsI, NaF, NaCl, NaBr and NaI being vacuum evaporated on thin Al-foils and irradiated by 14 N ions having an energy loss of 20–190 eV/A. A double-grid time-of-flight technique was used to determine the yields and the axial energy distributions of the alkali and halide ions ejected from the sample surface. In case of LiF, the most carefully investigated compound, the dependence of ion yields on the energy loss and the shape of the energy distributions clearly mark the onset of collisional sputtering towards low beam energies. Apart from F − , for all other investigated alkali and halide ions electronic sputtering is the dominant ejection process. Above 1 MeV bombarding energy, their mean axial energies were found to range from 0.26 to 2.0 eV, their energy distributions fall off with E − n , n having values between 3.0 and 4.5. In cases, where the nuclear energy loss is relatively low, the ion yields scale with about the second power of the electronic energy loss; the mean axial energies and the yields change only little or not with the charge state of the incident ions. These observations lead to the general conclusion, that electronic sputtering from alkali halides is established by cooperative processes and that probably electron-hole production in deep layers contribute essentially to formation and ejection of ions at the surface. The results are discussed in terms of exciton production and migration.

Electric field induced orientational order of gold nanorods in dilute organic suspensions

The electric field controlled alignment of gold nanorods offers a paradigm for anisotropic molecules with the potential for a wide variety of phases and structures. We experimentally study the optical absorption from gold nanorod suspensions aligned using external electric fields. We show that the absorption from these suspensions depends linearly on the orientational order parameter. We provide evidence that the critical electric field needed to orient the gold nanorods is proportional to the nanorod volume and depolarization anisotropy. Utilizing this critical field dependence, we demonstrate for suspensions with two different nanorod sizes that the alignment of each population can be controlled. We also develop a technique to determine the imaginary parts of the longitudinal and transverse electric susceptibilities of the nanorods. The ability to selectively address specific parts of the nanorod populations in a mixture using external fields may have significant potential for future display and optical...

Effects of charge state and number of constituents of MeV projectiles on secondary ion emission yields from LiF

Abstract Beams of MeV N+q (q =1–5) and N2+ ions were used to bombard a LiF thin film. Secondary ion (SI) desorption yields were measured by using the time-of-flight technique. The main peaks in the mass spectra correspond to Li+, Li2F+, H± and CpHs±. The emission of the first two ions is characteristic of the LiF sample while the others originate from its surface contamination. The yields of H± and CpHs± were found to be more sensitive to the incoming charge. The Li+ and Li2F+ yields were found to increase more strongly with the number of constituents in the incident ion. Enhancement in the yields of the CpHs± under diatomic ion bombardment was observed to be larger for low values of s.

Li enrichment of LiF surfaces bombarded by MeV nitrogen ion beams

Abstract The PDMS (Plasma Desorption Mass Spectrometry) technique was employed for monitoring surface modifications in a LiF sample under ion bombardment at different doses. The flux of the 2.5-MeV N + beam was kept at 10 10 particles/s (∼ 1 n nA) to induce radiation effects in the LiF sample and was decreased to ∼ 500 particles/s during the PDMS analysis. The resulting time-of-flight spectra exhibit peaks characteristic of the sample, Li + and Li 2 F + , and others such as H + and C 2 H n + , originated by the ionization of the adsorbed layer. It was observed that only the desorption yields corresponding to the LiF species present a dose dependence. This result is interpreted in terms of Li enrichment of the surface due to F,H center defect production.

COMBINED TOF-MS/RBS ANALYSIS OF LIF THIN FILMS BOMBARDED BY MEV NITROGEN IONS

Abstract Rutherford Backscattering Spectrometry (RBS) and Plasma Desorption Mass Spectrometry (PDMS) techniques were combined to verify compositional changes in a LiF thin film exposed to a 2.0 MeV nitrogen ion bombardment. These techniques can furnish complementary information concerning local relative changes in the Li + and F − concentrations, caused by the ion bombardment. The F − secondary ions are not observed in PDMS because they are not produced by electronic sputtering in LiF. The fluorine presence in the sample is assigned through the He recoil ion energy spectrum in the RBS technique. The film thickness reduction could be seen by RBS while the PDMS indicated a relative increase in the Li concentration on the surface. These results are consistent with a picture where the erosion caused by the ion bombardment occurs differently for the two target atoms. The fluorine is preferentially desorbed resulting in a lithium-enriched surface. This observation can be understood in terms of chemical reactions induced by the electronic excitation of the halogen sub-lattice.

MeV nitrogen bombardment of LiF: From the nuclear to the electronic sputtering regimes

Abstract The time-of-flight technique was used to measure the axial kinetic energy (Ez ) distribution of secondary ions emitted from a LiF surface bombarded with N3+ ions in the 0.30–6.0 MeV energy range. Desorption yields were also measured as function of the projectile energy. The analysis of the Li+ and F− secondary ions shows that, at low incident energies (below 0.50 MeV), these ions are sputtered from the surface by a collision cascade (nuclear sputtering). Such indication comes from the E z 2m−2 (where 0 < m< 1) asymptotic behavior of the axial kinetic energy distribution. The observed values for m are m ∼ 0 for Li+ emission and m ∼ 0.4 for F− emission. The mean axial kinetic energies are, respectively, σ 6eV and ∼ 9eV. At higher projectile energy (above 1 MeV), there is a change in the Li+ emission behavior: the high energy tail of the axial energy distribution becomes exponential, the desorption yield increases presenting a maximum near 2.0 MeV and the mean energy of emission decreases to ∼ 3 eV....