Secondary-ion emission from oxygen and hydrogen-covered beryllium surfaces: I. Coverage dependence

doi:10.1016/0039-6028(79)90361-3

Surface Science

Volume 90, Issue 2, 2 December 1979, Pages 564-578

https://doi.org/10.1016/0039-6028(79)90361-3 Get rights and content

Abstract

Secondary ion emission from a beryllium surface is studied in the presence of hydrogen and oxygen. For submonolayer coverages of oxygen, the adsorption follows site-exclusion statistics. On this basis, the Be⁺ secondary ion yield and energy distribution can be separated into oxygen-dependent and oxygen-independent processes governed by the local properties of the surface. Ionic molecular species with hydrogen or oxygen present include Be₂O⁺, Be₂O⁻, BeO⁻, and BeH⁺ but not BeO⁺ or BeOH⁺. This result is inconsistent with emission in which the oxide molecule is formed in the near-surface vacuum region by agglomeration of individually sputtered surface atoms. For BeH⁺emission the opposite conclusion is reached.

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Citation Excerpt :
This means that among NSIs either only one ion type could have linear yield dependence on concentration or all NSIs have nonlinear dependences on concentration. Since the H± and MenH± are the commonly used ion types [4,6–8,24] for hydrogen detection and analysis purposes we will consider these types in further discussion. Indeed, the obtained results show that in the low concentration range the yields of both positive and negative SIs containing one hydrogen atom depend on concentration very similarly and therefore linearly, as there is no indication of the opposite.
Secondary ion mass spectrometry is a powerful analytical tool that offers great capabilities for studying hydrogen interaction with metallic materials. Yet its utilization for the in situ studies of hydrogen interaction with a sample has not been well established. In this research we study the influence of hydrogen partial pressure (5 × 10⁻⁹ – 5 × 10⁻⁵ Torr) in the sample chamber, the sample temperature (305–900K), and primary ion current density on the emission intensity of various secondary ions for the hydrogen-storage TiFe alloy sample bombarded with 12 or 20 keV Ar⁺ primary ions. Presence of chemisorbed hydrogen on the alloy surface results in the presence of a variety of positive and negative hydrogen-containing secondary ions in mass spectra. Analysis of the influence of the varied experimental parameters has shown that the secondary ion yield changes reflect only changes of hydrogen concentration regardless of which varied parameter induced a concentration change. The yield dependences on concentration in general are different for different ions and tend to be more substantial for the ions containing a larger number of hydrogen atoms. The ions containing one hydrogen atom have similar and likely linear yield dependences when the concentration is low. At higher concentrations, the yield dependences for these ions diverge: their relation to concentration is discussed. For not hydrogen-containing secondary ions, hydrogen saturation of the surface resulted in an 80-400% increase of the yields of Ti⁺ and Ti-containing positive secondary ions, no yield change for other positive ions, and 2–3-times decrease of yields of negative ions. As for hydrogen interaction processes with the alloy surface, the obtained dependence of the ion yields only on hydrogen concentration indicates that the alloy surface is rather homogeneous i.e. without large parts that have different characteristics of interaction with hydrogen and different ratios of secondary ion yields. The results of the sample temperature influence indicate that the chemisorption probability of hydrogen molecules is temperature independent at least in the ∼300-500 K range. The substantial decrease in concentration at temperatures above 450 K is a result of associative desorption of hydrogen, although a possible contribution of other processes is not excluded by the presented results.
Formation of atomic secondary ions in sputtering
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Citation Excerpt :
Fig. 5 shows velocity resolved data acquired for Ta atoms sputtered from a dynamically oxidized tantalum surface. Similar data, albeit determined without the detection of sputtered neutral particles, were measured also for Be atoms sputtered from oxidized beryllium [46,47]. However, it should be noted that the reservations regarding this technique which have been described above must be even stronger for oxidized surfaces, since here molecular emission channels are strong or even dominating [48,49], making the data evaluation procedure on the basis of Eq. (10) practically useless.
Although being investigated for decades, the formation of atomic secondary ions in sputtering is still not completely understood. By briefly summarizing the available information on velocity and work function dependences of measured ionization probabilities, we show that the formation of practically all analytically useful ions is governed by a combination of non-adiabatic and substrate excitation related ionization mechanisms. We also demonstrate that the oxygen matrix effect can be well described by assuming a statistical distribution of oxygen atoms within the surface emission zone.
Effects of preadsorbed hydrogen on the adsorption of O<inf>2</inf>, CO and H<inf>2</inf>O on beryllium
2003, Surface Science
The adsorption of O₂, CO and H₂O on a hydrogen precovered beryllium surface was studied using direct recoils spectrometry and Auger electron spectroscopy. It was found that preadsorbed hydrogen, in addition to adsorption site blocking, changes the Be electronic structure in a way that hinders the adsorption of O₂ and CO on the surface. Apparently, repulsion between the preadsorbed hydrogen and adsorbed oxygen or CO creates depletion zones between them. For oxygen it also promotes agglomeration and penetration into the subsurface and leads to oxide nucleation at lower oxygen overages. For H₂O, only a minor delay of adsorption and hydrogen replacement was observed.
Oxidation of ion-bombarded vs. annealed beryllium
2002, Surface Science
Citation Excerpt :
They found that the initial oxidation process is by nucleation and growth of oxide islands, being somewhat faster on sputter-cleaned Be(0 0 0 1) compared to the annealed one [3]. These findings are inconsistent with the second order Langmuir isotherm oxidation kinetics, suggested by Krauss and Gruen for Ar+ sputtered beryllium [6]. Roth et al. [7] measured a parabolic reaction rate for oxidation of Be in oxygen and proved the common model for Be oxidation, namely Be2+ ions diffusing outward through the oxide to meet the oxygen atoms on the surface.
The mechanisms of O₂ adsorption and oxidation of ion-bombarded and annealed highly oriented polycrystalline beryllium surfaces were comparatively studied. It was found out that the basic mechanism of oxygen adsorption involves O clusters for both surfaces and at a certain oxygen coverage nucleation and growth of oxide islands take place. Also, in both cases, lateral growth of the islands dictates a relatively fast oxidation rate, which is decreased following coalescence, turning to a slow inverse logarithmic growth.
The introduction of surface and subsurface defects by the Ar⁺ ion-bombardment almost doubles the oxygen sticking coefficient, increases the oxide island thickness from ∼2 to ∼3 monolayers (prior to coalescence) and dictates a higher oxidation rate in the high oxygen exposure regime.
It was also found that annealing the surface, following sputtering, causes its smoothing, but in the initial heating process, in the range 300–500 K coarsening seems to occur.
'Infinite velocity method': a means of concentration calibration in secondary ion mass spectrometry?
1999, Surface Science
The so-called infinite velocity (iv) method, recently suggested by P. van der Heide as a means of quantifying, without standards, the signals measured by secondary ion mass spectrometry (SIMS), has been analysed critically. The iv method is essentially based upon two assumptions. (1) Secondary ion energy spectra can be described as a product of the isotopic concentration of the analysed species, the flux of sputtered neutrals, the instrument transmission, the detector efficiency, and the ionization probability, an ansatz that had already been used repeatedly in earlier work. (2) The ionization probability increases exponentially with decreasing inverse ion velocity, and in the limit of infinite velocity, all sputtered atoms are fully ionized. Hence, the matrix effect in SIMS is removed. Secondary ion yields at infinite velocity can be derived by extrapolation of measured energy spectra after eliminating the energy dependence of the sputtered atom flux and the instrument transmission, both considered to be the same for all elements and isotopes.
It is shown that, if the energy-dependent neutral flux and the instrument transmission were in fact universal correction functions, concentration calibration could be achieved by extrapolation of ion yield ratios, without even knowing the analytical form of these functions. Literature results as well as (re)analysed data of van der Heide provide clear evidence that the matrix effect is not removed at infinite velocity. Many of the previously ignored problems originate from the fact that the formalism used for data processing has never been outlined in a quantitatively correct manner. As a result, the importance of normalization factors associated with the use of the unknown input functions remained unnoticed (e.g. the surface binding energy and the shape of the energy and angular distributions). Element-specific differences in these parameters give rise to corresponding errors in concentration calibration. The frequently derived non-exponential inverse-velocity dependence of the ‘corrected’ ion yields can be reproduced if one considers rather small deviations from the arbitrary assumptions made in the data processing. It is concluded that, even if all errors in the basic formalism are removed, the iv method does not provide reliable data, neither on elemental concentrations nor on ionization or neutralization phenomena, the reason being that (1) none of the parameters and functions required for extracting ion fractions from measured energy spectra is known with the required accuracy, and (2) the matrix effect cannot be removed by extrapolation procedures.
Electron-exchange mechanisms of the secondary ion emission of metals and some incompatible experimental data
1998, Surface Science
Two electron-exchange theories of secondary ion emission are known. In the first one the ionization of the ejected atom occurs as a result of electron resonant tunneling through the potential barrier separating an atom and a metal. In the second the ionization occurs by the action of strong time-dependent perturbation due to the wave function of an atom and metal overlapping. Both theories predict different dependences that can be verified by experiments. Experimental data concerning the investigation of the secondary ion emission of metals have been analyzed to show (i) the form of the dependence of the probability of secondary particle ionization on its energy — it has been shown that this agrees with the first theory but does not agree with the second one; (ii) the dependence of the secondary ion emission yield on the metal work function — it has been found that this dependence can be described as incompletely by the second theory as by the first one united with the known thermodynamic non-equilibrium theory of Saha-Langmuir-Dobretsov.

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Secondary-ion emission from oxygen and hydrogen-covered beryllium surfaces: I. Coverage dependence☆

Abstract

J. Nucl. Mater.

Nucl. Instr. Methods

Surface Sci.

J. Nucl. Mater.

Surface Sci.

Surface Sci.

Surface Sci.

Surface Sci.

Surface Sci.

Surface Sci.

Surface Sci.

Surface Sci.

Nucl. Inst. Methods

Surface Sci.

Radiation Effects

Surface Sci.

J. Appl. Phys.

Surface Sci.

Surface Sci.

Radiation Effects

Fundamental Aspects of Ion Microanalysis