Ion Yield Ratio Research Articles

Isotopic analysis of a 6Li enriched lithium sample employing the Li 2BO 2+ ion beam method: verification of the theoretical accuracy

It was predicted recently by us (Int. J. Mass Spectrom. Ion Processes, 116 (1992) 87) that accurate measurements of the Li 2BO 2 + ion yield ratio(s) do not always imply accurate determination of the elemental isotopic abundance ratio(s). Further insight into this aspect is provided here by demonstrating that the results of the isotopic analysis of a 6Li enriched lithium sample using different pairs of isotopic Li 2BO 2 + ions as monitors show considerable variations, irrespective of the actual experimental errors. A theoretical explanation of the observations is presented. The present investigation provides the guidelines for selecting the proper monitor Li 2BO 2 + ion-pair(s) and discusses the other analytical requirements for making possible accurate isotopic analysis of lithium of any given isotopic composition. Experimental observations showing the effect of isotopic fractionation on the formation of the Li 2BO 2 + ions are also elaborated and discussed.

A study of matrix effects in CsI on Au by time-of-flight SIMS

We have studied the matrix change dependence of the positive (Cs +) and negative (I −) secondary ion intensities from an overlayer of ionic compounds (CsI) on an Au surface by TOF-SIMS. The change of the ion yield ratio of the positive to the negative is almost insignificant even if the coverage of the overlayer on Au changes from submonolayer (< 1 ML) to thick film ( > 10 ML). To interpret this phenomenon, tunneling and bond breaking models are applied. These results have led to the conclusion that the bond breaking model is suitable for an explanation of the secondary ionization probability of each of the positive and negative secondary ions for both submonolayer and thick ionic compounds (CsI) on an Au surface.

Surface characterization of a poly(styrene/p-hydroxystyrene) copolymer series using x-ray photoelectron spectroscopy, static secondary ion mass spectrometry, and chemical derivatization techniques

A series of poly(styrene/p-hydroxystyrene) copolymers was examined by x-ray photoelectron spectroscopy (XPS), static secondary ion mass spectrometry (SIMS), and chemical derivatization. Static SIMS distinguished polystyrene (PS) and poly(p-hydroxystyrene) (PHS) on the basis of fragments, 107 D (C7H7O+) and 119–121 D {[M–H]+,, [M]+, and [M+H]+, where M is the hydroxystyrene (HS) monomer}, that were present in the positive ion spectrum of PHS. The negative ion spectrum of PHS was structurally less informative (C− –C−4, CH−–C4H−, O−, OH−), with the exception of a peak at 119 D [M–H]−. While fluorinated fragments characteristic of trifluoroacetic anhydride (TFAA) (CF+3, CF−3, CF3 CO−2 ) were observed in both the positive and negative ion spectra of TFAA-derivatized PHS, fragments characteristic of the derivatized polymer (C7H6F+ , C7H6CO2CF+3 ) were observed only in the positive ion spectrum of TFAA-derivatized PHS. The quantitative relationship of positive ion intensities to XPS oxygen concentration for these copolymers was complicated by the absence of peaks unique to the styrene component. Quantitation based on normalization of unique hydroxystyrene peaks to total ion yield and peak ratio normalization of selected peaks was successful. Similarly, quantitative trends were observed for the TFAA-derivatized copolymer series when the peak intensity of the C7H6CO2CF+3 ion was normalized to the total ion yield.

Low pressure oxygen adsorption induced Re segregation on an annealed Pt-Re alloy: A sims and AES study

In agreement with a prediction based on the model of Tománek et al. surface segregation of Re on a Pt 60Re 40 alloy upon oxygen chemisorption was observed in the temperature range 300–900 K by means of SIMS (static) and AES. The influence of oxygen adsorption on the Re +/Pt + ion yield ratio due to alterations of the secondary ion emission probabilities is discussed in detail. This effect is argued to be superimposed on the effect of an altered alloy concentration ratio at the surface on S + (Re)/ S + (Pt).

Low pressure oxygen adsorption induced Re segregation on an annealed Pt−Re alloy: A sims and aes study

In agreement with a prediction based on the model of Tománek et al. surface segregation of Re on a Pt 60 Re 40 alloy upon oxygen chemisorption was observed in the temperature range 300–900 K by means of SIMS (static) and AES. The influence of oxygen adsorption on the Re + /Pt + ion yield ratio due to alterations of the secondary ion emission probabilities is discussed in detail. This effect is argued to be superimposed on the effect of an altered alloy concentration ratio at the surface on S + (Re)/ S + (Pt).

Quantitative SIMS measurements of Al( x)Ga(1− x)As as a function of alloy composition and ion beam energy

SIMS has been used to measure ion yield ratios from the matrix elements in a range of Al( x) Ga(1− x) As alloys at different primary ion energies. A linear relationship has been observed under both O 2 + and Ar + bombardment between the measured Al:Ga ratio and the matrix composition. Under O 2 + bombardment the ionization probability of As is enhanced by the presence of Al in the sample. The reverse is observed under Ar + bombardment.

A SIMS and classical dynamics study of the chemisorption of CO on Ni{7 9 11}

The dependence of secondary ion intensities on azimuthal angle of incidence of the primary ion beam in SIMS experiments on a stepped Ni{7 9 11} surface with adsorbed CO has been examined in detail. The surface was examined after various exposures to CO and at temperatures of 231 and 300 K. The angular anisotropies for the Ni+, Ni+2, Ni+3, Ni2CO+, and especially the NiCO+ species were found to be quite sensitive to surface structure changes suggested using other surface science techniques. Ion yield ratios as a function of exposure of CO to the surface, however, were found to be nearly insensitive to these structural changes. Of particular significance was the presence of a sharp feature in the NiCO+ ion yield at an Ar+ ion angle of incidence of φ=110° with the crystal temperature at 231 K and after a CO exposure adequate to populate the step edge sites. At higher exposures or temperatures, this feature was washed out when presumably mostly terrace sites are occupied. Using these ion yield versus azimuthal angle curves, it is also apparent that the saturation coverage structures at 231 and 300 K are different. By introducing 10−7 Torr of CO into the chamber at 300 K the ion yield curves are identical to those for the saturation coverage at 231 K, indicating similar surface structures. Classical dynamics calculations aimed at modeling the structure corresponding to a CO exposure of 0.6 L at 231 K support the idea that the CO molecules reside in twofold bridge sites along the bottom of the step edge.

Primary ion beam energy effects on secondary ion emission from Ni(001)c(2 × 2)-CO: Classical dynamics calculations and sims

The variation of the SIMS spectra of Ni(001)c(2 × 2)-CO induced by bombardment of Ar + primary ions with kinetic energy between 300 and 1200 eV has been examined in detail. The purpose of the study is to examine the influence of primary ion energy on a number of experimental observables and to test the accuracy of classical dynamics calculations for neutral particles in predicting the experimental results for ejected ions. The calculations were performed using two extreme forms of the Ar + -substrate interaction potential to examine the sensitivity of the results to the parameters that need to be included in the model. We find excellent agreement between the NiCO +/Ni + ion yield ratio measured between 300–1200 eV Ar + ion energy and the computational results if the Molière form of the ion-substrate potential is used and if the calculated results are corrected by including an image force. The calculated angular distributions of the ejected particles also agree well with those observed experimentally. From the calculations we see that the extent of CO fragmentation relative to the amount of molecular CO ejection is roughly constant in the 300–1200 eV beam energy range with a slight increase seen at lower (~ 300 eV) energies. The implications of these results are discussed in terms of our ability to study the chemistry and structure of surfaces with SIMS.

A static SIMS study of the chemisorption of CO on Ni(111)

SIMS spectra were obtained for CO adsorbed on Ni(100) at 300 K and on Ni(111) at 300 and 185 K. Changes in ion yield ratios with CO coverage are shown to correlate with various surface structures observed by LEED and vibrational spectroscopy. For example, there is a dramatic decrease in the NiCO +/Ni + ratio for CO on Ni(111) at 185 K at exposures corresponding to the coverage where CO molecules shift from threefold to twofold bridged sites and where the onset of the ( 3 × 3 )R30° LEED structure is observed. It is suggested that cluster formation is a very complex process. Thus it is not possible to make simple correlations between Ni 2CO + and bridge-bonded CO and NiCO + and linear bonded CO, even though the yields of these species are influenced by the surface structure.

Oxidation of polycrystalline indium studied by x-ray photoelectron spectroscopy and static secondary ion mass spectroscopy

The chemisorption of oxygen on clean polycrystalline indium surfaces is investigated by the combined techniques of x-ray photoelectron spectroscopy (XPS) and the static mode of secondary ion mass spectroscopy (SIMS). Oxygen uptake is characterized by a moderately rapid formation of In2O3, with no preoxidation adsorbed O2 phase discernable in the XPS spectra. A high binding energy O (1s) peak becomes prominent after one monolayer of In2O3 has formed, which we ascribe to adsorbed oxygen on the oxide. Formation of In2O3 continues into the bulk either via migration of the adsorbed oxygen through the oxide layer to the metal-In2O3 interface or by the outward migration of indium cations to the surface. Both the formation of oxide and the onset of adsorption are reflected in the secondary ion yield ratios InO−2/InO− and O−2/O−. The behavior of SIMS molecular cluster ion yields with oxygen exposure are consistent with recent theoretical calculations of the cluster formation process, in which considerable local atomic order of the surface is present in the cluster.

SIMS on ZnO surfaces: The influence of space charge accumulation layers on secondary ion yields and measurement of true hydrogen concentration

On single crystal surfaces of n-conducting ZnO rather strong space charge accumulation layers can be produced by adsorption of atomic hydrogen. It is shown that secondary ion yields depend strongly on this charge carrier concentration at and near the surface. By comparison to electric conductivity measurements it is demonstrated that surface concentrations of hydrogen are proportional to secondary ion yield ratios of type XH ±/X ±. In the experiments it was further noticed that Na, K, Ca represent common impurities on ZnO surfaces.