Secondary ion emission

In the case of fast ions, the terminology of secondary ion emission mass spectrometry (SIMS) is more obvious in that a primary incident beam of ions onto a target releases secondary ions after impact. 

Imaging SIMS, used for spatially resolved elemental analysis. A focussed ion beam is rastered over the surface so that each point on the target is individually bombarded in turn, so that secondary ion emission is localized. The intensity of a particular secondary ion is monitored for each position of the primary beam and the result shown at the corresponding point of a synchronized oscilloscope display or computerized data system. In this way, pixel by pixel across the sample surface and in depth as the material is removed, three-dimensional information on the sample composition may be obtained. 

A. Benninghoven, D. Jaspers, and W. Sichtermann. Secondary-Ion Emission of Amino Acids. Appl. Phys., 11(1976) 35-39. 

Todd, P.J. Secondary Ion Emission From Glycerol Under Continuous and Pulsed Primary Ion Current. Org. Mass Spectrom. 1988, 23, 419-424. 

Todd, P.J. Solution Chemistry and Secondary Ion Emission From Amine-Glycerol Solutions. J. Am. Soc. Mass Spectrom. 

When one considers the role of the matrix in the particle-induced emission of secondary ions it is no wonder that it is so difficult to unravel all the processes that take place. The matrix is the medium in which the primary excitation occurs. It must also disperse some of that energy to sites at the surface where secondary ion emission occurs. It must provide the species to be desorbed and at the same time mediate the ionization process. In an attempt to understand these complex coupled processes we have tried to simplify the system by first selecting a homogeneous substrate for the energy deposition and then studying the ionization-emission process for species that are present as a submonolayer on the surface (26). 

The construction and operation of LMI sources are briefly described in the first section of this paper. The potential use of LMI sources in fundamental investigations of secondary ion emission from liquid organic matrices is discussed in the following section. The few existing preliminary reports on LMI/SIMS combinations used or studied for analytical purposes are reviewed in the final section. 

One of the most significant uses of LMI sources in connection with SIMS of organic compounds may be as probes in performing measurements of secondary particle yields. Such measurements are important for understanding the processes of secondary ion emission from solid and liquid organic samples. Total particle yields reflect directly the dynamical aspects of emission processes variations in primary beam energy, incident flux density, and primary particle mass, for example, are all manifested in changes in total particle yields. The ratio of secondary ion yield to total particle yield and the ratio of secondary ion yields from two different species can be sensitive, quantitative monitors of the chemistry and kinetics, respectively, of ionization processes. 

Effects of Primary Beam Species Prediction of FABMS source sputtering yields can be generally supported by available data on secondary ion emission coefficients. For secondary ions detected at zero angle with respect to the surface normal, the secondary ion emission yield generally increases with the mass of the primary ions because of the... 

Physical Models. Two basic approaches are used to quantify secondary ion intensities physical models and empirical methods. The physical models consist of several theoretical or semi-empirical treatments developed to simulate secondary ion emission [3,48-50]. Although several models have been developed (see Werner L3j for a recent review) and continue to be applied, the use of calibration standards (empirical methods) consistently give better results, e.g., accuracies of a factor of 2-3 for physical models compared to 10-20% for empirical methods. 

SIMS has become a diverse tool in the study of many different substances other than metals and semiconductors. This part of the paper discusses the secondary ion emission of molecular and polyatomic ions from the surfaces of organic compounds including polymers and biomolecules. 

Sample Preparation. The methods of sample preparation affect the chemical and physical properties of the sample molecules and hence can profoundly influence the secondary ion formation/ emission process. In earlier molecular SIMS studies the samples were prepared by placing a dilute solution of the compound onto an acid-etched Ag foil [87, 88]. The acid etched surface provides, a substrate onto which thin layers of the compound can be deposited from solutions with extended concentration ranges. If on the other hand, the substrate was not etched and the concentration of the solution was too high, the adsorbed molecular film would grow too thick and consequently quench the secondary ion emission. 

To test these possibilities, secondary molecular ion emission from arachidic (AA) and behenic (BA) acids cast directly onto equivalently prepared silver substrates from solution was compared to the secondary ion emission from LB films of equivalent molecular concentrations (i.e. molecules/cm2) to determine the effect of orientation in molecular ion formation. The qualitative results are summarized in Table 3. For single LB layers of AA and BA on Ag, no (M+H)+ emission is observed, as was previously reported for stearate, oleate and linolenic systems (3a). The Ag cationized molecular species, (M+Ag)+ and (M-H+2Ag)+, are... 

Analysis of surface chemical structure requires use of the static SIMS technique to ensure that the major portion of the surface should not be affected by secondary ion emission. Time-of-flight SIMS (ToF SIMS) is the most widely used static SIMS technique. As its name indicates, ToF SIMS uses the ToF mass analyzer to measure mz l of secondary ions. ToF SIMS is a stand-alone instrument, not incorporated into or attached to other SIMS instruments as for dynamic SIMS. A typical structure is illustrated in Figure 8.13. 

Benninghoven, A., Jaspers, D., and Sichterman, W., Secondary-ion emission of amino acids, AppZ, Phys., 11, 35, 1976. 

Feld, H., Leute, A., Rading, D., Benninghoven, A., Chiarelli, M.P., and Hercules, D.M., Secondary ion emission from perfluorinated polyethers using megaelec-tronvolt and kiloelectronvolt ion bombardment, Anal Chem., 65,1947,1993. 

Benguerba, M., Brunelle, A., Della-Negra, S., Depauw, J., Joret, H., LeBeyec, Y., Blain, M.G., Schweikert, E. A., Assayag, G. B., and Sudreau, R, Impact of slow gold clusters on various solids nonlinear effects in secondary ion emission. Nucl. Instr. Meth. Phys. Res., B62, 8, 1991. 

Kotter, E, Niehuis, E., and Benninghoven, A., Secondary ion emission from pol5nner surfaces under Ar, Xe and SFJ primary ion bombardment, SIMS XI-Eleventh International Conference on Secondary Ion Mass Spectrometry, Orlando, EL, 1997. 

Secondary ion emission (SIE) involves bombarding the profile with high energy (4 keV) ions and monitoring the spattered adsorbate atoms with a mass spectrometer [71Abrl]. A broad-area beam is employed for bombardment a single slit placed at some distance from the initial step admits spnttered ions from only one location on the srrrface. The data fit to the time dependence expected from the diffusion equation with constarrt D. Spatial resolution of only 1 pm has been achieved. 

In the last years several techniques have been developed to improve the mass resolution in TOF-mass spectrometers such as the use of a reflecting field/2/ in the ion beam path or the method of velocity compac-tion/3/. Especially by using a Reflectron-Time-of-Flight (RETOF) ins-trument/4/ some shortcomings of the Time-of-Flight technique can be overcome like the limited mass resolution. The necessity of pulsed ion sources in Time-of-Flight mass spectrometry makes this technique excellent for pulsed ionization methods like laser ionization or secondary ion emission. 

Bolbach, G., Beavis, R., DeUa-Negra, S., Deprun, C., Ens, W., Lebeyec, Y., Main, D.E., Schueler, B., Standing, K.G. (1988) Variation of yield with thickness in SIMS and PDMS measurements of secondary ion emission from organized molecular films. Nucl. Ins. Meth. Phys. Res. B, 30,74-82. 

Stapel, D., Thiemann, M., Benninghoven, A. (2000) Secondary ion emission from polymethacrylate LB-layers under 0.5-11 keV atomic and molecular primary ion bombardment. Appl. Surf. Sci., 158,362-374. 

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