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Matrix Effects on the Electronic Partitioning of Iron Atoms Desorbed from Surfaces by Energetic Ion Bombardment Fred M. Kimock,' David L. Pappas, and Nicholas Winograd*
Department of Chemistry, T h e Pennsylvania State University, 152 Davey Laboratory, University Park, Pennsylvania 16802
The effect of the sample matrix on the electronic partitioning of sputtered Fe atoms Is examined. The surfaces studied are clean and air-exposed polycrystalline Fe, N1,Fe ( 111) and NBS 1264a steel. I n contrast to the yleld of secondary Fe' (which for all samples'ls Increased dramatically by Surface contamlnatlon), the fraction of eJectedground state Fe atoms Is relatively insensitive to surface contamination. When the clean targets are subjected to 900-eV argon Ion bombardment, the populations of ejected metastable exclted state Fe atoms are characterired by near-Boitrmann distributions with effective temperatures of about 700 f 100 K. The populations of Fe atoms sputtered from the alr-exposed surfaces are also near-Boitzmann In character, with effective temperatures up to 1400 f 200 K.
Ion beam methods have become increasingly popular probes for the characterization of solids and surfaces due to their surface sensitivity and mass selectivity. For example, for the analysis of dopants and impurities in semiconductors, secondary ion mass spectrometry (SIMS) is often employed because of its sensitivity and large dynamic range ( I ) . Unfortunately, the semiquantitative nature of the SIMS technique leads to difficulties in data interpretation. In SIMS analysis, it is difficult to quantitatively determine the concentrations of individual components because of the large differences in the sputtered ion fraction for each species and because the ion fraction for a given element is a strong function of the surface matrix. Recently, we have demonstrated that neutral species ejected from ion bombarded surfaces can be selectively and efficiently ionized by multiphoton resonance ionization (MPRI) (2-5). When operated with a low primary ion dose, the MPRI experiment may be more sensitive than static SIMS by several orders of magnitude and will be useful for monitoring the progress of surface chemical reactions (3) and as a structural probe ( 4 ) . Factors which influence the ultimate sensitivity of the technique have been discussed in detail (5). MPRI of sputtered neutrals promises to yield sub-part-per-billion (2000 K) to obtain a high yield of atoms. Unfortunately, these high temperatures lead to much greater excited state populations. Ion bombardment of solids is an efficient method for generating a reservoir of gas-phase atoms under conditions which appear to parallel a low filament temperature and hence low excited state populations. Thus, for MPRI spectrometry studies in geperal, energetic ion sputtering may be superior to thermal evaporation as an atomization source. It is important to point out that sputtering is a nonequilibrium process, so one would not necessarily expect the sputtered excited atoms to obey a Boltzmann distribution. We certainly place no physical significance to the chosen method of representing the data. Also, we note that these "effective Boltzmann temperatures" should not be confused with the very high surface temperatures (=lo4 K) which have been calculated aceording to some theories of surface ionization (23). Finally, the metastable excited states can be directly populated by the ejection event, as well as by cascade repopulation from upper electronic levels. Thus, the origin of the measured populations is uncertain. From the point of view of quantifying the MPRI method, however, the determination of these effective Boltzmann temperatures may be useful, especially since this temperature seems not to be strongly influenced by the surface matrix.
CONCLUSION Several factors are known to affect the yield of ground state neutral atoms from ion bombarded surfaces. First, the total number of desorbed neutrals will be influenced by changes in surface binding forces. This factor would be expected to alter the yield on the order of 50%. Secondly, the total flux of secondary particles may be partitioned among several channels including excited states and secondary ions. The loss of ground state atom population to molecule formation may also be important. Whether these factors contribute significantly to an overall matrix effect will depend on the nature of the electronic structure of the atom itself. For example, it is expected that elements with high ionization potentials, with first excited states well above the ground state, and which exhibit a small probability to form molecules will be quite immune to changes in the nature of the matrix. Elements such as He and Ne represent rather trivial examples of this extreme situation. At this point, it is not yet clear how strongly these variables will complicate our ability to make quantitative measurements. It will be necessary to focus our attention on each parameter and carefully decide when to worry and when not to worry. In this work, we have examined the magnitude of the matrix effect for Fe when found in six different environments. The results suggest that the fraction of secondary ions and highlying excited state atoms rarely represents more than several percent of the total sputter yield. Further, the ground state fraction and low-lying excited state fractions are only weakly influenced by the surface matrix. Thus, it appears that even though the Fe electronic structure is quite complex, Fe itself is an excellent candidate for quantitative andysis using MPRI techniques. It will be of interest to test the generality of these observationsthroughout the periodic table in future research. ACKNOWLEDGMENT The authors acknowledge the many beneficial discussions with J. P. Baxter, G. A. Schick, and P. H. Kobrin regarding this work. LITERATURE CITED (1) Clegg, J. 9.; Scott, G. B.; Hallals, J.; Mlrcea-Roussel, A. J . Appl. Phys. 1981, 52, 1110-1112. (2) Wlnograd, N.; Baxter. J. P.; Klmock, F. M. Chem. Phys. Left. 1982, 88, 581-584.
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(3) Kimock, F. M.; Baxter, J. P.; Winograd, N. Surf. Scl. 1983, 124,L41L46. I Baxter, J. P.; Kobrin, P. H.; Schick, G. A.; Winograd, N., unpublsihed work. Kimock, F. M.; Baxter, J. P.; Pappas, D. L.; Kobrin, P. H.; Winograd, N. Anal. Chem. 198q956, 2782-2791. Parks, J. E.; Schmitt, H. W.; Hurst, G. S.;Fairbank, W. M. Thin Solid Films 1983, 108,69-70. Pellln, M. J.; Yourlg, C. E.; Calaway, W. F.; Gruen, D. M. Surf. Sci. 1984, 144,619-637. Coburn, J. W.; Kay, E. Appl. fhys. Letf. 1971, 19, 350-352. Coburn, J. W.; Taglauer, E.; Kay, E. J . Appl. Phys. 1973, 45, 1779-1786. Oechsner, H.; Stumpe, E. Appl. fhys. 1977, 14, 43-47. Oechsner, H.; Schoof, H.; Stumpe, E. Surf. Scl. 1978, 76, 343-354. Oechsner, H.; Ruhe, W. Stumpe, E. Surf. Sci. 1979, 85, 289-301. Oechsner, H.; Wucher, A. Appl. Surf. Sci. 1982, IO, 342-348. Muller, K. H.; Oechsner, H. Mlcrochim. Acta 1983, IO, 51-60. Stumpe, E.; Oechsner, H.; Schoof, H. Appl. fhys. 1879, 20,55-60. Kimock, F. M. Ph.D. Thesie. The Pennsylvania State University, University Park, PA, 1985.
(17) Hurst, G. S.; Payne, M. G.; Kramer, S. D.; Young, J. P. Rev. Mod. fhyS. 1979, 51, 787-819. (18) Fuhr, J. R.; Martin, G. A.; Wiese, W. L.; Younger, S. M. J . fhys. Chem. Ref. Data 1981, IO,305-565. (19) Norskov, J. K.; Lundquist, B. I. fhys. Rev. B 1979, 19,5661-5665. (20) Benninghoven, A.; Mueller, A. fhys. Lett. A 1972, 40A, 169-170. (21) Fassett, J. D.; Moore, L. J.; Travis, J. C.; Lytle, F. E. Int. J. Mass Spectrom. Ion Processes 1983, 54, 201-216. (22) Schweer, B.; Bay, H. L. Vlde Couches Mlnces Suppl. 1980, 201, 1349. . 1355 ._. (23) Wittmaack, K. Nucl. Instrum. Methods 1980, 168,343-356.
RECEIVED for review May 20, 1985. Accepted July 17, 1985. The authors are grateful for the financial support of the National Science Foundation (Grant No. CHE 81-08382),the Office of Naval Research (Grant No. N00014-83-K-0052),and the Air Force Office of Scientific Research (Grant No. AFOSR-82-0057).
Ion Sampling for Inductively Coupled Plasma Mass Spectrometry Jose A. Olivares and R. S . Houk* Ames Laboratory-USDOE and Department of Chemistry, Iowa State University, Ames, Iowa 50011
The sampling of ions from an atmospheric pressure inductlvely coupled plasma for mass spectrometry (ICP-MS) with a supersonic nozzle and skimmer is shown to follow simliar behavior found for neutral beam studies and for Ion extractkn from other plasmas and flames. I n particular, highest Ion beam intensity is found If the skimmer tlp is close to the Mach disk and at a calculated skimmlng Knudsen number close to the recommended value of 1. Our ICP-MS instrument with an off-axls detector and conventional cylindrical electrostatic ion focuslng in the transition regime gives Intense count rates of 1-5 MHz/(mg L-') of anaiyte superimposed on a background of 1-la kHz. Finally, the dependence of count rates for metal oxide and doubly charged ions on ICP operating parameters and sampling Interface configuration are discussed for this instrument.
The first analytical mass spectrometerfor ion sampling from an inductively coupled plasma (ICP-MS) (1) used a stagnant layer type sampling interface (Figure l),subsequentlyreferred to as boundary layer sampling. In this technique ions and neutrals were extracted from an aerodynamic boundary layer that formed in front of the sampling cone. The difficulties associated with this type of sampling were quickly realized by the observation of metal oxide and metal hydroxide ions, deposition of salts on the orifice, and short orifice lifetime. Supersonic nozzle beam sources for dynamic sampling of ions and neutrals from flames and plasmas have been in use since the pioneering work of Kantrowitz and Grey (2) in 1951. The version of this technique used in ICP-MS employs a large sampling orifice (diameter 0.5-1 mm), through which the source gas flows under continuum conditions into an evacuated expansion stage. The gas flow rate into this orifice is high enough to puncture the external boundary layer so that salt deposition and oxide ion formation are much less severe than with the original pinhole orifice (diameter 50-75 pm). The
large sampling orifice can also be made with relatively thick metal near the tip and is thus resistant to erosion or expansion. Part of the supersonicbeam that forms in the expansion stage is then transmitted through a conical skimmer into a second vacuum stage. Gray and Date (3) and Douglas et al. (4) applied supersonic nozzle interfaces to ICP-MS,finding overall improved performance in ion sampling. Yet, little has been described in the literature on the adaptation of supersonic nozzles to ICPs with most of the published work emphasizing the analytical capabilities of ICP-MS. Another area of research in ICP-MS where common information is lacking, is ion focusing, detection, and background suppression. Most instruments employ a Bessel box (5, 6) or a modification of this for ion focusing in the transition flow region behind the skimmer. This device consists of an arrangement of three lenses, a cylinder, and two rings on each end. The cylinder contains, a t its center, a round plate with a diameter nearly equal to or less than the inside diameter of the end rings. The arrangement, when used with appropriate voltages applied to the lenses, provides an effective energy analyzer. The Bessel box was originally implemented for ICP-MS partly because it was believed that ion energy analysis was necessary. But, when this was found not to be the case (3) this lens arrangement remained in use because the center plate provides an effective shield for blocking ICP photons from reaching the detector. The central axis of a Bessel box is physically blocked so that the ion transmission efficiency of this device is relatively low compared to an open ion lens focusing system. Therefore, our research has been geared at producing a detector arrangement which minimizes the detection of background photons and can collect a major portion of the mass analyzed ion beam while using conventional electrostatic ion focusing in the transition flow region. In this paper we describe in detail the construction and performance characteristics of a supersonic nozzle for ICP-MS with ion sampling in the continuum flow regime, the performance of an off-axis detector arrangement for photon
0003-2700/65/0357-2674$01.50/00 1985 American Chemical Soclety
