Analysis of Solution-Deposited Alkali Ions by Cluster Surface

Chemical Analysis of Complex Surface-Adsorbed Molecules and Their Reactions by Means of Cluster-Induced ... Analytical Chemistry 2018 90 (5), 3328-333...
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Anal. Chem. 2003, 75, 5124-5128

Analysis of Solution-Deposited Alkali Ions by Cluster Surface Collisions F. Eusepi, A. Tomsic, and C. R. Gebhardt*

Max-Planck-Institut fu¨r Quantenoptik, Hans-Kopfermann-Strasse 1, D-85748 Garching, Germany Sulfur dioxide clusters (SO2)x of mean size 〈x〉 ) 1.7 × 103 collide at a velocity of 1.6 km s-1 with a metal surface that has been pretreated with dilute salt solutions containing Li+, Na+, K+, Cl-, and Br- ions in different concentrations. In the scattered gas plume, positive and negative cluster fragments of the formal composition (SO2)x(Li+, Na+, K+, SO2-) are detected. While the amount of charge observed in the various cationic channels correlates with the concentration of the respective cations in the solutions, no cluster fragments carrying chloride or bromide anions have been observed. This indicates, that the observed free charge carriers are not formed by a direct pickup of ions from the surface. Based on the assumption that the physical state for alkali adsorbates on metal surfaces is independent of the charge state of the adsorbing precursor, the findings are explained in terms of a known charge separation effect in cluster surface collisions involving neutrally deposited alkali adsorbates. The observation of abundant positively and negatively charged cluster fragments upon the collision of neutral molecular clusters with a target surface, termed the cluster electric effect (CEE), is an intriguing phenomenon.1-5 For the combination of polar molecule clusters such as (SO2)x with alkali-spiked targets, the proposed charging mechanism is depicted in Figure 1.1 Upon impact, the cluster picks up an alkali surface adsorbate, which desorbs neutrally. Inside the polar molecule cluster, it ionizes spontaneously6 forming an electron-cation pair, which is subsequently separated by the collision-induced cluster fragmentation. By dosing the surface with alkali atoms from an oven source, the ion yield can be increased.1 The alkali metals in general represent a scientifically and technologically important group of elements.7,8 Sodium and potassium range among the 10 most abundant elements in the * Corresponding author. E-mail: [email protected]. Fax: +49-89-32905200. (1) Gebhardt, C. R.; Schro ¨der, H.; Kompa, K. L. Nature 1999, 400, 544-547. (2) Christen, W.; Kompa, K. L.; Schro¨der, H.; Stu ¨ lpnagel, H. Ber. Bunsen-Ges. Phys. Chem. 1992, 96, 1197-1199. (3) Vostrikov, A. A.; Dubov, D. Y.; Predtechenskiy, M. R. Chem. Phys. Lett. 1987, 139, 124-128. (4) Andersson, P. U.; Pettersson, J. B. C. Z. Phys. D 1997, 41, 57-62. (5) Gebhardt, C. R.; Witte, T.; Kompa, K. L. ChemPhysChem 2003, 4, 308312. (6) Hertel, I. V.; Hu ¨ glin, C.; Nitsch, C.; Schulz, C. P. Phys. Rev. Lett. 1991, 67, 1767-1770. (7) Bonzel, H. P., Bradshaw, A. M., Ertl, G., Eds. Physics and chemistry of alkali metal adsorption; Material Science Monographs 57; Elsevier: New York, 1989.

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Figure 1. Schematic representation of the charge separation in cluster surface collisions based on the pickup of alkali surface adsorbates.

earth crust. Due to their high chemical reactivity and electropositive character, the alkali metals generally occur as compounds. As solvated ions they play an important role in the biochemistry of cells. In the semiconductor industry, alkali ion contaminations are a concern, e.g., in the context of electric instabilities in metal oxide-silicon structures.9 The prominent role of the alkali elements goes hand in hand with the need for analytical methods to quantify their presence. Presently used techniques include optical and X-ray spectroscopy, mass spectrometry, and ion-selective electrodes. Here, we investigate whether the CEE described above can be used as an analytical tool for the determination of alkali surface contaminations. We extend the correlation found between ion yield and neutrally deposited alkali atoms from an oven source1 to the case where the alkali surface adsorbates are formed via alkali ions that are deposited from salt solutions on the sample surface. EXPERIMENTAL SETUP The experiments are performed in a conventional two-chamber, high-vacuum molecular beam apparatus interconnected by a skimmer and pumped by turbomolecular pumps and LN2 cold traps.1,5 The collision target consists of a heatable stainless steel plate that was electrolytically coated with gold and is mounted in the second chamber perpendicularly to the beam 300 mm downstream from the nozzle. A bias grid placed 10 mm in front of the target allows for measuring of the total charge yield per pulse by pushing one ion polarity against the target for neutralization. It also serves to transfer fragment ions into the detection volume of our pulsed Wiley-McLaren-type time-of-flight mass spectrometer oriented perpendicularly to the beam axis 100 mm upstream from the target. The collision target was prepared by placing half of the target surface in the electrolytic solution A and (8) Borgsted, H. U.; Mathews, C. K. Applied Chemistry of the Alkali Metals; Plenum Press: New York, 1987. (9) Greeuw, G.; Verwey, J. F. J. Appl. Phys. 1984, 56, 2218-2224. 10.1021/ac0345143 CCC: $25.00

© 2003 American Chemical Society Published on Web 09/05/2003

Table 1. Ion Concentrations c in the Salt Solutions Used To Prepare the Collision Target and the Resulting Ion Yields Y in the Mass Spectraa

solution A (mmol/l) solution B (mmol/l) side A: ion yield (arb units) side B: ion yield (arb units) correlation coefficient m

Li+

Na+

K+

Cl-

Br-

44.8 25.0 10 4.9 0.22

9.88 9.82 7.8 8.0 0.80

4.21 8.65 3.4 9.0 1.0

44.8 25.0 0 0

14.1 18.5 0 0

a The correlation coefficient m is calculated based on the assumption of a linear dependence Y ) mc. The ion yields have been corrected for the mass-dependent detector efficiency10 and normalized so that for potassium m is unity.

Figure 3. Left panel: neutralization current IT into the target measured upon impact of the SO2 cluster beam on the left side (solid line) and on the right side of the target (dashed line). Right panels: mass spectra of the positively and negatively charged cluster ions generated by the cluster surface impact on the respective target sides. The peak triplets in the cation spectra correspond to Li+, Na+, and K+ solvated by an increasing number of SO2 molecules. The anionic spectra are based on a SO2- core ion.

Assuming a log-normal distribution (eq 1) for the neutral cluster Figure 2. Experimental retarding field measurement (circles) and fitted model function (solid line) together with a sketch of the setup.

later on the remaining half in solution B (see Table 1). The dry target was transferred into the apparatus and heated to 7 × 102 K, to desorb residual water and volatile surface adsorbates. To produce clusters, a gas mixture of 2% partial pressure SO2 in He at a stagnation pressure of 10 bar is adiabatically expanded through a pulsed nozzle (nominal diameter 0.5 mm, typical pulse width 300 µs) located 7 cm upstream of the skimmer. The formed cluster beam has a narrow velocity distribution centered around a value of 1.6 km s-1.11 The beam velocity corresponds to a kinetic energy of 0.8 eV/SO2 monomer. Using a retarding field technique,12 the cluster size distribution of the incoming beam has been determined: Following ionization of the beam by electron attachment with 9-eV electrons, the ion current into a Faraday collector is monitored as a function of the increasing height of a potential step established by a set of grids in front of the collector (Figure 2). The measured cumulative beam energy distribution can be converted into the corresponding cumulative cluster size distribution using the kinetic energy per SO2 monomer. In the employed seeded beam expansion, a typical terminal speed ratio, i.e., the ratio between mean flow velocity and most probable thermal velocity, well above 10 is achieved.11 Therefore, we can neglect the influence of the velocity distribution in the cluster beam on the obtained cluster size distribution. (10) Twerenbold, D.; Gerber, D.; Gritti, D.; Gonin, Y.; Netuschill, A.; Roessel, F.; Schenker, D.; Vuilleumier, J.-L. Proteomics 2001, 1, 66-69. (11) Miller, D. R. In Atomic and Molecular Beam Methods; Scoles, G., Ed.; Oxford University Press: New York, 1988; Vol. 1, Chapter 2, pp 14-53. (12) Hagena, O. F.; Obert, W. J. Chem. Phys. 1972, 56, 1793-1802.

h(x) ) exp(- (ln(N) - m)2/(2s2))/(x2πNs)

(1)

I(x) ∝ 1 - erf((ln(N) - (m + 2/3s2))/(x2s))

(2)

size distribution and taking into account an x2/3 dependence of the geometrical attachment cross section, the signal can be fitted by the model function eq 2 giving directly the log-normal distribution parameters m ) 7.0, s ) 0.93 and hence the mean cluster size 〈x〉 ) exp(m + 0.5s2) ) 1.7 × 103 (Figure 2). Compared to a model function based on a pure log-normal distribution, the inclusion of the size-dependent attachment cross section shifts the distribution to smaller sizes, m ) m ˜ - 2/3s2. RESULTS Upon impact of the incoming neutral cluster beam on the target surface, the abundant formation of positive and negative free charge carriers is observed. The left panel of Figure 3 gives the neutralization current into the target for a grid bias voltage of (200 V. The time profile follows closely the time profile of the molecular beam. The total amounts of positive and negative charge carriers are of the same order of magnitude, which indicates an intracluster charge-transfer reaction leading to the formation of geminate ion pairs as the relevant charging mechanism.1,5 Alternative surface ionization processes would generally produce a predominant amount of either positive or negative charge carriers.13 The right panels of Figure 3 show the mass spectra related to these free charge carriers for a bias voltage of (5 V applied to the transfer grid. The spectra are baseline corrected and normalized to unit total charge yield. All spectra show peak progressions typical for (13) Kawano, H.; Page, F. M. Int. J. Mass Spectrom. Ion Processes 1983, 50, 1-33.

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Figure 4. Schematic model for the observed sensitivity of the cluster electric effect to solution-deposited alkali ions. The physical state of the alkali surface adsorbate on a metal is the same for neutral or ionic adsorbing species.

cluster mass spectrometry with peak separations of one SO2 mass unit, indicating a growing solvation shell around an embedded core ion. The absence of water in the solvation shells corresponds to the elevated target temperature: water molecules from the dilute salt solutions employed to pretreat the target have largely evaporated off, leaving behind their ionic solute in the form of surface adsorbates. From the mass spectra in Figure 3, the core ions for the positive charge carriers are readily identified as Li+, Na+, and K+; the anionic progression is generated by a SO2- core ion. The mass spectra do not show negatively charged species based on Cl- and Br- core ions. Since the electron affinity of Cl with 3.61 eV and Br with 3.36 eV exceeds that of SO2 with 1.11 eV, the lack of detected Cl- and Br- cannot be explained in terms of an electron transfer from these anions to the SO2 solvent molecules. The different behavior of alkali cations and halogen anions therefore excludes a direct pickup of solution-deposited core ions from the surface as the mechanism for the observed charging of the cluster fragments. Microscopic Model. The concomitant presence of the alkali cations and of the SO2- anion in the mass spectra indicates that the previously described mechanism based on the pickup of alkali surface adsorbates is responsible for the observed charging phenomenon. Thus, the electron that is stabilized as SO2- anion stems from the spontaneous ionization of the alkali atom inside the polar cluster environment. While it has been shown previously1 that this type of cluster impact charging can be induced by dosing the target surface with neutral alkali atoms from an oven source, the present experiments show that deposition of solvated alkali ions on a metal surface also brings about the effect. We therefore infer that the physical state of those alkali surface adsorbates that induce the CEE bear no memory of whether they have been formed via adsorption of a neutral alkali atom or an alkali ion (see Figure 4). Alkali adsorbates on a variety of surfaces have been studied intensively.7,14-16 Upon adsorption of an alkali atom on a metal surface, the s orbital of the alkali atom broadens and shifts in energy. Concomitantly, the alkali valence electron is partially transferred into the substrate, leaving the alkali atom with a positive partial charge and establishing a surface dipole (Figure 4). The formed alkali adsorbate is known to induce charge separation in cluster surface collisions1. Figure 4 schematically rationalizes that the physical state of the alkali adsorbate can also be reached by depositing alkali ions from solutions. Especially in dilute solutions spread out on a surface in thin films, no recombination between the solvated positive and negative ions (14) Diehl, R. D.; McGrath, R. Surf. Sci. Rep. 1996, 23, 43-171. (15) Madey, T. E.; Yakshinskiy, B. V. J. Geophys. Res. 1998, 103, 5873-5887. (16) Kuchler, M.; Rebentrost, F. Phys. Rev. Lett. 1993, 71, 2662-2665.

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upon evaporation of the solvent is to be expected, since the dominant interaction is that with the surface. Upon evaporation of the solvent, partial neutralization of the remaining cations is reached via resonant charge exchange with the electronic system of the metal surface. The high electron transition rates between the adsorbate and the surface establish the equilibrium occupations for the electronic state independent of their initial filling; i.e., the adsorbate loses the memory of its initial charge state.17 Thus, eventually the same physical state is reached as for surface adsorbates formed via neutral precursors. The absence of halogen core ions in the anion spectra indicates that the observed charging is not due to direct pickup of ionic species from the surface. However, similar to the alkali case, a neutral desorption channel for the electronically equilibrated halogen surface adsorbates is possible. In this case, a subsequent charge-transfer reaction in the polar cluster environment, in the form of an electron transfer from sulfur dioxide to halogen or vice versa, would require much more energy than can be recovered by the solvation of the formed charge pair. Thus, contrary to the alkali atoms, the halogen atoms would remain neutral inside the cluster. Moreover, due to their lower binding energy to the cluster fragments as compared to the polar cluster constituents, they would evaporate out of the impact heated fragments, making it impossible to detect them as part of the solvation shell. Presently, we have no experimental evidence that the halogen surface adsorbates are desorbed at all during the cluster surface collision. Correlation. Table 1 contains the total charge yields for the respective cations and target sides, which have been obtained by integrating the peaks belonging to each progression in the mass spectra. Since the spectral heights of Figure 3 are influenced by the decreasing detection efficiency toward higher masses of our MCP detector, a zero-order correction has been applied.10 Table 1 shows a clear correlation between the amount of observed charge in the cationic progressions and the concentration of the respective core ion in the salt solutions used to pretreat the target surface. While the sodium ion yield and the sodium ion concentration in the solutions stay constant in both experiments, the drop in the lithium ion yield reflects the reduced Li+ concentration in solution B, and the increase in the observed potassium ions is a consequence of the higher K+ concentration in solution B. This correlation is reflected in the changing peak height ratios of the alkali progressions in the respective mass spectra of Figure 3. Assuming a linear dependence Y ) mc between the ion yield Y and the ion concentration c in the solution, a correlation coefficient m is calculated (Table 1). The parameter m sums up the complex transfer function from solvated ions in the solution to mass spectrometrically detected charged cluster fragments. It depends on how the alkali adsorbate coverage on the surface correlates with the ion concentration in the solution, on how efficient the pickup into the cluster is for the various adsorbate species, and on how efficiently the charge separation takes place. It is also affected by the desorption of alkali adsorbates due to the elevated target temperatures15 and by the experimental parameters of cluster size and impact velocity. The value of m exhibits a clear ion specificity and increases in the lyotropic series from lithium to potassium. This trend correlates with the decreasing binding (17) Keller, C. A.; DiRubio, C. A.; Kimmel, G. A.; Cooper, B. H. Phys. Rev. Lett. 1995, 75, 1654-1657.

Figure 5. Comparison between incoming neutral cluster size distribution (gray) and the scattered ion size distribution of anions (SO2)xSO2- (green) and cations (SO2)xK+ (magenta). The scattered distributions have been corrected for the detector efficiency.10 Solid lines give fitted log-normal envelopes.

Figure 6. Peak height distributions for the various core ion progressions observed in the spectra. Spectral heights have been corrected for mass-dependent detector sensitivity, and the distributions have been normalized to unity.

energy of the alkali adsorbate, the decreasing vibrational frequency of the complex, the increasing size and mass of the adsorbate, and the increasing energy transfer in a central collision with a SO2 molecule in the same direction of the series. Based on a scan of this parameter space, a more detailed analysis of the yield dependence should allow one to gain insight into the cluster impact-induced desorption process. Size Distributions. Figure 5 compares the size distributions for the incoming neutral clusters and the observed scattered cluster ions on a log scale. Like the incoming beam, the scattered size distributions are also well represented by a log-normal distribution, e.g., with parameters m ) 1.84, s ) 0.68 for the K+(SO2)x progressions and m ) 2.67, s ) 0.65 for the SO2-(SO2)x progressions obtained from target side B (see also Figure 6). The deviation from the ideal log-normal shape of the potassium progression in general and the deviation toward small solvation shells in both cases can be rationalized in terms of the growing strength of the ion-molecule interaction. The pronounced shift in the mean cluster size from the incoming to the scattered

clusters is a consequence of the collision-induced cluster fragmentation and can be rationalized by comparing the kinetic energy per particle of 0.8 eV in the incoming cluster to the binding energy of a cluster constituent. In a cluster in which a large part of the constituents are at the surface, the latter can be approximated by the standard enthalpy of vaporization ∆Hvap ) 0.26 eV, which is a factor of ∼3 smaller than the kinetic energy per monomer. Even if part of the kinetic energy is transferred into the target surface, extensive fragmentation of the cluster is to be expected. Fragment size distributions induced by cluster surface collisions have been investigated experimentally and by means of molecular dynamics simulations.18-25 For the neutral fragments, generally monotonically decreasing size distributions have been found, which often follow a power law f(x) ∝ x-R.18,26,27 Contrarily, for the charged cluster fragments, the present experiments show a log-normalshaped size envelope with a clear maximum at intermediate fragment sizes. At least partly, this behavior can be explained in terms of the increasing binding energy of the solvent molecules to the core ion toward smaller solvation shells. Thus, the formation of small cluster fragments requires more energy and therefore becomes less probable compared to the situation in a neutral fragment. The factor of more than 2 between the mean size of the solvation shell around the cation and the anion, which equals to 〈x〉 ) 8 and 〈x〉 ) 18, respectively, is tentatively explained in the following way. The cation as massive particle cannot diffuse into the impacting cluster at the picosecond time scale of the collision and therefore is confined to the hottest zone of the cluster close to the collision surface. Contrarily, the electron is highly mobile and can enter deeper into the compressed cluster where there is more phase space and where the cluster matter is colder,18 so that there is a higher probability for it to end up on large fragments. That indeed the difference in mobility between electron and the alkali cations and hence their different position in the fragmenting cluster accounts for the difference in the envelope of the scattered cluster ions is supported by recent experiments on the surface collision of (HNO3)x clusters.5 There, the observed core ions of the positively and negatively charged cluster fragments are NO3- and NO2+, which are formed by the self-ionization reaction of nitric acid in the incoming cluster. Both core ions stem from the volume of the impacting cluster, and accordingly, no difference between the envelope of (HNO3)xNO3- and (HNO3)xNO2+ is observed in the experiments. Magic Numbers. Figure 6 gives peak integrals for the individual progressions in Figure 3. For each core ion, the most intense data set from the two experiments has been chosen. On (18) Tomsic, A.; Schro ¨der, H.; Kompa, K. L.; Gebhardt, C. R. J. Chem. Phys., in press. (19) Hendell, E.; Even, U.; Raz, T.; Levine, R. D. Phys. Rev. Lett. 1995, 75, 26702673. (20) Raz, T.; Even, U.; Levine, R. D. J. Chem. Phys. 1995, 103, 5394-5409. (21) Christen, W.; Even, U.; Raz, T.; Levine, R. D. Int. J. Mass Spectrom. Ion Processes 1998, 174, 35-52. (22) Cleveland, C. L.; Landman, U. Science 1992, 257, 355-361. (23) Petterson, J. B. C.; Markovic´, N. Chem. Phys. Lett. 1993, 201, 421-426. (24) Vach, H.; Benslimane, M.; Chaˆtelet, M.; Martino, A. D.; Prade`re, F. J. Chem. Phys. 1995, 103, 1972-1980. (25) Farizon, B.; Farizon, M.; Gaillard, M. J.; Gobet, F.; Guillermier, C.; Carre´, M.; Buchet, J. B.; Scheier, P.; Ma¨rk, T. D. Eur. Phys. J. D 1999, 5, 5-8. (26) G.-Q-Xu.; Holland, R. J.; Bernasek, S. L.; Tully, J. C. J. Chem. Phys. 1989, 90, 3831-3837. (27) Andersson, P. U.; Tomsic, A.; Andersson, M. B.; Pettersson, J. B. C. Chem. Phys. Lett. 1997, 279, 100-106.

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Table 2. Intensity Irregularities Observed in the Mass Spectra of Alkali Ions Embedded into Sulfur Dioxide Cluster Fragmentsa Li+(SO2)n 4, 6, (12), (14)

n a

Na+(SO2)n

K+(SO2)n

SO2-(SO2)n

6, (8), 12

6, (8)

15, 16

Values in parentheses represent weak irregularities.

top of a log-normal-like envelope there are distinct intensity enhancements for certain solvation shell sizes that indicate an increased stability for the given cluster configuration. These irregularities form during the evaporative cooling of the impact heated cluster fragments.28,29 Table 2 summarizes the magic numbers observed in the spectra. Due to the convolution with the log-normal envelope, some of the structural stabilities especially for small solvent shells might be obscured, so that the list is not necessarily complete. While the structure and the stability of alkali cations embedded in water and ammonia have been studied in detail theoretically and experimentally,30-35 data with respect to sulfur dioxide are scarce.31 Whereas the hydration shell of sodium ions was found to follow a smooth envelope in the range below 17 water molecules,36 in the case of solvation with SO2, pronounced enhanced stabilities are observed for x ) 6 and 12. For the Li+ core ion, the same irregularity at x ) 6 is observed, accompanied by an additional feature at x ) 4. In the potassiumbased progressions, x ) 6 is the only pronounced intensity anomaly. The less structured solvation shell around the potassium ion as compared to the sodium ion is in accordance with similar findings for the solvation by water.34 To correlate the observed magic numbers to structural features of the respective cluster ions, theoretical structure calculations are necessary similar to the case with water as solvent.34,36 The late onset of strong magic numbers in the system SO2-(SO2)x is consistent with previous equilibrium measurements37 that did not observe shell structures in the range 2 e x e 10. The higher stability of the SO2--SO2 ion, which was found to have a partial covalent bond,37 is masked by the overall envelop in the present experiments. Since spectra similar to Figure 6 can be obtained for the class of polar molecule clusters1 and given the ease with which various alkali ions can be deposited (28) Klots, C. E. J. Chem. Phys. 1985, 83, 5854-5860. (29) Echt, O.; Kreisle, D.; Knapp, M.; Recknagel, E. Chem. Phys. Lett. 1984, 108, 401-407. (30) Dzˇidic´, I.; Kebarle, P. J. Phys. Chem. 1970, 74, 1466-1474. (31) Castleman, A. W., Jr.; Peterson, K. I.; Upschulte, B. L.; Schelling, F. J. Int. J. Mass Spectrom. Ion Phys. 1983, 47, 203-206. (32) Keesee, R. G.; Castleman, A. W., Jr. J. Phys. Chem. Ref. Data 1986, 15, 1011-1071. (33) Fuke, K.; Hashimoto, K.; Iwata, S. In Advances in Chemical Physics; Priogine, I., Rice, S. A., Eds., J. Wiley & Sons: New York, 1999; Vol. 110, Chapter 7. (34) Lee, H. M.; Kim, J.; Lee, S.; Mhin, B. J.; Kim, K. S. J. Chem. Phys. 1999, 111, 3995-4004. (35) Schulz, F.; Hartke, B. ChemPhysChem 2002, 3, 98-106. (36) Hartke, B.; Charvat, A.; Reich, M.; Abel, B. J. Chem. Phys. 2002, 116, 35883600. (37) Vacher, J. R.; Jorda, M.; Luc, E. L.; Fitaire, M. Int. J. Mass Spectrom. Ion Processes 1992, 114, 149-162.

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from solution, cluster surface collisions could be employed to investigate the microsolvation of alkali ions in a great variety of solvents. Outlook: Analytics. Given the correlation between the mass spectrometrically observed ion yield and the concentration of the respective ion in the solution used to pretreat the target, the combination of cluster surface collisions with pretreated target surfaces can be employed to analyze the content of alkali ions in electrolytic solutions such as water samples or even blood and urine samples in the medical sector. Quantitative measurements should be possible in combination with an internal standard, e.g., by adding a known concentration of Cs+ ions to the analyte. Based on the observed ion currents of nanoamperes for the millimole per liter sample concentrations even under the present unoptimized conditions, the analysis of micromole per liter concentrations seems feasible. Since the solvent is evaporated off during the target surface preparation, the ultimate detection limit is given by the surface density of the alkali adsorbates on the target and not by their concentration in the liquid sample. With sensitive ion counting methods, the detection of 105 alkali atoms on the cluster beam cross section of roughly 1 cm2 should be achievable, which correlates to a sensitivity in the attomole regime. Even if the sensitivity is reduced by confining the beam for spatially resolved measurements, the present method could be applied to analyze alkali contaminations on semiconductor surfaces. SUMMARY AND CONCLUSION We have shown that the collision of sulfur dioxide clusters with a metal surface that has been pretreated with alkali ioncontaining solutions leads to the formation of positively and negatively charged cluster fragments. Based on the assumption that the physical state of alkali surface adsorbates on metal surfaces is independent of whether the adsorbing particle was in its neutral or ionic state, the results can be explained in terms of the known charging effect via the pickup of alkali surface adsorbates by polar molecule clusters. The correlation between the alkali concentrations in the solution and the observed ion yield in the mass spectra hints at a potential analytical application of this effect to analyze the alkali content of electrolytic solutions via cluster surface collisions. Further work is needed to explain the dependence of the proportionality coefficient m between ion concentration and ion yield on experimental parameters such as cluster size, cluster velocity, and surface properties, as well as to understand potential matrix effects arising from spectator ingredients in the sample solution. ACKNOWLEDGMENT We thank H. Schro¨der and K. L. Kompa for their continuous support. This work was supported by the German BMBF under the EEF program (02EEF137) and by the DFG within the SFB377. Received for review May 15, 2003. Accepted July 14, 2003. AC0345143