A Comparative Study of Secondary Ion Emission from Water Ice under

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A Comparative Study of Secondary Ion Emission from Water Ice under Ion Bombardment by Au+, Au3+, and C60+† Xavier A. Conlan,‡ John S. Fletcher, Nicholas P. Lockyer, and John C. Vickerman* Surface Analysis Research Centre, Manchester Interdisciplinary Biocentre, School of Chemical Engineering and Analytical Sciences, The UniVersity of Manchester, Manchester M1 7DN, U.K. ReceiVed: June 27, 2009; ReVised Manuscript ReceiVed: October 28, 2009

Secondary ion emission from water ice has been studied using Au+, Au3+, and C60+ primary ions. In contrast to the gas phase in which the spectra are dominated by the (H2O)nH+ series of ions, the spectra from ice using all three primary ions are principally composed of two series of cluster ions (H2O)nH+ and (H2O)n+. Dependent on the conditions, the unprotonated series can dominate the spectra. Since in the gas phase (H2O)n+ is unstable with respect to the formation of the protonated ion series, the presence of the solid must provide a means to stabilize their formation. The cluster ion yields under Au+ bombardment are very low and can be understood in terms of sputtering on the borderline between linear cascade and thermal spike behavior. There is a 104 increase in yield across the whole spectrum compared to Au+ when Au3+ and C60+ species are used as primary ions. The character of the spectra differed between these two primary ions, but insights into the mechanism of secondary ion emission for both is discussed within an energy deposition framework provided by the fluid flow-based mesoscale energy deposition footprint (MEDF) model that predicts a cone-shaped zone of activation and emission. C60+ differs from Au3+ in that it delivers its energy closer to the surface, and it is argued this has consequences for the cluster ion distribution and yield. Increasing the ion dose by sputtering suppresses the yield of (H2O)n+ and increases the yield of the protonated ions in the small cluster region, whereas the yield in the large cluster regime is suppressed significantly. The three primary ions show rather different behavior, and this is discussed in the light of the sputtering models. Finally, negative ion spectra including cluster ions have been observed for the first time. C60+ delivers the highest yields, but these are less than 10 times the positive ion yields, probably because the O and OH fragment ions on which the clusters are based are easily neutralized by protons. Introduction With the emergence of metal cluster and polyatomic ion beams such as Aun+, Bin+, SF5+, and C60+, the exploitation of the mass spectral capability of molecular secondary ion mass spectrometry (SIMS) to study biological systems is an increasingly active area of research worldwide. The vacuum requirements of the technique mean that care has to be taken over sample preparation to ensure that the sample is maintained in a state as close to natural as possible. Since all biological systems are composed predominantly of water, the frozen hydrated state would seem to be optimum. To freeze a biological sample and analyze it successfully presents a number of challenges. The freezing process must be very fast to prevent crystallization of the water that may damage the biological structure, and the mass spectral analysis has to cope with the high yield of secondary ions from the water ice itself. It is therefore important that we have a clear understanding of sputtering and secondary ion formation characteristics of water ice under bombardment by the types of primary ion most commonly used. This paper reports on a study of the analysis and effects of prolonged sputtering of ice using Au+, Au3+ and C60+ primary ions. As we shall see, the SIMS spectra from ice consist of an extensive series of water cluster ions. The generation of water †

Part of the “Barbara J. Garrison Festschrift”. * To whom correspondence should be addressed. E-mail: John. [email protected]. ‡ Present address: Institute for Technology Research and Innovation, Deakin University, Geelong, Victoria 3217, Australia.

cluster ions from gas phase water and from ice has been studied over many years. The structure of liquid water is of course dominated by the hydrogen bond. Many studies have revealed much about the structure and properties of water. Recently it has been shown that quite extensive cluster formation occurs within water itself.1–3 All the solid phases of ice involve the water molecules being hydrogen bonded to four neighboring water molecules. In all cases, the two hydrogen atoms are equivalent, with the water molecules retaining their symmetry, and they all obey the “ice” rules: two hydrogen atoms near each oxygen, one hydrogen atom on each O · · · O bond. There is no strong evidence that the H-O-H angle in any ice phase is very different from that in the isolated water molecule. In the studies reported here, it is very likely that the ice is in the metastable ice 1c form. It is formed by condensation of water vapor, at ambient pressure below -80 C. It converts, irreversibly but extremely slowly in the temperature range 170-220 K, to hexagonal ice (ice 1 h) with about 50 J mol-1 heat evolution.4 It consists of a face-centered cubic lattice with half the tetrahedral holes filled. The water molecules have a staggered arrangement of hydrogen bonding with respect to all of their neighbors. All atoms have four tetrahedrally arranged nearest neighbors and 12 second neighbors. A number of studies have modeled the structure of amorphous and cubic ice surfaces.5,6 The models suggest that the surface consists of a quasi-liquid bilayer with surface species that have one of three structures: a four-coordinate water, and three-coordinate waters with either hydrogen or oxygen dangling bonds. The surface is imagined

10.1021/jp906030x  2010 American Chemical Society Published on Web 12/28/2009

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as a nonperiodic outer bilayer of water molecules forming ring structures of a broad size distribution. We are interested in the formation of secondary ions from water ice. However, water molecules ionize within liquid water endothermically due to electric field fluctuations caused by nearby dipole librations resulting from thermal effects, and favorable localized hydrogen bonding. Ions may separate by means of the Grotthuss or proton-hopping mechanism, but normally recombine within a few femtoseconds. The concentrations of H3O+ and OH- are taken as the total concentrations of all the small clusters including these species. Both ions create order and form stronger hydrogen bonds with surrounding water molecules. Other water molecules are required to promote the hydrolysis and the following equation depicts the most important, 4 H2O f H5O2+ + H3O2-. The hydronium ion, H3O+ is the most stable hydrated proton species in liquid water, being slightly more stable than dihydronium (H5O2+). The ion clusters with 2 and 3 shared hydrogens from the H3O+, namely H7O3+ and H9O4+ are also significantly stable and are involved in hydronium ion transfer in water.7 The tetrahedral ion H7O4-, HO- · · · (HOH)3 is probably the most stable hydrated hydroxide ion being slightly favored over H3O2-.8 Studies of water cluster ions in the gas phase have used molecular beams with electron beam and photon induced ionization; laser desorption with photon ionization and sputtering in SIMS and fast atom bombardment (FAB).9–15 From both experimental and theoretical studies, much is known about the structure and energetics of the water clusters (H2O)n-1H+ emitted. There are magic numbers of especially stable ions at n ) 4, 21, and others have been reported at 24, 26, 28, and 30.10,16 In molecular beam studies using electron beam or photon ionization protonated ions are almost exclusively formed.9,10 These ions are thought to be formed by rapid proton transfer:

(H2O)n + hv f [(H2O)n+]* + e- f (H2O)n-1H+ + OH• + eThe precise variation of ion yields with cluster size depends on the conditions used to form the molecular beam. The partial vapor pressure of water and the stagnation conditions play a critical role. The cluster size is determined by the efficiency with which clusters can increase in size and remain stable. At high pressures cluster-water molecule collisions will be high, favoring both increasing water cluster size and collisional cooling. Such conditions favor the observation of large clusters, the yield increasing from n ) 2 to the n ) 10-20 range. At low stagnation pressures and lower partial pressures of water, the yield of large clusters is much reduced, the yield generally decreasing as n increases. However, almost invariably the clusters at n ) 4, 21, and 28 are found to be particularly stable. Both experimental and theoretical studies have provided support for these observations. The lowest energy structures of protonated clusters seem to be linear up to n ) 3; star-shaped (with H3O at the center) for n ) 4; then cyclic structures begin to take over, with pentamers becoming the favored base structure.17 After n ) 4, the first hydration shell is complete, and there is a sharp drop of 30 to 40 kJ/mol in the binding energy of water molecules in the cluster.16,18–20 The n ) 21 structure has attracted a great deal of study. It is believed to be have a clathrate structure consisting of a symmetrical cage with 12 fivemembered rings formed by 20 water molecules on its surface with one additional molecule trapped inside. This structure seems to be some 8-12 kJ/mol more stable than the structures

on either side, due to the fact that it has 30 saturated hydrogen bonds, compared to 28 for (H2O)22.9,21 However, there are suggestions that the proton is on the surface of the cage structure.22 (H2O)n+ ions are not observed generally in gas phase studies. Ab initio calculations have shown that the dimer cation changes its geometry relative to the neutral dimer and is unstable relative to the fragmentation channel to H3O+ + OH.23 (H2O)n+ ions can be observed in the gas phase if there is a third body in the molecular beam that can help remove the excess energy.24 Thus near-threshold photoionization at 11.83 eV on supersonic beams of water-argon mixtures have produced (H2O)n+ ions by the mechanism (Ar)m(H2O)n + hv f (H2O)+ n + m(Ar) + e

These experiments delivered protonated ions alongside the unprotonated ions. The yield of unprotonated ions increases with stagnation pressure. At 5 atm (with saturated vapor pressure of water at 25 C), the yield of the two ion series was similar as a function of cluster size, although the maximum yield was at n ) 3 for the unprotonated and n ) 4 for the protonated. Electrospray studies of water also exclusively generate H+(H2O)n clusters. Here the maximum cluster yield was for n ) 4, followed by a minimum at n ) 9; the yield intensities then rose again reaching a maximum at n ) 21, with a small local maximum at n ) 28; thereafter the intensities fell slowly to n ) 201.25 Negative ion OH- clusters, OH-(H2O)n, are observed in the gas phase, although they have been studied a good deal less than the protonated clusters. Yang and Castleman showed that X-(H2O)n clusters (where X ) OH, O, O2, and O3) could be produced in a high pressure source by discharge ionization of a mixture of He and water.26 SIMS studies of ice surfaces have been reported using He+ and Ar+ primary ions. The yield of positive cluster ions was studied. Using a quadrupole analyzer and ion beam currents in the region of 10 nA cm-2, Rabelais et al. observed that there were three predominant ion cluster series: (H2O)nH+, (H2O)n+, and (H2O)n-1OH+.11 In their case, the most intense component was the (H2O)nH+ series, but the other two are clearly present. In contrast, the spectra observed by Donsig and Vickerman using pulsed 10 keV Ar+ and a time-of-flight (ToF)-SIMS analyzer, reported that only the (H2O)nH+, (H2O)n+ series were observed.12 This study also investigated the effect of exposing the ice to HCl vapor. This resulted in a doubling of the (H2O)nH+ yields relative to (H2O)n+ yields. A similar result was obtained in a FAB MS study of acidified water.13 The Winograd group studied the sputtering of ice in some detail using the same primary ions as those used in this paper. The main purpose of this work was not to study the cluster ion distributions, rather in very careful work they measured the relative sputter yields of Au+, Au2+. Au3+, and C60+ and showed very significant increase through this series from 94 molecules per Au+ impact to almost 2000 per C60+ impact.14 In a study of a frozen aqueous solution of histamine supported on silver, a cluster distribution is observed under C60+ bombardment.15 Again both protonated and nonprotonated cluster ions are observed, although the protonated cluster ions appear to predominate possibly because of the presence of the solute histamine. Experimental Section Materials. Seven microliter droplets of HPLC grade water, pH 7.0 (BDH, U.K.), were placed on silicon wafers, and a

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second silicon wafer was placed on top of the water droplet forming a water layer between the wafers. The silicon, sourced from Advent, was washed and ultrasonicated twice in ethanol before use. The lower silicon wafer was secured on a sample stub by two screws. The sample stub was placed in a bath of liquid nitrogen. After 3 s, the water layer was frozen, the top wafer was rapidly removed leaving an ice film. This freeze-fracture approach was used to minimize the possibility of contamination during freezing and to expose a virgin ice surface immediately before insertion into the vacuum system. The ice films were about 0.5 mm thick. The sample stub was rapidly inserted into the SIMS instrument and placed onto the sample stage, which had been precooled to a temperature of about 100 K. ToF-SIMS Analysis. All experiments were carried out in the Bio-ToF SIMS instrument described elsewhere.27 The instrument is equipped two Wien filtered primary ion beam systems, one based on a C60+ ion source, and the other using a gold/germanium liquid metal ion source.28,29 The depth profile studies of ice films were carried out using each of the three ion primary beams C60+, Au+, and Au3+ at 15 keV impact energy. The depth profiles were performed using a computer-controlled automated sequence of etching followed by spectral acquisition cycles. The primary ion current was measured into a screw hole on the sample stub. Au+ etch used a DC beam current of 8.78 nA into an etch area of 1500 × 1500 µm; the Au3+ sputtering was carried out with 0.75 nA DC beam current into an etch area of 800 × 800 µm and the C60+ etch used 0.97 nA DC beam current into an etch area of 1000 × 1000 µm. All the analysis spectra we obtained from an area of 400 × 400 µm in the center of the respective etch areas, a 40 ns pulsed ion beam was used for all spectral acquisition. The total ion doses for the entire profile sequences were ∼5-6 × 1014 ions cm-2. Each spectrum was acquired after an etch dose of ∼3 × 1013 ions/ cm2. Spectra were obtained using a total ion dose of 5 × 1010 ions cm-2 for Au+ and Au3+ analysis and a dose of 2.5 × 1010 ions cm-2 for C60+ analysis. For most of the studies, the same ion beam was used for analysis as was used for sputter etching. However, two series of mixed beam studies were carried out. In one series, C60+ was used to sputter etch followed by analysis using Au3+, in the second series the reverse arrangement was used. For these dual beam experiments alignment of the two ion guns was confirmed using secondary electron imaging and SIMS imaging of a copper finder grid. In all cases, charge compensation was carried out using a low energy electron gun (25 eV) with the electron dose for the experiment not exceeding the Gilmore limit of 6 × 1018 electrons/m2 to avoid electroninduced damage.30 Results This study compares the positive and negative ion spectra obtained using 15 keV Au+, Au3+, and C60+ as analytical primary beams on the fractured virgin ice surface and then the spectra obtained after sputtering the ice with each of these ions with a dose of up to 5 × 1014 ions cm-2. Figure 1a displays the positive ion spectrum obtained from the virgin surface using the Au+ ion beam. It can be seen that there are sequences of ions of composition (H2O)n+, (H2O)nH+, and (H2O)n-1OH with n ) 1 to >50. The very weak peaks in the region of m/z ) 30 and 40 are observed in some samples and not in others. They are thought to arise from condensation of small amounts of contaminants from the residual gases in the analysis chamber. The negative ion spectrum obtained from the virgin surface is shown in Figure 1b. The principal ions are O- and OH- with sequences of cluster ions (H2O)nO- and (H2O)nOH-. The

Conlan et al. negative ion series are very much less intense than the corresponding positive ion series. The Au3+ and C60+ also generate similar series of both positive and negative ions, although the absolute and relative intensities differ quite markedly. We will now report the detailed results for each of the primary ions. 1. Analysis and Sputtering of Ice with Au+ Primary Ions. There are similarities between the positive ion spectrum in Figure 1a and those observed in earlier SIMS studies; however, there are distinct differences.11 In the case using pulsed 15 keV Au+ ions, we observed three series of cluster ions. The (H2O)n+ series is the most intense, and (H2O)nH+ and (H2O)n-1OH+ are of similar intensity. As mentioned above, Rabelais et al. observed the same three ion cluster series, even though their conditions must have been well above the static limit. In their case, the most intense component by far was the (H2O)nH+ series, but the other two are clearly present. In contrast, Donsig and Vickerman using pulsed 10 keV Ar+ and a ToF-SIMS analyzer, and reported spectra and an ion cluster distribution very similar to that observed here using Au+ except that only the (H2O)nH+, and (H2O)n+ series were observed.12 The variation of yield with cluster size is particularly interesting. In Figure 2 the yields of (H2O)n+ and (H2O)nH+ are plotted up to n ) 40 (m/z ) 720/721). In contrast with all gas phase data, the nonprotonated ion series (H2O)n+ always shows a ∼3 times higher yield than the protonated series (H2O)nH+. The mechanism of gas phase ion formation suggests that the nonprotonated ions are unstable with respect to protonated ions. In the gas phase, the nonprotonated ions only became stable in the presence of third-body argon.24 Clearly the formation process here must be different, perhaps excess energy is lost either to the surface or in collisions in the emission zone during the process of formation and emission. The variation of yield with n also displays similarities and differences to the gas phase data. There is a relatively high yield at n ) 1, this is rarely seen in the gas phase. There are then local maxima around n ) 3 to 4 and n ) 21. As outlined in the Introduction, it is well-known that a cluster of four water molecules is especially stable, so this maximum is not surprising.7 There is also much data in the literature regarding the special stability of the n ) 21 cluster.9,21 A clathrate structure has been proposed, but it has always been discussed with respect to the protonated species. Here we observe a maximum in both series, but it is the (H2O)21+ that shows the greater yield. Turning to the negative ion spectrum in Figure 1b, although there has been a good deal of interest in the formation of water cluster ions based on -OH, there are no reported studies of the negative secondary ion spectra from ice. This may be because under atomic ion bombardment, the most intense ions by far are O- and OH-, with little else apparent in the spectrum. The spectrum obtained using Au+ shows a strong yield of O- and OH-. The cluster ions, however, are ∼103 weaker than the corresponding positive ions. The ion series are based on OHand O-, the former being the stronger. There seems to be some evidence that there is a maximum for (H2O)3OH-, but the yields are very small. The ice films were then sputtered, and the spectra were monitored as a function of the primary ion fluence up to 5.5 × 1014 ions cm-2. Both sputtering and analysis were carried out using the Au+ primary ion. Figure 3 shows the positive ion spectrum of the ice after 3 × 1014 ions cm-2. The negative ion spectrum only consists of O- and OH-. While the n ) 1 positive ions show the highest yield, there have been a number of



Figure 1. (a) Positive ion spectrum of water ice using Au+ primary ions, before sputtering. (b) Negative ion spectrum of water ice using Au+ primary ions, before sputtering.

changes consequent on the sputtering. The most notable is the fact that the protonated ion clusters now show the highest yield for all clusters other than n ) 21 (see Figure 2). The yield of the protonated series after sputtering is higher than either of the series before sputtering in the range n ) 2-18, but beyond that the yield is lower. There is also an overall fall in the yield of the n ) 1 ions; however, the local maximum at n ) 4 is more marked for the protonated series and has almost been lost

in the nonprotonated series. The other major change is that the local maximum in (H2O)21+ is very much reduced and not evident at all in (H2O)21H+. The change in cluster ion yield develops with ion fluence. The variation of the ratio (H2O)nH+/ (H2O)n+ for n ) 1, 3, 4, and 10 as a function of primary ion dose density are shown in Figure 4. It can be seen that, within the sputtering range studied, the relative yield of (H2O)nH+ is still increasing, and, while the n ) 1 ions are the most intense,

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Figure 2. Variation of ion cluster yields (H2O)n+ (squares) and (H2O)nH+ (triangles) as a function of n before (filled symbols) and after sputtering ice with 3 × 1014 cm-2 Au+ primary ions (open symbols).

Figure 3. Positive ion spectrum of ice after sputtering by 3 × 1014 Au+ cm-2.

the degree of protonation increases with cluster size but seems to plateau around n ) 10. 2. Analysis and Sputtering of Ice with Au3+ Primary Ions. The general form of the spectrum produced by Au3+ is similar to that shown in Figure 1a, but there are a number of important differences in the absolute and relative yields of the ions. The (H2O)n+ series and (H2O)nH+ show comparable yield, whereas (H2O)n-1OH+ is much lower. Figure 5 shows the variation in yield as a function of n for the two principal series of ions (H2O)n+ and (H2O)nH+. It is immediately obvious that the yields are dramatically higher, on the order of 10-4, compared to 10-8 with Au+. This mirrors the nonlinear yield increase observed elsewhere when gold clusters are used as primary ions.31,29 Second, the cluster variation in both positive and negative ions with n is rather different. The highest yield is at n ) 3, not as with Au+ at n ) 1. Overall, the

two series of cluster ions show very similar yields up to n ) 15; however, beyond n ) 15, although the yields are low, the (H2O)n+ series yield is approximately twice that of the (H2O)nH+, and at n ) 21 it is about 3 times. As with Au+, there is a very sharp maximum at the nonprotonated ion, (H2O)21+. The negative ion spectrum generated by Au3+ shows a larger proportion of cluster ions relative to the yield of O- and OH(not shown). However, the yield of negative clusters is at least 10 times less than the positive ions. Again there is a local yield maximum for the cluster with three water molecules around the OH-. The ice films were then sputtered with the Au3+ beam, and the spectra were monitored as a function of the primary ion fluence up to 6.5 × 1014 ions cm-2. Both sputtering and analysis were carried out using the Au3+ primary ion. The negative ion



Figure 4. Plot of intensity ratio (H2O)nH+/(H2O)n+ for n ) 1, 3, 4, 10 as a function of primary ion dose density.

Figure 6. Plot of intensity ratio (H2O)nH+/(H2O)n+ for n ) 1, 3, 4, 10 as a function of primary ion dose density under Au3+ sputtering.

spectrum again consisted of little more than the O- and OHions and a few very weak cluster ions. The positive ion yield as a function of n is also plotted for the sputtered surface in Figure 5. The yield of protonated ions has increased very markedly in the region below n ) 10 and is much higher than the (H2O)n+ series with a high maximum in the n ) 3-4 region. However, although most of the yield in (H2O)21+ has been lost, in contrast to the data for Au+, beyond n ) 20 the (H2O)n+ series shows a higher yield than the protonated series, which also falls below the yields of both protonated and nonprotonated series observed before sputtering. The change in cluster ion yield develops with ion fluence. The variation of the ratio (H2O)nH+/ (H2O)n+ for n ) 1, 3, 4, and 10 as a function of primary ion dose density are shown in Figure 6. In contrast to the Au+ data, the ratios reach a steady state after ∼2 × 1014 Au3+ impacts cm-2. For the ions reported in Figure 6, the maximum ratios of (H2O)nH+ to (H2O)n+ are also around 3 times higher than those observed under Au+ sputtering, and the highest relative yield of (H2O)nH+ is at n ) 3 and 4. 3. Analysis and Sputtering of Ice with C60+ Primary Ions. C60+ provides yet a further increase in ion yield (× 5-10) compared to Au3+. The main ion series observed are again

(H2O)n+ and (H2O)nH+ and they are of similar yield until n ) 20; thereafter the (H2O)n+ series shows 1.5-2 times higher yield than (H2O)nH+ (see Figure 7). The ion distribution with n is very different below n ) 9 compared to both Au+ and Au3+. The n ) 1 ions are the most intense, and there is no local maximum in the region of n ) 4. However, there is a strong local maximum again at n ) 21 for (H2O)n+. In contrast to the two gold primary ions, the negative ion spectrum shows a well-developed set of cluster ions that are visible in the n ) 40 region. The fragment ion (O-, OH-) yields are of the same order as the clusters. The negative ion cluster yields are only about a factor of 10 less than the positive ions. It is interesting that the fragment ions appear at m/z 16, O-; 17, OH-; 18, OH2-; 19, H3O-. The latter is unusual and, although contamination by fluorine is not suspected, it cannot be ruled out. Whether the negative charge resides on oxygen or on a hydride ion is a matter of conjecture. Whichever, the fragments seem to be the nucleus for a series of cluster ions. The highest yield series is (H2O)nOH-, but (H2O)nO- and (H2O)nOH3- are of the similar yield. In this case, the n ) 2 cluster shows the local maximum.

Figure 5. Variation of ion cluster yields (H2O)n+ (squares) and (H2O)nH+ (triangles) as a function of n before (filled symbols) and after sputtering ice with 3 × 1014 cm-2 Au3+ primary ions (open symbols).

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Figure 7. Variation of ion cluster yields (H2O)n+ (squares) and (H2O)nH+ (triangles) as a function of n before (filled symbols) and after sputtering ice with 3 × 1014 cm-2 C60+ primary ions (open symbols).

Figure 8. Plot of intensity ratio (H2O)nH+/(H2O)n+ for n ) 1, 3, 4, 10 as a function of primary ion dose density under C60+ sputtering.

The ice films were then sputtered with the C60+ beam, and the spectra were monitored as a function of the primary ion fluence up to 5 × 1014 ions cm-2. Both sputtering and analysis were carried out using the C60+ primary ion. The positive ion yield for the sputtered surface after a dose of 3 × 1014 ions cm-2 is also plotted as a function of n in Figure 7. As in the case of Au3+ below n ) 15, sputtering reduced the yield of the (H2O)n+ series significantly, whereas the protonated series (H2O)nH+ increased somewhat, although, in relative terms, not as much as with Au3+. Beyond n ) 15, the yield of both series fell by more than a factor of 2 relative to the virgin ice surface. The (H2O)21+ species lost most of its intensity, but beyond n ) 20 the unprotonated series again shows the greatest yield. After sputtering, there is still a detectable yield of negative ion clusters. Sputtering with C60+ results in an almost instantaneous and dramatic change in the ratio of protonated to nonprotonated ions below n ∼10 (see Figure 8). Whereas steady state was never attained by Au+ sputtering within the fluence range used, with Au3+ it was reached after ∼2 × 1014 impacts cm-2, and with C60+ it is attained in ∼3 × 1013 impacts cm-2. This may be merely a consequence of a much larger sputter yield or a

dramatically higher density of protons in the emission zone. We will return to a consideration of the differing processes occurring under bombardment of the three ions later. To provide further insights into this issue, it is instructive to investigate whether analysis with a different primary ion from that used for sputtering results in different spectra. In other words, if the ice surface is sputtered with C60+, does a Au3+ primary ion “see” a different surface from that generated by Au3+ sputtering? Similarly, if the surface is sputtered by Au3+, does C60+ see a different surface from that produced by C60+ sputtering? Broadly, the spectrum observed using Au3+ as the analysis beam even after sputtering by C60+ closely corresponds to that generated by Au3+ for both sputtering and analysis. There is a pronounced maximum yield at n ) 3. Where C60+ is used for analysis after Au3+ sputtering, while the H3O+ ion shows the highest yield as was observed when C60+ was used for analysis and sputtering, there is a clear inflection at n ) 3 in the yield as a function of n, suggesting some influence of the Au3+ sputtering mechanism on the observed yield. Discussion The spectral data reported above suggests that the three primary ions “see” the surface of the ice differently and affect the surface differently. The secondary ions that are observed will be influenced in a complex manner by the structure of the ice, the mechanism of particle emission specific to the primary ion considered, the mechanism of ionization, the internal energy and stability of the secondary ion cluster emitted, and the interactions that the emerging secondary ion may have with the surface molecules or gas phase molecules as it leaves the surface and travels through the analyzer to the detector. We first compare the observations with what is known about the gas phase spectra from water vapor. Comparison with Gas Phase Spectra. Perhaps the most striking observation from the present data is the high yields of (H2O)n+. Conventional 70 eV electron ionization of gas phase water in a mass spectrometer generates H2O+ and OH+. However, almost invariably the mass spectra of water vapor consists of exclusively (H2O)nH+ ions whether produced by electron, photon, field ionization, or Penning ionization. A

Secondary Ion Emission from Water Ice number of very weak (H2O)n+ are sometimes observed. As outlined in the Introduction, the mechanism of ion formation in the gas phase is believed to be by the decomposition of an unstable (H2O)n+ to form (H2O)n-1H+ + OH · . (H2O)n+ are only observed if there are third bodies in the gas phase to take away the excess energy.24 The SIMS analysis of virgin ice by all three primary ions results in an (H2O)n+ series that shows the highest yield. In addition to the (H2O)n-1H+ series, there is also in some cases a (H2O)n-1OH+ series. There are a number of possible mechanistic variants consequent upon the presence of the surface during cluster emission. The clusters observed may be those that already exist, or are preformed on the surface; they may grow by collisions with water molecules in the surface region, or in the gas phase above the surface as they emerge. The fact that (H2O)n+ is unstable when formed in the gas phase suggests that these clusters must be formed within the surface. They must lose energy to the solid and are thereby stabilized. The low intensity shoulders on the low mass sides of the peaks in the larger clusters (see, e.g., n ) 10 in Figure 1) suggests that metastable decay of energetic clusters plays a significant role in the cluster ion formation. Such decays involve the loss of water molecules from the cluster and have been observed in the gas phase to be slow, occurring on microscale time-scales.32 The mechanism by which the protonated clusters are formed will also be influenced by the surface and will not be restricted to the fragmentation of unstable (H2O)n+. Other workers have mainly observed that the protonated cluster ions predominate. The pH of the water will be a factor, and Donsig and Vickerman showed that exposing an ice film to acid vapor dramatically increased the relative yield of the (H2O)n-1H+ series.12 The presence of a solute may well have an influence. A cluster series dominated by protonated cluster ions is reported by the Winograd group from a frozen solution containing histamine.15 The OH+ related clusters are unlikely to be formed by fragmentation of (H2O)n+. The sputtering process will generate water fragments: charged H and OH ions around which the water molecules are likely to cluster to form the emitted ions. Thus ionization of water clusters could proceed by proton or OH+ attachment. As will be seen later, negative ion clusters are formed based on OH-, O-, and even H2O-. It is likely that these too are formed by the attachment of a negative fragment ion to a cluster or vice versa. Sputtering Models. With the advent of metal cluster and polyatomic ion beams, there has been a great deal of interest in the mechanism of sputtering with these ions. The metal cluster ions deliver significantly higher yields than their corresponding monatomic beams. Polyatomic ion beams not only deliver higher yields, for many molecular materials, they generate much lower bombardment-induced chemical damage such that, in some cases, the static limit can be lifted in molecular SIMS analysis. Two basic approaches have been used to try to understand and describe the sputtering mechanisms. In reviewing the literature on monatomic and cluster ion beams, Seah has shown that sputtering by monatomic primary ions is very well described by Sigmund’s linear cascade model, whereas the much higher yields generated by cluster and polyatomic beams are well described by Sigmund and Claussen’s thermal spike model, which reflects the higher energy density deposited in the impact region by these larger metal cluster and polyatomic ions.33 Wucher has shown that sputtering of indium by monatomic gold displays transitional behavior between linear cascade and thermal spike character, whereas the gold cluster ions show clear thermal spike behavior.34 On this basis, the mechanism of


Figure 9. Schematic of the fluid dynamics track model. There is an implied cylindrical symmetry around the left edge of the figures. (a) Incident MeV particle and excitation track of radius Rcyl. (b) Arrows showing the general initial direction of motion of the particles. (c) Schematic showing a conical volume of ejected material (in red). Reproduced with permission of the American Chemical Society from Russo, M. F., Jr.; Garrison, B. J. Anal. Chem., 2006, 78, 7206-7210.

sputtering of ice reported here by all three primary ions might be expected to reflect thermal spike behavior. The sputtering by Au+ may be on the boarderline between linear cascade and thermal spike. The other popular approach is to apply molecular dynamics (MD) simulations to seek to understand the processes from a molecular level. Simulations of organic molecular models have shown that sputtering by an energetic atomic primary species is predominantly as a consequence of a cascade of collisions initiated within the solid, some of which reach the surface and result in the emission of atomic and small molecular species. The emission of larger molecular species occurs as a consequence of relatively rare cooperative events where two or more cascades emerge at the surface together.35 Water ice has been a popular model system for MD simulations comparing the effects of Au3+ and C60+ sputtering, and Garrison and coworkers have shown that a different approach is required because the impacts of these multiatomic particles give rise to so much motion in the surface and subsurface region. The activity has the character of a thermal spike, and the MD simulations become computationally too demanding. A new model was developed that combined an initial phase of MD simulations that modeled the initial impact of the primary particle with the substrate, with the subsequent events having the character of fluid flow off the surface as a consequence of an energetic impact.36 The formalism of the model was derived from the work of Jakas et al. in which the ejection of material due to MeV particle bombardment originates from an energetically excited region whose behavior is described by fluid mechanics.37 It is known as the mesoscale energy deposition footprint (MEDF) model. In Figure 9 the yellow track illustrates the excitation region of radius Rcyl along the path of the impacting projectile. In this region, substrate material is quickly displaced and energized. Due to the rapid expansion, material that is less than or equal to a depth of Rcyl from the surface is able to travel in the direction of the surface and escape. Anything deeper than Rcyl will not escape, but may expand radially due to collisions with other bulk species. After initial impact and subsequent excitation, the excitation diffuses outward such that the boundary of the energized track and surface becomes blurred. Material that is in the near-surface region after some time can be energized and has the chance to escape. This diffusion of energy leads to a second radius, Rs, from which additional material can be removed. Rs is connected to Rcyl by the relation Rs ) E˜Rcyl where E˜ is the average excitation energy within the track relative to the cohesive energy of the substrate material. E˜ and Rcyl are estimated from the MD simulations carried out for the first 250 fs of the bombardment event. It should be emphasized that the principal aim behind the development of this model was to try

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Figure 10. Snapshots of the reaction zones created by 15 keV C60 and Au3 at 0.5 ps. The red triangle outlines the conical ejection region as identified by the MEDF model and implicitly includes the blue and yellow regions. The blue region represents the energized track created by projectile bombardment, and the yellow region is indicative of the reaction region created. Reproduced with permission of the American Chemical Society, from Ryan, K. E.; Wojciechowski, I. A.; Garrison, B. J. J. Phys. Chem. C, 2007, 111, 12822-12826.

to predict the relative sputtering yields from ice under Au3 and C60 bombardment. In this it was successful, but using the conceptual energy deposition framework of this model we can also derive insights into the molecular interactions arising from Au3 and C60 bombardment of ice.38 Figure 10 shows the energized tracks for both C60 and Au3. The impact of these projectiles results in their fragmentation. It can be seen that, because the energy per gold atom is so much greater than the energy per carbon atom, the Au3 energy is deposited deep into the substrate such that much of the energy is ineffective in generating sputtering, although it will generate subsurface chemical damage. C60 on the other hand deposits most of its energy close to the surface.39 Figure 10 also shows how the impact energy is partitioned in the particle emission zones of the ice substrate. The blue and yellow regions are where 90% of the energy of the incident particle is deposited. For C60, this has a radius of 2.5 nm, and the conical emission volume has a base radius of 6.2 nm and a depth of 2.5 nm. Most of the higher energy activity is in the blue and yellow regions. Simulations suggest that these regions are where fragmentation and reactions between molecules can occur. In the case of C60, a high proportion of the molecules may react, and there is certainly a high density of hydrogen atoms generated in this region. It has been shown that the pink outer regions are where molecules may be emitted at a later stage and with significantly lower internal energy.40 It can be seen in Figure 10 that the volumes of both the blue the pink regions are significantly smaller for the Au3 case, and much of the reaction is deeper in the solid. While the MEDF and the thermal spike models approach the issue of sputtering from somewhat different directions, in terms of predicting sputter yields, they reach similar conclusions; however, the MD/MEDF approach provides insight into how the energy deposited during primary particle impact is partitioned such that two emission zones can be identified. These zones display rather different molecular reactivity that should be relevant to understanding the cluster yield variations observed under Au+, Au3+, and C60+ bombardment. The rest of the discussion will seek to apply these insights to the discussion of the results. Analysis and Sputtering Using Au+. Although the mass and energy of the atomic primary ion, Au+, used here are different from the early SIMS data, the mechanism of emission may not be too qualitatively different. The high energy Au+ ions are expected to deposit their energy deep in the ice, initiating collision cascades. The detected emitted ions will predominantly result from particles emitted some distance from the impact point as a result of some of these cascades returning to the surface.

Conlan et al. Substantial subsurface disruption would be expected, but at this stage before any sputter etching of the surface, the emitted secondary ions should in some way reflect the chemical structure of the ice surface. We note that the n ) 1 ions are the most intense, but then there is a local maximum at n ) 4. While the local surface structure that consists of four coordinate water molecules and three coordinate molecules with hydrogen and oxygen dangling bonds may well play an important role in the ion structures formed, the mechanism by which the ionized clusters are formed, and their gas phase stability will be crucial in determining the ions detected. The stability of ion clusters has been studied in some depth. The cluster dissociation energies drop from around 120 kJ/mol at n ) 2 to ∼75-88 kJ/mol for n ) 3 and 4 followed by a fall to 40 kJ/mol at n ) 5.16,18–20 The stability up to n ) 4 is explained by the fact that a complete solvation shell of three water molecules surrounds the central ion. The dissociation energies for n > 5 all lie below ∼32 kJ/ mol, apart from the rise at n ) 21. If gas phase stability is a crucial parameter, the yield variations should follow the stability curve. In qualitative terms they do. As is observed, the sputtering mechanism for atomic Au+ would be expected to yield a high yield of single n ) 1 species. MD simulations have shown that the emission of large cluster ions requires the correlated motion of a number of cascades such that a number of soft impacts contribute to the “lift-off” of the molecule.41 Seah and Wucher have suggested that sputtering with Au+ may result in a sufficient density of collision cascades to produce thermal spike behavior that can give rise to the emission of the larger clusters.33,34 However, the yield of larger clusters is very small, more than 104 lower than observed with Au3+ and C60+, so the mechanism must be on the borderline of thermal spike and linear cascade. However, the yield distribution as a function of n does seem to reflect the cluster stability variation, so the internal energy of the emitted ions may not be unusually high. After prolonged sputtering with Au+, the overall secondary ion yields are a little lower, but the most noticeable change is that the (H2O)nH+ series shows significantly higher yield in the range n ) 3-16 and the maximum at n ∼ 4 is maintained. Beyond n ) 16, the (H2O)nH+ is marginally more intense than the (H2O)n+ but the n ) 21 species is not present in (H2O)nH+ and is all but lost in (H2O)n+. This suggests that Au+ sputtering must dissociate H2O in the surface region to generate protons that serve as nucleation points to form (H2O)nH+ that enhances yield for n < 16, and the damage generated by the atomic sputtering reduces the already low yield of larger clusters. Analysis and Sputtering with Au3+. Au3+ delivers a greater than 104 increase ion secondary ion yield compared to Au+. Before sputtering, the two water cluster ion series are of comparable yield with a local maximum in the n ) 3 region, below n∼ 10, (H2O)nH+ is somewhat higher. Above n ∼ 15 (H2O)n+ is higher with a strong n ) 21 maximum. This is in line with expectations. The cluster primary ion has been observed in other studies to give rise to a nonlinear increase in yield.42,28,31 As discussed above, because of the lower energy per component atom (5 keV/Au), the cluster ion deposits its energy closer to the surface and is more efficient in sputtering larger secondary ion species whose yield has increased by 104. Using Au3+ the highest yield cluster is n ) 3. Again, the yield distribution may reflect the predominant local surface bilayer structure of 3 and 4 coordinate water molecules and/or the higher stability of these cluster ions. It is interesting that the yield of these clusters is even higher than the single water ions. This suggests that the sputter mechanism either does not favor the release of single water ions, or the internal energy imparted to

Secondary Ion Emission from Water Ice the emitted clusters is sufficiently low that these clusters are less likely to decompose to generate H3O+. It is clear that the mechanism of sputter emission is different from atomic Au+. The ion distribution does not follow the ion stability pattern observed for the gas phase, although it is similar to the distribution observed below n ) 9 in electrospray MS.25 The electrospray mechanism involves the formation of ions by evaporation from large clusters or droplets. Thermal spike behavior is to be expected, and the high cluster yields reflect this; however, the MEDF model outlined above is based on fluid flow, and this would also suggest that the impact of cluster and polyatomic primary ions may favor the emission of large clusters. As the model suggests, the Au3 cluster deposits its energy along a track several nanometers wide and activates an approximately cone-shaped volume (mini-droplet) at the surface/ bulk interface. The model suggests that a crater several nanometers deep is activated in the ice surface, there is emission of clusters in a quasi-fluid effect, and large clusters flow off the sample perhaps breaking up as they leave.36 In the central region (blue region in Figure 10), it has been argued that molecular dissociation will occur, and small protonated species may be formed.39,40 The higher yields of the unprotonated large clusters suggest a significant peripheral emission region (the pink areas in Figure 10), where disruption is lower and the proton concentration is low. The two series (H2O)n+ and (H2O)nH+ are the main ions observed, so stabilization of the (H2O)n+ ions must be occurring; however, metastable decay processes are again suggested by the shoulders at the low mass sides of the peaks in the higher clusters. After prolonged Au3+ sputtering, the (H2O)nH+ yields are much higher for n ) 1-15 than any of the yields before sputtering. Concurrently, the (H2O)n+ yields are dramatically reduced. There is a high maximum of (H2O)nH+ around n ) 3-4. Beyond n ) 15, the protonated yields fall below the yield of (H2O)n+ observed before sputtering and beyond n ) 20 below the yields of (H2O)n+ after sputtering, and the local maximum at n ) 21 is greatly reduced. Analysis of the reaction dynamics in the central impact region of the MEDF model for Au3 suggests that sputtering generates a good deal of molecular fragmentation and reaction, which results in a great number of free hydrogens, together with some subsurface disruption as the high energy Au atoms penetrate deeper in the solid.39 Subsequent hits will result in a surface that is significantly disrupted and the density of fragments and protons would be expected to be higher than in a virgin surface. This disruption may account for the steep fall in the (H2O)n+ yield, although the increased density of protons must contribute to the observed large increase in protonated cluster yield for n < 15. However, the fall in yield beyond n ) 20 by a factor of 2-3 suggests that the concentration of sites that would give rise to the larger clusters is reduced, which implies that the sputtering results in some longer range damage to the crystal structure. Analysis and Sputtering with C60+. C60+ provides a further 3-5 times increase in ion yield from the virgin surface compared to Au3+. The main ion series observed are again (H2O)n+ and (H2O)nH+. However, the ion distribution with n is very different below n ) 9 compared to both Au+ and Au3+. In the C60+ case, the energy per atom is much less (250 eV/C). All the energy of the 60 impacting carbon atoms will be deposited very close to the surface. The sputter yield is very high, close to 2000 molecules per impact, compared to around 800 for Au3+.14,38 This is reflected in the higher yield of ions for n < 4 and the 2-3 times increase for clusters from n ) 21 to 40. As we observed for the case of Au3+, the yields and cluster

J. Phys. Chem. C, Vol. 114, No. 12, 2010 5477 distribution support the idea that the mechanisms of emission as a result of C60+ sputtering have a fluid flow character. The MEDF model based on this idea suggests that an area of about 6 nm is affected by the 15 keV C60 impact, and all the emitted particles come from within 5 nm of the surface. The reaction dynamics analysis suggests a dense area of activity among water molecules in the crater region.39 The protonation of clusters is likely to involve protons formed during the C60 impact. Figure 7 shows that the protonated ions predominate up to n ) 15. However, beyond n ) 20, the unprotonated ions are the majority species. The (H2O)nH+ may be expected to arise from the blue and yellow higher energy zones of the crater in the MEDF model (Figure 10), while the larger yield of n > 20 unprotonated clusters may arise from the lower energy pink region of the crater, which, according to the analysis referred to above, will have a far lower density of protons. Because most of the ion energy is deposited close to the surface, this region is significantly larger in area and volume than in the Au3+ case, hence the higher yield of large clusters can be rationalized. It is significant that the cluster distribution shows a clear discontinuity at n ) 21. There is some evidence of this intensity discontinuity in the Au+ data (at 104 lower yield), although it is quite obvious from Au3+ (see Figure 5). There is now a good deal of gas phase data to suggest that this large cluster is stable relative to its neighbors by several kilojoules per mole because of its closed-shell clathrate structure based on a dodecahedron of 20 water molecules surrounding a H3O+ ion or a H2O molecule with a mobile proton on the surface.9,21,16 However, in contrast to the gas phase data, it is the unprotonated (H2O)21+ cluster that shows the greatest yield in these studies. It seems to be clear that the stabilization of particular clusters occurs after ionization by a slow metastable decay process that results in the evaporation of water molecules to favor the emergence of the n ) 21 species. Careful examination of the intensities in this region shows that the n ) 22 ion is significantly depleted relative to n ) 21. In gas phase studies, the dissociation rate of this cluster has been measured to be more than twice that of the n ) 21 ion.32 However, despite the similarities with the water vapor behavior, the fact that there is a significant yield of (H2O)21+ confirms that the proximity of the solid ice surface must play a role in stabilizing these species. After sputtering with 3 × 1014 C60+ cm-2, an increase in the protonated yield for n < 10 was observed, but the increase was relatively small compared to that observed after a similar ion dose of Au3+. However Figure 7 shows that, as in the case of Au3+, there was a very considerable decrease in the yield of (H2O)n+. Comparison of the increase in the (H2O)nH+/(H2O)n+ ratio as a function of ion dose for Au3+ and C60+ plotted for small clusters in Figures 6 and 8 shows that they reached steady state by about 3 × 1013 ions cm-2 under C60 bombardment, whereas 3 × 1014 cm-2 was required for Au3+. This suggests that the density of action in the C60 impact region is greater, and, although the larger sputter dose reduces the (H2O)n+ yield possibly due to fragmentation, the (H2O)nH+ may have saturated. Beyond n ) 10, the post sputtering yields of both ion series fall significantly below the presputtering yields. Additionally, beyond n ) 20 the (H2O)n+ yield predominates as it does from the virgin surface. Although the yield of these cluster ions has fallen by a factor of 3-4 as a consequence of sputtering, the yield is at least twice that using Au3+. The prolonged sputtering has clearly disrupted the surface and, as a consequence, the probability of emission of large clusters; however, the fact that these clusters are still evident suggests some of the structure of the surface is preserved.

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A comment is in order on the experiments in which prolonged sputtering was carried out with either Au3+ or C60+ and analysis was performed with the other ion. Where Au3+ was the analytical ion, the spectra were broadly similar to those obtained in the Au3+ study. This implies that C60+ does not change the surface seen by Au3+. In other words, the spectra are dominated by the sputtering characteristics of the analytical ion. However, where C60+ was the analytical ion after Au3+ sputtering, something of the Au3+ spectral character was evident. As in the C60+ study, H3O+ was the most intense ion; however, there was a distinct intensity inflection at n ) 3 in the (H2O)nH+ series, which suggests some favoring of these small clusters relative to the C60+ only case. The other contrast is that the yield of large cluster ions beyond n ) 27 was close to zero. This data suggests that Au3+ sputtering changes the ice surface so that it is different from that observed by C60+ after C60+ sputtering. These two observations suggest that if Au3+ is involved, whether as a sputtering ion or as an analytical ion, it will have an influence on the observed spectrum in a way that C60+ does not. This may be rationalized if it is remembered that Au3+ deposits its energy deeper than C60+ and the emission zone is cooler. Taken together, these two effects on the one hand may favor the formation of small stable clusters in the emission crater that can be sampled by a C60+ analytical ion; on the other hand, deeper structural disruption may be caused such that there are fewer large clusters for C60+ to sample in the surface region. Negative Ion Spectra. Although there has been a good deal of interest in the formation of water cluster ions based on OH-, there are no detailed studies of the negative secondary ion spectra from ice. This may be because, under atomic ion bombardment, the most intense ions by far are O- and OHwith little else apparent in the spectrum. The spectrum shown in Figure 1b and those referred to subsequently demonstrate that negative cluster ions can be generated. However, the yield is far less than the positive ions discussed above. Because of their high proton affinity, significant loss of OH-based negative ions will result from the energetically favorable recombination with the many very mobile protons that will inhabit this system.43,44 While a mechanism for formation of the positive ion clusters may be via the dissociation of ionized metastable (H2O)n+ clusters, this does not involve the formation of negative ions, rather the release of an electron. The formation of clusters based on negative ions probably requires the formation of OH-, O-, and HO2- by high-energy fragmentation of water molecules in the sputtering process. These species will provide “nucleation” sites for water molecules. However, despite the fact that this will help to stabilize and shield the charge, the vast majority of the small negative ions are probably lost by proton capture. Studies show that the first three water molecules are most strongly bound to the three lone pairs around the OH oxygen. A cluster stability variation as a function of n similar to the positive ion clusters is then observed. The spectrum obtained using Au+ shows a strong yield of O and OH-. The cluster ions, however, are ∼103 weaker than the corresponding positive ions. The ion series are based on OH- and O-, the former being the stronger. There seems to be some evidence that there is a maximum for (H2O)3OH- but the yields are very small. The spectrum generated by Au3+ shows a rather larger proportion of cluster ions to the yield of O- and OH-. Clearly, if the generation of these small ions is required in order to form the nucleus of the clusters, the absolute yield is not determinative for the yield of clusters. The ability of the primary ion to lift

Conlan et al. out clusters must also play a role. We know that Au3+ is more effective in this regard. However, in this case, the yield of negative clusters is at least 10 times less than the positive ions. It is probably not possible to draw any real conclusions about the mechanism. However again there is a local yield maximum for the cluster with three water molecules around the OH-8. The spectrum observed using C60+ provides the highest cluster ion yield. The highest yield series is (H2O)nOH-, but (H2O)nOand (H2O)nOH3- are of the similar yield. The contrast with the Au3+ observation is the higher yield of larger clusters. Perhaps the arguments above based around the conceptual framework of MEDF model also provide some insights here. In the negative ion series, there are no ions equivalent to the unprotonated (H2O)n+ series, so to form ions requires the formation of fragments for water molecules to cluster around. These fragments are most likely formed in the blue and yellow zones of the model crater. Clearly the density of fragments must be larger than with Au3+, but will fall rapidly in the pink region where large water clusters are emitted; hence it is perhaps understandable that the overall yield of large negative clusters is 10 times less than in positive ion spectra. Conclusions 1. Ion formation as a consequence of sputtering water ice is different from ion formation in the gas phase. In contrast to the gas phase in which the spectra are dominated by the (H2O)nH+ series of ions, spectra are principally composed of two series of cluster ions (H2O)nH+ and (H2O)n+. Dependent on the conditions, the unprotonated series can dominate the spectra. Since in the gas phase (H2O)n+ is unstable with respect to the formation of the protonated ion series, the presence of the solid must provide a means to stabilize their formation. 2. Although a series of water cluster ions are observed as a consequence of Au+ sputtering, the yield is 104 lower than with Au3+ and C60+. Because the yields of these ions that depend on cooperative sputtering events in the emission zone are very low, it is thought that the mechanism of sputtering of ice using Au+ probably lies at the borderline between linear cascade and thermal spike sputtering. Prolonged sputtering results in surface disruption, an increase in the (H2O)nH+ yield, and a decrease in the low yield of large clusters. 3. Analysis of water ice using the Au3+ cluster and the polyatomic C60+ species resulted in a 104 increase in yield across the whole spectrum compared to Au+. The high yields can be qualitatively understood using a thermal spike mechanism or the MEDF model. The character of the cluster ion spectra differed between the two primary ions. It is argued that insights into the possible reasons for this difference can be obtained by considering the energy deposited by the two ions in the emission zone suggested by the MEDF model. Because C60+ deposits its energy closer to the surface, the central hot region is thought to be denser and probably of higher energy, but shallower than that for Au3+; however, the cooler peripheral region of the emission cone is larger for C60+. It is suggested that from the central zone C60+ delivers a high yield of small cluster ions, dominated by the n ) 1 ion, whereas, because for Au3+ this zone is perhaps somewhat cooler and deeper, this may explain why the small cluster ion yield is dominated by the n ) 3 to 4 clusters. The cooler peripheral emission zone is larger for C60+, and this

Secondary Ion Emission from Water Ice would provide some rationalization for the significantly higher yield of (H2O)n+ where n > 20 compared to Au3+. 4. Increasing the ion dose by sputtering suppresses the yield of (H2O)n+ and increases the yield of the protonated ions in the n < 10 region, whereas the yield in the large cluster regime is suppressed significantly. This is in line with the effect of multiple impacts generating many zones of high reactivity. High densities of surface protons will be generated enhancing the protonated ion yield, but the general longer range structure will be damaged. While the effect of C60+ is greater in enhancing the low mass cluster yield in the surface region, Au3+ appears to have an effect deeper in the solid and may generate deeper chemical damage. 5. Negative ion spectra including cluster ions have been observed for the first time. C60+ delivers the highest yields, but these are less than 10 times the positive ion yields, probably because the O- and OH- fragment ions on which the clusters are based are easily neutralized by protons. Acknowledgment. The financial support of the UK Engineering and Physical Sciences Research Council under its Life Sciences Initiative is gratefully acknowledged. References and Notes (1) Bartha, F.; Kapuy, O.; Kozmutza, C.; Van Alsenoy, C. J. Mol. Struct. (THEOCHEM) 2003, 666-667, 117–122. (2) Chumaevskii, N. A.; Rodnikova, M. N. J. Mol. Liq. 2003, 106, 167–177. (3) Chaplin, M. F. Biophys. Chem. 2000, 83, 211–221. (4) Starr, F. W.; Angell, C. A.; Stanley, H. E. Physica A 2003, 323, 51–66. (5) Materer, N.; Starke, U.; Barbieri, A.; Van Hove, M.; Somorjan, A.; Kroes, G. J.; Minot, C. J. Phys. Chem. 1995, 99, 6267–6269. (6) Van Hove, M. J. Phys. Chem. B 2004, 108, 14265–14269. (7) Zavitsas, A. A. J. Phys. Chem. B 2001, 105, 7805–7815. (8) Asthagiri, D.; Pratt, L. R.; Kress, J. D.; Gomez, M. A. Chem. Phys. Lett. 2003, 380, 530–535. (9) Hermann, V.; Kay, B. D.; Castleman, A. W. Chem. Phys. 1992, 72, 185. (10) Radi, P. P.; Beaud, P.; Franzke, D.; Frey, H. M.; Gerber, T.; Mishcler, B.; Tzannis, A.-P. J. Chem. Phys. 1999, 111, 512. (11) Lancaster, G. M.; Honda, F.; Fukuda, Y.; Rabelais, J. W. J. Am. Chem. Soc. 1979, 101, 1951. (12) Donsig, H. A.; Vickerman, J. C. J. Chem. Soc. Faraday Trans. 1997, 93, 2755. (13) Boryak, O. A.; Kosevich, M. V.; Stepanov, I. O.; Shelkovsky, V. S. Int. J. Mass Spectrom. 1999, 189, L1.

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A Comparative Study of Secondary Ion Emission from Water Ice under

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