Collisions of Silylium Cations with Hydroxyl-Terminated and Other Self

Oct 26, 2000 - Nathan Wade, Chris Evans, Federico Pepi,† and R. Graham Cooks* ... energy (20-90 eV) collisions with hydroxyl-terminated (HO-SAM), ...
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J. Phys. Chem. B 2000, 104, 11230-11237

Collisions of Silylium Cations with Hydroxyl-Terminated and Other Self-Assembled Monolayer Surfaces: Reactions, Dissociation, and Surface Characterization Nathan Wade, Chris Evans, Federico Pepi,† and R. Graham Cooks* Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907 ReceiVed: July 5, 2000; In Final Form: September 14, 2000

Silylium cations, SiCl3+ and Si(CH3)3+, undergo dissociative ion/surface reactions in the course of lowenergy (20-90 eV) collisions with hydroxyl-terminated (HO-SAM), hydrocarbon (H-SAM), and fluorocarbon (F-SAM) self-assembled monolayer surfaces. Formation of the substitution product, SiCl2F+, upon collision of SiCl3+ with the F-SAM surface is the result of a transhalogenation reaction. In an analogous fashion, one observes substitution of a chlorine in the SiCl3+ projectile ion by either an OH group from the HO-SAM surface or a CH3 group from the H-SAM surface to form the scattered reaction products, SiCl2OH+ and SiCl2CH3+, respectively. The concomitant transfer of a Cl atom from the projectile ion into the surface is indicated by the sputtered ion, CH2Cl+. The scattered product SiCl(OH)2+ involves disubstitution, and reaction with more than one chain at the surface. These and related reactions involve the activation of C-O, C-F, C-C, C-H, and O-H bonds at the appropriate surface, and they occur after, or in concert with, surfaceinduced dissociation of the polyatomic projectile. Surface effects on the dissociation of projectile ions are studied using the Si(C2H5)4•+ ion, and threshold values for translational to internal energy (T w V) conversion for this ion are measured as 13%, 13%, and 20% for the H-SAM, HO-SAM, and F-SAM surfaces, respectively. At higher collision energies, (>40 eV), the HO-SAM surface demonstrated greater internal energy conversion efficiency than the H-SAM surface. The process of neutralization and the accompanying release of chemically sputtered ions also served to distinguish the three surfaces. Decreased neutralization at the F-SAM surface is associated with increased amounts of dissociatively and reactively scattered product ions. Thermodynamic estimates regarding charge exchange between the surface and the projectile ion are consistent with the relative amounts of chemically sputtered products observed for each of the surfaces.

Introduction A variety of processes may occur when low-energy (1-200 eV) polyatomic ions collide with a stationary target. They include surface-induced dissociation1-4 (SID), charge exchange,5 chemical reaction,6-10 chemical sputtering,11 electronic excitation,12 and ion soft-landing.13,14 Inelastic collisions which lead to dissociation of the target ion, viz. SID, have received considerable attention among the low-energy ion/surface collision processes. This experiment serves as an attractive alternative to collision-induced dissociation (CID),15 the gas-phase procedure for characterizing ion structures and connectivities, because it provides large internal energies to the primary ion with relatively narrow distributions. The value of SID in the structural analysis of complex molecules, such as peptides, is increasingly being realized.16-18 Ion/surface reactive collisions, in which atoms or groups of atoms are exchanged between the surface and the projectile ion, are evident from the observation of scattered ions into which have been incorporated atoms or groups of atoms derived from the surface,19 or by the independent detection of ions representative of the products of the complementary surface chemical modification.20 These chemical reactions are performed by novel methods and have potential applications in selective surface * To whom correspondence should be addressed. E-mail: cooks@ purdue.edu † On leave from Dipartimento di Studi di Chimica e Technologia delle Sostanze Biologicamente Attive, University of Rome “La Sapienza”, 00185 Rome, Italy.

transformations. Important questions surrounding these interfacial reactions include their dependence on thermodynamics, reaction dynamics and mechanisms, excited-state effects, and the role of electron transfer. Systematic variation in the nature of the projectile ion and surface is widening the scope of this field.21 In a few cases molecular details of these reactions have been elucidated through the study of analogous ion/molecule reactions.22 In many of these studies, valuable information on the chemical composition at the outermost layers of the surface adsorbatessbefore or after ion beam treatmentsis provided by in-situ chemical sputtering. In this process, charge exchange occurs between the surface and an incoming projectile ion, and the ionized surface species then dissociates and releases a representative ion into the gas-phase.11 This study focuses on the interactions of silylium cations with various organic monolayer surfaces, particularly the hydroxylterminated self-assembled monolayer (HO-SAM) surface. Novel ion/surface reactions are sought and examples involving C-O, C-C, C-H, and O-H bond dissociations are encountered. Further, the scattered ion reaction products, as well as sputtered ions derived directly from the surface, are found to be characteristic of the HO-SAM structure and differentiate it from other organic SAM surfaces. Surface-induced dissociation (SID) is also examined to see how changes in surface functionality affect translational to internal energy conversion. The energetics of each of these processes is studied and the dependence on thermochemistry and possible mechanisms are discussed.

10.1021/jp002405b CCC: $19.00 © 2000 American Chemical Society Published on Web 10/26/2000

Collisions of Silylium Cations with SAMs The HO-SAM surface has been examined in several contexts and has been considered as a possible template for the construction of molecular films for electronics and sensor applications.23 Here, it is examined principally as a substrate that participates in low-energy ion/surface reactive collisions. Self-assembled monolayer surfaces have proven to be useful for the study of low-energy ion/surface collisions, and hydrocarbon (H-SAM)24 and fluorocarbon (F-SAM)25 surfaces in particular have been utilized. Of the two, the F-SAM surface is the more rigid surface, and it produces more fragmentation of projectile ions. In addition, the higher work function of the F-SAM surface results in less charge exchange. The reactive functional group of the HO-SAM surface differentiates it from either of the previously studied surfaces. Activation of the C-O bond occurs in the course of both ion/surface reactions and chemical sputtering. Carbon-oxygen bonds are ubiquitous in nature and cleavage of this bond (∼91 kcal/mol for ethanol)26 is important in chemistry. This importance is perhaps most evident in the reactions of alcohols at transition metal surfaces which have been investigated in detail due to their role in partial oxidation and Fischer-Tropsch catalysis.27,28 Oxygenated species are also incorporated into many polymeric surfaces and contribute to the performance of these materials in terms of adhesion and wettability.29 The amount and type of these functionalities on the surface is controlled through such means as oxidative addition reactions,30 photolysis,31 or plasma treatment.32 The silicon-hydroxyl bond in trialkylsilanols is interesting from both a theoretical and practical viewpoint. Practically, intermolecular dehydration of the related monoalkyl and dialkyl hydroxysilanes is important as a route to silicone polymers.33 Formation of Si-O bonds has also been utilized for the synthesis of nanostructures on wafers,34 and this chemistry has been used to graft functional groups to silica supports for chromatographic applications.35 The trimethylsilyl cation is of particular interest as an analogue of the methyl cation and for its possible isolation in the condensed phase.36 In the gas-phase, investigations of analogous association reactions37,38 involving Si(CH3)3+ have been conducted, including reactions with alcohols.39 Experimental Section Alkanethiolates were bonded to a gold film through the sulfur linkage and allowed to self-assemble for use as collision surfaces in these experiments. The three surfaces examined are HO(CH2)11S-Au (HO-SAM), CH3(CH2)11S-Au (H-SAM), and CF3(CF2)7(CH2)2S-Au (F-SAM). Substrates were prepared by thermal evaporation of chromium then gold onto silicon wafers (International Wafer Service). The molecular assemblies were constructed by immersing the substrates in dilute (1 mM) ethanol solutions of either 11-mercaptoundecanol, dodecanethiol, or the disulfide (CF3(CF2)7(CH2)2S)2 for a period of at least 1 week at room temperature. Detailed information concerning the preparation and properties of the surfaces has been provided elsewhere.40 The surfaces were rinsed in ethanol and dried under argon before being introduced into a high-vacuum scattering chamber. All experiments were performed in a custom-built, hybrid mass spectrometer with geometry BEEQ (B ) magnetic sector, E ) electric sector, Q ) quadrupole mass analyzer), a detailed description of which has been provided.41 Ions of interest were generated by electron ionization (70 eV) of gaseous samples of tetrachlorosilane, tetraethylsilane, and tetramethylsilane (Aldrich, Milwaukee, WI), independently introduced into the ion source (10-5 Torr nominal sample pressure). The resulting ions were

J. Phys. Chem. B, Vol. 104, No. 47, 2000 11231 accelerated to 2 keV translational energy, and mass and energy selected, respectively, by the magnetic and electrostatic analyzers of a double focusing mass spectrometer. Projectile ions of interest were decelerated to low translational energies, then allowed to collide with the surface in a UHV scattering chamber maintained at a nominal base pressure of 2 × 10-9 Torr (typical operating pressures were below 5 × 10-9 Torr). For the ion/surface scattering experiments, the sample was rotated so that the primary ion beam was incident at 55° to the normal, while scattered ions were collected over a wide range of angles centered about the specular angle. Collection angles were not varied but ion trajectory simulations show that under the conditions used, the analysis system accepts a wide range of scattering angles, approximately 30° on either side of the specular angle. The angles selected for the ion/surface scattering experiments were representative values chosen to allow experiments to be compared with earlier data. The kinetic energy analyzer was set in a low-resolution mode so as to pass ions of a broad range of energies in the range of a few electronvolts. These conditions are known to efficiently transfer the products of inelastic, reactive and chemical sputtering processes.41 The selected conditions give maximum scattered ion transmission without significant angular or velocity discrimination, although such experiments have been performed earlier,41 using the same instrument. Scattered ions were analyzed using a quadrupole mass analyzer preceded by an electrostatic analyzer, used as a kinetic energy to charge filter. Results and Discussion Energy Dependence of Collision Processes Observed with SiCl3+. Figure 1 shows energy resolved mass spectra (ERMS) for 20-90 eV collisions occurring between SiCl3+ and each of three surfaces, namely the H-SAM, HO-SAM, and F-SAM surfaces. These plots summarize the extent to which the main ion/surface interaction processes occur as a function of collision energy. For the F-SAM surface, it is evident that dissociation plays a significant role even at collision energies as low as 30 eV. It is observed that SID product ions constitute approximately half of the total ion abundance in a mass spectrum collected at this energy. By comparison, SID for both the H-SAM and HO-SAM surfaces is a minor process over much of the collision energy range. The enhanced ability of the F-SAM surface relative to the H-SAM surface to cause conversion of translational energy of a projectile ion into internal energy, thus promoting fragmentation, is well-known.42,43 It has been surmised that the greater effective mass of the terminal group (CF3 versus CH3) along with the order and rigidity of the monolayer is the reason for the increased efficiency. The behavior of the HO-SAM and H-SAM surfaces contrasts with that of the F-SAM surface in another respect. Not illustrated in the ERMS plots (Figure 1) are the absolute intensities of the total scattered ions produced upon collision with each of the surfaces. At each of the collision energies, approximately five times as many ions scatter from the F-SAM surface as the HO-SAM and H-SAM surfaces. Given the wide angular and velocity acceptance conditions of the analysis system, this effect is reasonably ascribed to differences in neutralization efficiency at the two surfaces and not to differences in the velocities of the secondary ions. The ratio of ion yields from the two surfaces is in agreement with results43 of Wysocki et al., who demonstrated scattered ion efficiency values of 60-70% for fluorinated SAM surfaces and 3-15% for hydrocarbon SAM surfaces. It has been proposed that the primary pathway for ion neutralization involves electron transfer,

11232 J. Phys. Chem. B, Vol. 104, No. 47, 2000

Wade et al. H-SAM, and CF3+ (m/z 69) from the F-SAM. These products are formed in processes having heats of reaction that can be evaluated, at least relatiVe to each other, by considering the corresponding gas-phase enthalpies of a simple model system (eqs 1-3) where the data are taken from standard sources46

C3H7OH + SiCl3+ f CH3CH2• + SiCl3• + CH2OH+ ∆Hrxn ) 2.84 eV (1) C4H10 + SiCl3+ f CH3CH2• + SiCl3• + CH2CH3+ ∆Hrxn ) 3.39 eV (2) C3F8 + SiCl3+ f CF3CF2• + SiCl3• + CF3+ ∆Hrxn ) 5.39 eV (3) The predicted relative enthalpies of charge exchange between SiCl3+ and each of the surfaces correlate inversely with the relative amounts of chemically sputtered products observed for each of these systems. These results suggest a dependence of charge exchange (the first step in chemical sputtering of surface entities) on the difference in ionization energies of the surface absorbate and the projectile ion. A further observation, made for the HO-SAM surface, is that chemically sputtered products are more abundant in the case of collisions involving the SiCl 3+ ion than those involving the ion, Si(CH3)3+. This result is consistent with the thermochemistry of charge exchange as shown by a comparison of eq 1 with eq 4. Figure 1. Energy-resolved mass spectra showing the scattered ions produced upon collision of SiCl3+ at a H-SAM, a HO-SAM, and a F-SAM surface over a range of collision energies, 20-90 eV. Product ions are categorized as elastic scattering ([), surface-induced dissociation (9), chemical sputtering (2), and ion/surface reactions (b).

and that the thermochemistry of the event can be related to differences in the ionization energies of the projectile ion and the collision target.44 Thus, the decreased amounts of neutralization observed from projectile ions at the fluorinated surface are attributed to the higher work function of this surface (ionization energy (IE) ∼ 13.5 eV for a typical fluorocarbon, IE ∼ 11 eV for the corresponding hydrocarbon). Not surprisingly, the degree of neutralization is not solely dependent on the differences in ionization energy of the projectile and surface molecule. For example,45 a higher yield of scattered ions is observed when Si(CH3)3+ ions are scattered from a Ni(111) surface containing adsorbed C2Cl4 units than a Ni(111) surface containing adsorbed C3F6 units, whereas the ionization energy of C3F6 is more than 1 eV greater than that of C2Cl4. It is evident from the ERMS plots that chemical sputtering as a result of collisions with SiCl3+ varies with the choice of the surface as follows, F-SAM , H-SAM < HO-SAM. For the HO-SAM and H-SAM surfaces, the disappearance of elastically scattered ions is accompanied by the simultaneous increase in chemically sputtered products. At collision energies greater than 60 eV the abundance of chemically sputtered products is greater for the HO-SAM than the H-SAM surface. For the F-SAM surface, elastically scattered products decline quickly with increasing collision energy, however this decrease is associated with increases in SID and ion/surface reaction products, not chemical sputtering. We infer that neutralization is unfavorable at this surface. From the mass spectra, one can deduce that the primary products of chemical sputtering from each surface are CH2OH+ (m/z 31) from the HO-SAM, CH2CH3+ (m/z 29) from the

C3H7OH + Si(CH3)3+ f CH3CH2• + Si(CH3)3• + CH2OH+

∆Hrxn ) 4.86 eV (4)

Figure 2 displays the scattered ion mass spectra generated from the SiCl3+ ion impinging on the HO-SAM surface at a series of collision energies, 50-70 eV. Below 30 eV, SiCl3+ scatters elastically from the HO-SAM surface. At higher energies the major fragmentation pathway of SiCl3+ is the loss of two chlorine atoms giving rise to the ion SiCl+ (m/z 63). The atomic ion Si•+ is not observed at any of the examined collision energies, and, especially at the higher energies, formation of the odd-electron SiCl2•+ (m/z 98) species is a minor product. Ion/surface reaction products, e.g., SiCl2H+ and SiCl2OH+, which will be discussed later, are also present in Figure 2. The threshold energy for these products is about the same as for the SID products, i.e., 30 eV. Their abundances increase with collision energy until about 70 eV, then charge exchange processes begin to dominate. There is no evidence in any of the systems examined of multiply charged ions, which would require a highly endoergic process. Further, almost all ions appearing in mass spectra taken using the HO-SAM surface are even-electron ions. Similar results are observed for the H-SAM surface, but this situation is in contrast to what is observed for collisions at the F-SAM surface. In Figure 3, which shows 60 eV collisions of SiCl3+ with the F-SAM surface, SiCl2•+ is one of the more abundant ions and ion/surface reaction products include SiClF•+ and SiF2•+. Chemically sputtered ions can be used to identify the chemical composition of the sample surface, and the appearance of the ions, H3O+ (m/z 19), CH2OH+ (m/z 31), and C2H4OH+ (m/z 45) are clear indications of the presence of the terminal OH group. The onset of these chemically sputtered ions is about 40 eV collision energy, and their abundances increase with collision energy. Other chemically sputtered ions have m/z values of 15, 27, 29, 39, 41, 43, 55, 57, and 69, and presumably they are

Collisions of Silylium Cations with SAMs

Figure 2. Scattered ion mass spectra recorded upon collision of SiCl3+ with a HO-SAM surface at an energy of (a) 50 eV, (b) 60 eV, and (c) 70 eV.

Figure 3. Scattered ion mass spectra recorded upon collision of SiCl3+ with a F-SAM surface at a collision energy of 60 eV.

CnH2n+1+ and CnH2n-1+ ions indicative of fragments from the OH-terminated alkane structure and/or from hydrocarbon molecules which are contaminants at the surface. Hydrocarbons arising from low levels of vacuum pump oil present in the scattering chamber typically are observed from metal surfaces. Contamination is also observed on the F-SAM surface (Figure 3), however to a far smaller extent. The cleanliness of the F-SAM surface has often been noted6 as a desirable property for its use in ion/surface collision experiments. Larger amounts of hydrocarbon contamination were observed on the HO-SAM surface. Figure 4 displays scattered ion mass spectra for SiCl3+ collisions on the H-SAM surface at 50-70 eV. Comparisons with Figure 2 demonstrate clearly the differences between ion interactions at the H-SAM and HO-SAM surfaces. Chemically sputtered ions at m/z 19, 31, and 45 are not present in the H-SAM case, while ions at m/z 15, 27, 29, 41, etc., which represent fragments from the alkane monolayer, are now

J. Phys. Chem. B, Vol. 104, No. 47, 2000 11233

Figure 4. Scattered ion mass spectra recorded upon collision of SiCl3+ with a H-SAM surface at an energy of (a) 50 eV, (b) 60 eV, and (c) 70 eV.

dominant features of the spectra. It is probable that contaminant hydrocarbon exists on this monolayer as well, however spectra recorded from this surface were highly reproducible. The greater degree of neutralization of projectile ions at the HO-SAM surface compared to the H-SAM surface is best illustrated by comparing the 70 eV collision data shown in Figures 2 and 4. For the HO-SAM surface, the abundance of the scattered projectile ion and dissociated ions relative to the observed chemically sputtered products is much smaller than for experiments involving the H-SAM surface at the same collision energy. It also appears that more internal energy is deposited into this projectile ion at the HO-SAM surface than the H-SAM surface at a collision energy of 60 eV. The ratio of the intact ion to fragment ion SiCl3+/SiCl+ is 0.7 for the H-SAM surface and 2.5 for the HO-SAM surface. SID of Si(C2H5)4•+. The radical cation, Si(C2H5)4•+, has been studied previously by several methods of activation and has proven useful as a thermometer ion for the characterization of the distribution of internal energy in an activated ion population.47 The majority of fragments arising from this ion occur via a single series of reactions (loss of ethyl radical followed by sequential ethylene eliminations), for which the activation energies are known. Figure 5 represents the products of collisions of Si(C2H5)4•+ with the H-SAM, HO-SAM, and F-SAM surfaces at an energy of 20 eV. More extensive fragmentation is observed for the F-SAM surface, as demonstrated by the greater abundance of the lower mass, higher activation energy, ions. For example, Si(C2H5)H2+ (m/z 59) is observed upon collision of the molecular ion with the F-SAM surface but not with either of the other two surfaces. Comparison between the HO-SAM and H-SAM surfaces shows minimal differences. Onset energies for the m/z 59 fragment were determined by plotting the abundance of that ion, as observed

11234 J. Phys. Chem. B, Vol. 104, No. 47, 2000

Figure 5. Scattered ion mass spectra recorded upon collision of Si(C2H5)4•+ at (a) a H-SAM surface, (b) a HO-SAM surface, and (c) a F-SAM surface at a collision energy of 20 eV.

from the mass spectra recorded upon collisions of the molecular ion at various collision energies. Extrapolating that data to zero abundance yielded threshold values of 27, 26, and 17 eV for dissociation at the H-SAM, HO-SAM, and F-SAM surfaces, respectively. The activation energy, eo, for formation of the Si(C2H5)H2+ ion has been estimated at 3.5 eV,47 and this translates into T w V conversion values at threshold of 13%, 13%, and 20%, respectively. These data agree with those of Morris et al.,48 who reported a T w V conversion factor of 19% for the F-SAM surface, and a value of 12% for hydrocarbon covered surfaces. (Small corrections for the pre-collision internal energy of the projectile ion are not made but would lower the values slightly.) Below collision energies of 40 eV, the HO-SAM and H-SAM surfaces were barely distinguishable in terms of T w V conversion ability. At energies 40-90 eV, however, the differences are greater, with larger amounts of internal energy conversion for ions incident on the HO-SAM surface. Figure 6 displays scattered ion mass spectra of Si(C2H5)4•+ colliding with the two surfaces at 80 eV. Measuring accurate internal energy distributions for the Si(C2H5)4•+ ion at higher collision energies is more difficult since the ion produces many low mass fragment ions which are the result of competitive and consecutive fragmentation processes with varying entropy requirements. However, it is evident that more internal energy is transferred upon collision with the HO-SAM surface due to the greater relative abundances of the ions, Si(C2H5)+ (m/z 57), SiH3+ (m/z 31), and SiH+ (m/z 29). Suggested reasons49 for the increased internal energy transfer when comparing different surfaces include greater momentum transfer as a result of the larger effective mass of the surface group and greater rigidity of the surface possibly caused by repulsive forces between neighboring groups on a single chain. Two new peaks, at m/z 44 and 47, are observed in the spectrum for the HO-SAM surface (Figure

Wade et al.

Figure 6. Scattered ion mass spectra recorded upon collision of Si(C2H5)4•+ at (a) a H-SAM surface, (b) a HO-SAM surface.

6b). These fragment ions, apparently SiO•+ and Si(OH)H2+, are the result of ion/surface reactions and will be discussed later. Reactive Collisions of SiCl3+. Low-energy ion/surface reactions are sensitive to the outermost atomic layers of a surface. Wysocki et al. demonstrated that ion/surface reactions were constrained, in at least one experiment,50 solely to the first atomic layer of an isotopically labeled surface. Furthermore, ion/surface collisions have been used to distinguish isomeric surfaces, e.g., p-chlorobenzenethiol from m-chlorobenzenethiol adsorbates,51 as well as surfaces adsorbates which differ in their geometric orientation.52 The sensitive nature of these collisions is again evident in the reactions of SiCl3+ with the HO-SAM surface. Though this surface differs from the H-SAM surface only in the nature of the terminal functional group, reaction products from each of these surfaces are quite different as will now be shown. Activation of the surface C-O bond is demonstrated by the scattered ion/surface reaction product, SiCl2OH+ (Figure 2). Formation of this ion, formally the result of a substitution of chlorine by hydroxyl at the cation center, is observed at collision energies below 40 eV. Its abundance is greatest at around a collision energy of 60 eV. Pick-up of an OH functionality has been observed earlier in the course of reactions of Cs+ ions with adsorbed water on Ni(100) surfaces53 but this is the first instance of a substitution reaction involving hydroxyl. Though some water is believed to be present on the HO-SAM surface, due to the appearance of a peak at m/z 18 at some collision energies, the participation of the OH-terminated alkane is evident in large abundances of chemically sputtered ions. Similar reactions, attempted at other surfaces which contained similar amounts of water contamination, resulted in no OH pick-up, establishing that the SiCl3+ reaction does involve the HO-SAM structure. The Cl-for-OH group transfer reaction, is analogous to the transhalogenation reaction observed with halogen-

Collisions of Silylium Cations with SAMs containing ions and the F-SAM surface.54 However, in this case a nucleophilic mechanism is clearly favored. Figure 3 illustrates formation of the transhalogenation product SiCl2F+ after collision of SiCl3+ with the F-SAM surface at 60 eV. The ion/surface reaction product is practically the most abundant ion in the mass spectrum. For the systems examined here, ion/surface reaction products are formed in greater abundance at the F-SAM surface than from the other two surfaces. This is probably a consequence of (i) more internal energy being deposited into the projectiles upon impact, (ii) less neutralization of the projectile ion, and (iii) multiple fluorine atoms being available at a single carbon atom. Besides observing the substitution of a fluorine atom into the scattered ion, it has been shown55 through subsequent surface analysis that a halogen atom from the projectile ion is incorporated into the terminal group at the F-SAM surface, generating the group -CF2X (X ) Cl, Br). Similar surface modifications as a result of ion/surface reactions have been demonstrated for the H-SAM or HOSAM surfaces. Chlorine-for-alkyl substitution reactions are also observed in the course of the interaction of the SiCl3+ ion with both the H-SAM and HO-SAM surfaces in the form of the product ion, SiCl2CH3+. A Cl for H substitution product, SiCl2H+, is observed also when these surfaces are examined. Similar ion/ surface reactions have been observed when CF3+ or BBrn+ (n ) 0-2) ions56,57 were collided at hydrocarbon-covered surfaces. In the case of B+, multiple H abstraction was favored over single H abstraction. Hydrogen and alkyl group abstraction was first observed when hydrocarbon surfaces were interrogated with radical aromatic cations or heteroatom-containing ions, such as the molecular ion of benzene19 or acetone.58 Observation of the low abundance SiCl2CH3+ product from the HO-SAM surface may involve reaction with adventitious hydrocarbon, since a methyl group is not likely to be abstracted from the HO-SAM structure. Pick-up of an H atom to form SiCl2H+ may involve either the OH chain or the adventitious hydrocarbon. Other ion/surface reaction products observed include SiClH2+ from the H-SAM surface and HO-SAM surface and SiCl(OH)H+ and SiCl(OH)2+ from the HO-SAM surface. In a previous study, which involved multiple abstractions of hydrogen and deuterium atoms from a mixed monolayer surface, it was concluded that those reactions involved a single surface chain.59 Formation of the SiCl(OH)2+ ion is an excellent demonstration that an ion/surface reaction can involve more than just a single surface chain, a result which was also shown, much less directly, by the formation of WFn+ (n ) 1-5) upon scattering W+ and W(CO)m+ (m ) 0-6) ions from an F-SAM surface.25 In each of the above reactions, it is not always known whether these products are formed during dissociation at the surface or if unimolecular dissociation occurs after release of an addition product from the surface. In either case, it is evident that formation of even-electron product ions is considerably more favorable than formation of odd-electron ions, a result which is suggestive of Lewis acid/base chemistry, viz formal nucleophilic substitution, whether the reaction is concerted or not. In the silylium ion system, formation of the odd-electron species, SiCl(OH)•+ or SiClH•+, is not observed. It also seems unlikely that formation of these products occurs through a charge exchange mechanism, for example that suggested for H addition reactions to radical aromatic cations,8 since ions such as OH+ are not observed in the scattered ion mass spectrum. Furthermore, reaction products actually decrease in the mass spectrum as a function of collision energy under conditions when chemically sputtered ions, such as CH3+, increase.

J. Phys. Chem. B, Vol. 104, No. 47, 2000 11235

Figure 7. Scattered ion mass spectra recorded upon collision of (a) Si(CH3)3+ with an HO-SAM surface at a collision energy of 50 eV and (b) Si(CD3)3+ with an HO-SAM surface at a collision energy of 70 eV.

Another reaction product observed for both the H-SAM and HO-SAM surfaces is CH2Cl+. Similar reaction products have been observed6 upon collision of AlCl3+ ions with an F-SAM surface to yield product ions, CFCl•+ and CF2Cl+. These reactions were reported to demonstrate interaction of Cl+ with the surface. The observation of the ionized surface-derived species CH2Cl+ complements the formation of SiCl2H+ or SiCl2OH+ and is the expected product of the proposed transhalogenation reaction. The reaction product is produced more abundantly from the H-SAM surface than from the HO-SAM surface. The scattered ion/surface reaction product SiClO+ differs from the product ions discussed so far since it is evidently an unsaturated ion. It is likely to result from unimolecular dissociation through the loss of water from the ion/surface reaction product, SiCl(OH)2+ or the loss of H2 from SiCl(OH)H+. Notice, however, that at a collision energy of 50 eV, SiClO+ is present, but neither SiCl(OH)2+ or SiCl(OH)H+ have appeared yet. Reactive Collisions with Si(CH3)3+. The silicon-based Lewis acidity37 of the Si(CH3)3+ ion and differences in the ionization energy and fragmentation routes of this ion, as compared to SiCl3+, make its ion/surface reactions of interest. Figure 7 illustrates mass spectra resulting from collisions of Si(CH3)3+ and Si(CD3)3+ with the HO-SAM surface at energies of 50 and 70 eV, respectively. When comparing reactions of Si(CH3)3+ with SiCl3+ (Figure 2), both impacting the HO-SAM surface, there are noticeable differences. The alkyl ion Si(CH3)3+ appears to be the more fragile ion, since the elastically scattered signal represents only a small peak at a collision energy of 50 eV, whereas the elastically scattered SiCl3+ ion is the base peak at this same energy. The depletion of the scattered Si(CH3)3+ ion at this collision energy is apparently due to SID and the formation of several low mass fragment ions, two being Si-

11236 J. Phys. Chem. B, Vol. 104, No. 47, 2000 (CH3)+ (m/z 43) which results from sequential loss of methyl groups and Si(CH3)H2+ (m/z 45) which results from loss of C2H4 with rearrangement. Other SID products include Si(CH3)2•+, SiH3+, SiH+, and Si•+. The other difference observed between the ions, Si(CH3)3+ and SiCl3+, already discussed, but now illustrated, is the smaller abundance of charge exchange products observed upon collision of the Si(CH3)3+ ion with the HOSAM surface. At 70 eV collision energy (Figure 6b), sputtered ion products CH2OH+ (m/z 31) and C2H5+ (m/z 29) are practically absent, whereas in the case of 70 eV collisions of SiCl3+ (Figure 2C), these sputtered ion products represent the base peak in the mass spectrum. This result is attributed to the lower recombination energy of the Si(CH3)3+ ion, 6.5 eV, compared to that for SiCl3+, 8.5 eV, which was estimated by considering appearance energies and heats of formation from experimental gas-phase data.46 Several of the same types of reaction products observed from SiCl3+ collisions with the HO-SAM surface are also observed with the Si(CH3)3+ ion. For instance, alkyl-for-OH or alkylfor-H substitution reactions are indicated by the reaction products, Si(CD3)2OH+ and Si(CD3)2H+. However, the greater ease of fragmentation of the Si(CH3)3+ ion yields several new ion/surface reaction products as well, including SiO•+, SiOH+, SiD2H+, SiD2OH+, and Si(CD3)H2+. The onset of these ion/ surface reaction products was similar to that of Si•+, and this gives some measure as to the amount of internal energy required for these dissociative reactions to occur. In the case of SiCl3+, the greater Si-Cl bond energy (97 kcal/mol vs 73 kcal/mol for Si-C60) means that sufficient internal energy to produce fragments is not available and neither Si+• nor the ion/surface reaction products SiO+• and SiOH+ are observed. Conclusion Silylium cations were chosen for this study in an attempt to explore Lewis acid/base chemistry in the context of low energy ion/surface collision processes. This examination of the HOSAM surface as a substrate in low-energy ion/surface collisions has led to the discovery of several new types of ion/surface reactions. Simple abstraction reactions by the projectile ion, to form such products as SiCl3OH•+, were not observed, presumably due to the small internal sink for redistribution of the energy released into the tetravalent ion upon new bond formation. For the same reason, dissociative ion/surface reactions, in which bonds to the projectile ion are broken while new bonds are formed with the surface, are expected and observed. Atom and group transfer reactions occur and yield such substitution products as SiCl2OH+, SiCl2CH3+, and SiCl2H+. These reactions are probably nucleophilic substitutions at the ionized silicon. They involve incorporation of the Cl atom into the surface into the scattered ion, evidence for this being found in the formation of CH2Cl+, which accompanies the other ion/surface reaction products in the mass spectra. Observation of the disubstitution product, SiCl(OH)2+, demonstrates that reaction can occur with multiple surface chains. Beyond this, SiO•+ and SiClO+ are examples of such product ions in which extensive fragmentation and rearrangement must occur. In general, processes occurring at the hydroxyl surface were compared and contrasted with the H-SAM and F-SAM surfaces. Efficiencies of conversion of translational energy of a projectile ion into internal energy, as determined by the extent of fragmentation for that ion, upon impact with a stationary target can be used as a defining quality of that surface in terms of its relative “hardness or softness.” For all ions included in this study the F-SAM surface provided greater internal energy

Wade et al. uptake for the projectile ion. Possible reasons for this effect may be a result of the greater effective mass at this surface, CF3 vs CH3, or the greater rigidity of the perfluorinated alkyl chain. For perhaps the same reasons, the HO-SAM surface was shown to produce greater fragmentation of projectile ions than the H-SAM surface, at least for collision energies greater than 40 eV. The abundance of scattered ions recorded at each of the three surfaces gave a rough measure of the amount of projectile ions experiencing neutralization and showed that charge exchange was more prominent at the HO-SAM and H-SAM surfaces than at the F-SAM surface. The relative amounts of neutralization occurring for different systems can be explained by considering the difference in ionization energies of the projectile ion and the surface molecule. In considering the chemical sputtering process, which can only occur through neutralization of the projectile ion, it is shown that the relative amounts of chemically sputtered products observed from each of these systems are inversely correlated with the calculated enthalpic energies required for these processes. The reactions encountered in this study, like those encountered in previous studies of hyperthermal ion reactive collisions,61 are thought to involve ions that are not equilibrated with the surface. This issue is likely to be the subject of much future interest.62 Acknowledgment. This work was supported by National Science Foundation (CHE-9732670). References and Notes (1) Mabud, M.; Dekrey, M.; Cooks, R. G. Int. J. Mass Spectrom. Ion Processes 1985, 67, 285-94. (2) De Maaijer-Gielbert, J.; Gu, C.; Somogyi, A.; Wysocki, V. H.; Kistemaker, P. G.; Weeding, T. L. J. Am. Soc. Mass Spectrom. 1999, 10, 414-22. (3) Kubista, J.; Dolejsek, Z.; Herman, Z. Eur. Mass Spectrom. 1998, 4, 311-19. (4) Chorush, R.; Little, D.; Beu, S.; Wood, T.; McLafferty, F. Anal. Chem. 1995, 67, 1042-46. (5) Inta, I.; Assuncao, M.; Wisckerke, A.; Teodoro, O.; Kleyn, A. W.; Moutinho, A. Rom. J. Phys. 1997, 42, 519-23. (6) Pradeep, T.; Ast, T.; Cooks, R. G.; Feng, B. J. Phys. Chem. 1994, 98, 9301-11. (7) Winger, B.; Julian, R.; Cooks, R. G.; Chidsey, C. J. Am. Chem. Soc. 1991, 113, 8967-69. (8) Phelan, L.; Hayward, M.; Flynn, J.; Bernasek, S. L. J. Phys. Chem. B 1998, 102, 5667-72. (9) Boyd, K.; Marton, B.; Todorov, S.; Al-Bayati, A.; Kulik, J.; Zuhr, R.; Rabalais, J. W. J. Vac. Sci. Technol., A 1995, 13, 2110-22. (10) Ada, E.; Kornienko, O.; Hanley, L. J. Phys. Chem. B 1998, 102, 3959-66. (11) Vincenti, M.; Cooks, R. G. Org. Mass Spectrom. 1988, 23, 31726. (12) Tsukuda, T.; Yasumatsu, H.; Sugai, T.; Terasaki, A.; Nagata, T.; Kondow, T. Surf. ReV. Lett. 1996, 3, 875-79. (13) Miller, S.; Luo, H.; Pachuta, S.; Cooks, R. G. Science 1997, 275, 1447-50. (14) Tsekouras, A.; Iedema, M.; Cowin, J. P. J. Chem. Phys. 1999, 111, 2222-34. (15) Shukla, A.; Futrell, J. Mass Spectrom. ReV. 1993, 12, 211-55. (16) Gu, C.; Somogyi, A.; Wysocki, V. H.; Medzihradszky, K. Anal. Chim. Acta 1999, 397, 247-56. (17) Fuhrer, K.; Schultz, A.; Stone, E.; Gillig, K.; Ruotolo, B.; Park, Z.; Russell, D. 48th ASMS Conference on Mass Spectrometry and Allied Topics, Long Beach, CA, June 2000. (18) De Maaijer-Gielbert, J.; Chalmers, M.; Mohammed, S.; Brancia, F.; Ferro, M.; Gaskell, S.; Gora, L.; Smith, C. 48th ASMS Conference on Mass Spectrometry and Allied Topics, Long Beach, CA, June 2000. (19) Cooks, R. G.; Ast, T.; Mabud, M. Int. J. Mass Spectrom. Ion Processes 1990, 100, 209-65. (20) Pradeep, T.; Feng, B.; Ast, T.; Patrick, J.; Cooks, R. G.; Pachuta, S. J. Am. Soc. Mass Spectrom. 1995, 6, 187-94. (21) Shen, J.; Evans, C.; Wade, N.; Cooks, R. G. J. Am. Chem. Soc. 1999, 121, 9762-63.

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