An overview of gaseous ion/surface collisions is presented, with special emphasis on the behavior of polyatomic projectile ions at hyperthermal collision energies (1–100 eV) and the instrumentation needed for such studies. The inelastic and reactive processes occurring during ion/surface collisions are described in terms of several archetypes, viz., elastic and quasielastic scattering, chemical sputtering leading to release of surface material, inelastic scattering leading to surface-induced dissociation (SID) of the projectile, ion/surface reactions, and soft landing. Parameters that are important in ion/surface interactions are discussed, including the interaction time, the conversion of translational to internal energy, the translational energies of the scattered ions, the effects of scattering angle, and the influence of the nature of the surface. Different types of tandem mass spectrometers, built specifically to study ion/surface collision phenomena, are discussed and the advantages and disadvantages of the individual designs are compared. The role of SID as a technique in bioanalytical mass spectrometry is illustrated and this inelastic collision experiment is compared and contrasted with gas-phase collision-induced dissociation, the standard method of tandem mass spectrometry. Special emphasis is placed on reactive scattering including the use of ion/surface reactions for surface chemical analysis and for surface chemical modification.

Collisions of ions with surfaces at chemically relevant energies: Instrumentation and phenomena

ion mechanism, discussed in section 1.1.2. Figure 27 illustrates the reactive scattering mechanism with four representative snapshots of a Cs scattering trajectory in a classical MD simulation. The abstraction reaction is driven by the ion−dipole attraction force between the Cs ion and an adsorbate molecule. The impinging projectile first releases part of its initial energy to the surface (Figure 27b) even without direct collision with the adsorbate. Subsequently, the projectile pulls the adsorbate gently away from the surface in its outgoing trajectory (parts c and d of Figures 27 in sequence), leading to the formation of a Cs−molecule complex. The velocity of the outgoing Cs must be slow enough to accommodate the inertia of the adsorbate. As a result, adsorbates of low mass and small binding energy are efficiently abstracted. A heavier projectile like Cs transfers more energy to the target surface, and its lower velocity in the outgoing trajectory enhances the efficiency of reactive scattering events. Detailed aspects of Cs reactive scattering and its application for surface analysis have been reviewed. Table 9. Hyperthermal Energy Collisions at Condensed Molecular Solids method (projectile ion) system aim/observations refs reactive scattering and LES (Cs) H2O−D2O rate and activation energy of self-diffusion and H/D exchange of water 462, 476, 479, 496 H3O −water ice affinity of protons for the ice surface and proton transfer mechanism 478−480 H3O −H2O−D2O hydronium ion-mediated proton transfer at the ice surface 495 OH−H2O−D2O hydroxide ion-mediated proton transfer at the ice surface 497 HCl−water ice molecular and ionized states of HCl on ice 457, 477 Na−water ice hydrolysis of Na 484 H3O −NH3−water ice incomplete proton transfer from H3O to NH3 on the ice surface 454, 458 H3O −amine−water ice proton transfer efficiency on ice is reversed from the order of amine basicity 502 CO2−Na−water ice CO2 hydrolysis is not facilitated by a hydroxide ion 463 NO2−water ice NO2 hydrolysis produces nitrous acid 465 SO2−water ice SO2 hydrolysis occurs through various intermediates 511 C2H4−HCl−water ice electrophilic addition reaction mechanism at the condensed molecular surface 466 ethanol/2-methylpropan-2-ol−water ice SN1 and SN2 mechanisms at the condensed molecular surface 505 NH3−water ice and UV irradiation ammonium ion formation 608 CH3NH2−water ice and UV irradiation protonated methylamine formation 483 CH3NH2−CO2−water ice and UV irradiation glycine and carbamic acid formation 464 NaX−water ice (X = F, Cl, Br) surface/bulk segregation and transport properties of electrolyte ions 472−474 reactive scattering (Cs) CO and CO2 on Pt(111) mechanism of Cs + reactive ion scattering 89 Ar, Kr, Xe, and N2 on Pt(111) adsorbate mass effect on the reactive ion scattering cross-section 609 C2H4 on Pt(111) dehydrogenation mechanism of ethylene to ethylidyne 459, 610 C2D4 and H on Pt(111) ethylidene intermediate in H/D exchange reaction with ethylene 80, 610 reactive scattering (H) water ice and alcohol H2 + formation 469 CS (Ar) water ice−chloromethanes (CCl4, CHCl3, CH2Cl2) except CCl4, others undergo diffusive mixing 174 water ice−simple carboxylic acids structural reorganization on the ice film 175 water ice micropore collapse in the top layers of the ice film 176 water ice−butanol 494 Figure 27. Illustration of the reactive scattering mechanism of a Cs ion in four snapshots of a scattering trajectory from a Pt(111) surface: (a) initial positions before impact, (b) impact of the Cs and energy release to the surface, (c) Cs pulling the adsorbate away in its outgoing trajectory, (d) slow outgoing Cs dragging the adsorbate along and forming a Cs−molecule association product. Reprinted with permission from ref 88. Copyright 2004 John Wiley and Sons, Inc. Chemical Reviews Review dx.doi.org/10.1021/cr200384k | Chem. Rev. 2012, 112, 5356−5411 5388 Figure 28 shows an example of reactive collision mass spectra, which were obtained on a D2O ice film exposed first to 0.5 L of HCl gas and then to varying amounts of NH3 gas at 140 K. The spectra show peaks at higher masses than Cs (m/z 133), viz., CsNH3 + at m/z 150, Cs(D2O)n + (n = 1, 2) at m/z 153 and 173, and CsHCl at m/z 168, indicating the presence of the corresponding molecules on the surface. The intensities of H/D-exchanged species represent their original concentrations on the surface, because H/D isotopic scrambling does not occur during the ion/surface collision time (<1 × 10−12 s). The conversion efficiency of a neutral adsorbate (X) into a gaseous ion (CsX) ranges from ∼10−4 for chemisorbed species to ∼0.1 for physisorbed small molecules. Typical product ion signal intensities for ice film surfaces are much stronger than those for chemisorbed species. Also, it is worthwhile to point out that reactive collisions of Cs are ineffective for detecting large molecules such as polymers or long-chain SAM molecules. The mass spectra in Figure 28 also show LES signals corresponding to pre-existing ions on the surface. The hydronium ions seen are produced by the spontaneous ionization of HCl on the ice surface, and they undergo proton transfer reactions with NH3 to generate ammonium ions. The spectra show characteristic H/D isotopomers of each species produced by H/D exchange reactions with D2O molecules. The LES signals due to preformed hydronium and ammonium ions exhibited sputtering thresholds at Cs impact energies of 17 and 19 eV, respectively. On the other hand, on pure H2O and NH3 surfaces, these ions were emitted only above ∼60 eV due to their formation during secondary ion emission. It was also found that ultra-low-energy (a few electronvolts) collision of H with the ice surface can produce H2 +. The reaction proceeds more efficiently on amorphous solid water than crystalline water, reflecting differences in the surface concentration of dangling O−H bonds. Simple alkanols also behave in the same manner. The combined occurrence of reactive scattering and LES provides a powerful means to probe both neutral molecules and ions on surfaces and, therefore, to follow reactions on ice surfaces such as the ionization of electrolytes and acid−base reactions, which are described below. 7.2. Surface Composition and Structure Impurities in ice become concentrated in the quasi-liquid layers in the surface and at grain boundary regions due to the “freeze concentration effect”, and this has important consequences for atmospheric reactions on ice surfaces. However, there appear to be numerous exceptions to this general trend, where the surface segregation behavior of the dissolving species and their bulk solubility are determined by thermodynamic factors specific to individual chemical species. A good example is the formation of stable bulk phases of clathrate hydrates. Chemical specificity in the segregation phenomena can be studied by monitoring the surface populations of the dissolving species during the slow annealing of ice samples. Kang and coworkers examined these propensities in Na and halide ions at the surface and in the interior of ice films. They ionized NaF, NaCl, and NaBr molecules on ice films by the vapor deposition of the salts, and the variation in the surface population of the ions was monitored as a function of the ice temperature for 100−140 K by using LES. As shown in Figure 29, the LES intensities of Na and F− ions decrease with an increase in temperature above ∼120 K, whereas the Cl− and Br− intensities remain unchanged. The results indicate that Na and F− ions migrate from the ice surface to the interior at the elevated temperatures. The migration process is driven Figure 28. Cs reactive scattering and LES spectra monitoring the H3O −NH3 reaction on ice. The D2O film [3−4 bilayers (BLs), 1 BL = 1.1 × 10 water molecules cm−2] was exposed first to 0.5 L of HCl to generate hydronium ions and then to NH3 at varying exposures: (a) 0.02 L, (b) 0.3 L, (c) 0.7 L. The sample temperature was 100 K. The Cs collision energy was 30 eV. Reprinted with permission from ref 454. Copyright 2001 John Wiley and Sons, Inc. Figure 29. Surface populations of Na (□), F− (▲), Cl− (◇), and Br− (●) ions as a function of the ice film temperature measured from LES intensities of the ions. NaF, NaCl, and NaBr were deposited for a coverage of 0.8 ML for each salt on a D2O ice film grown at 130 K. The LES signals were measured at the indicated temperatures of salt adsorption. The LES intensities are shown on the normalized scale with the intensity at 100−105 K as a reference. The Cs beam energy was 35 eV. The figure is drawn on the basis of the data in refs 473 and 474. Chemical Reviews Review dx.doi.org/10.1021/cr200384k | Chem. Rev. 2012, 112, 5356−5411 5389 by the ion solvation energy, and it requires that surface water molecules have enough mobility to facilitate ion passage at temperatures above 120 K. It is worth noting that such a segregation behavior for ice agrees with the negative adsorption energy of these ions at water surfaces predicted by the Gibbs surface tension equation and MD simulations. An interesting property of hydronium ions observed in recent studies is that they preferentially reside at the surface of ice rather than in its interior. Evidence of this property has come from a variety of experimental observations over the past decade. The adsorption and ionization of HCl on an ice film promotes H/D exchange on the surface. However, vertical proton transfer to the film interior is inefficient. Continuous exposure of HCl gas on the ice film led to saturation in the hydronium ion population at the surface, and the amount of HCl uptake required for this saturation was independent of the thickness of the ice film. These observations suggest that protons stay at the ice surface and hardly migrate to the interior. This behavior can be attributed either to the active trapping of protons at the surface or to the lack of proton mobility to the ice interior. The observation of asymmetric

Low-energy ionic collisions at molecular solids.

AuAg alloy clusters protected with octadecanethiol (ODT) monolayers form crystalline solids whose powder X-ray diffractograms can be indexed to single cubic unit cells. These solids can also be regarded as superlattices of alloy clusters. The bulk Au:Ag ratio is significantly different from the metal salt feed ratio used for the synthesis of the alloy clusters. The surface plasmon resonance band of the alloy clusters shift monotonically with change in composition and only one distinct band is seen in the composition range, indicating that the single metal core containing alloy clusters are not forming. The shifts observed can be linearly related to the compositions. Atomic emission spectroscopy gives similar compositions as determined from absorption spectroscopy. The monolayer coverage determined from thermogravimetry increases with an increase in silver concentration. Thermal stability of the monolayers is comparable to pure Au and Ag clusters and no change in desorption temperature is observed with composition. Both monolayer melting and superstructure melting transitions are seen in the differential scanning calorimetry of the solids. Infrared spectroscopy suggests that the monolayers are essentially solidlike and the conformations are similar to the monolayers on pure Ag clusters. Some of the bands exhibit factor group splitting in the Au-rich composition, which is observed in other compositions upon annealing.

Crystalline solids of alloy clusters

Self-assembled monolayers (SAMs) formed from alkyl-terminated, CF 3 -terminated, CF3CF2-terminated, and C 1 0 F 2 1 -terminated alkanethiolates on gold substrates were examined by low-energy (eV) ion-surface collisions to determine the impact of varying the degree of fluorination of the thiolate tail group. The fluorine-terminated SAMs are compared and contrasted to previously examined CF 3 -terminated Langmuir-Blodgett films. Polyatomic (M + . for benzene and pyrazine) and atomic (Mo + . and Cr + . ) ions were collided with the different SAM films at various collision energies (20-70 eV). Data indicate that substitution of CH 3 with CF 3 as the terminal group has a substantial influence on the ion-surface interactions, including energy transfer (fragmentation), electron transfer (neutralization), and atom/group transfer (reaction). However, slight penetration into a depth of the film is apparent and illustrated with the formation of certain ion-surface reaction products and theintensities ofthose products. This penetration has a noticeable effect on low-energy ion-surface collisions. Also, results for a series of CF 3 (CH 2 ) n S-Au films (where n = 12, 13, 14, 15) illustrate that ion-surface reactions vary with the orientation of the fluorine atoms that are present in the terminal group. The results from collisions of low-energy ions with these relatively well-defined surfaces allow further characterization of the collision events that have the potential to become valuable tools in surface analysis.

Low-Energy Ion−Surface Collisions Characterize Alkyl- and Fluoroalkyl-Terminated Self-Assembled Monolayers on Gold

Using a multi-sector ion-surface scattering mass spectrometer, reagent ions of the general form SiR(3) (+) were mass and energy selected and then made to collide with a hydroxy-terminated self-assembled monolayer (HO-SAM) surface at energies of approximately 15 eV. These ion-surface interactions result in covalent transformation of the terminal hydroxy groups at the surface into the corresponding silyl ethers due to Si--O bond formation. The modified surface was characterized in situ by chemical sputtering, a low-energy ion-surface scattering experiment. These data indicate that the ion-surface reactions have high yields (i.e. surface reactants converted to products). Surface reactions with Si(OCH(3))(3) (+), followed by chemical sputtering using CF(3) (+), yielded the reagent ion, Si(OCH(3))(3) (+), and several of its fragments. Other sputtered ions, namely SiH(OCH(3))(2)OH(2) (+) and SiH(2)(OCH(3))OH(2) (+), contain the newly formed Si--O bond and provide direct evidence for the covalent modification reaction. Chemical sputtering of modified surfaces, performed using CF(3) (+), was evaluated over a range of collision energies. The results showed that the energy transferred to the sputtered ions, as measured by their extent of fragmentation in the scattered ion mass spectra, was essentially independent of the collision energy of the projectile, thus pointing to the occurrence of reactive sputtering.A set of silyl cations, including SiBr(3) (+), Si(C(2)H(3))(3) (+) and Si(CH(3))(2)F(+), were similarly used to modify the HO-SAM surface at low collision energies. A reaction mechanism consisting of direct electrophilic attack by the cationic projectiles is supported by evidence of increased reactivity for these reagent ions with increases in the calculated positive charge at the electron-deficient silicon atom of each of these cations. In a sequential set of reactions, 12 eV deuterated trimethylsilyl cations, Si(CD(3))(3) (+), were used first as the reagent ions to modify covalently a HO-SAM surface. Subsequently, 70 eV SiCl(3) (+) ions were used to modify the surface further. In addition to yielding sputtered ions of the modified surface, SiCl(3) (+) reacted with both modified and unmodified groups on the surface, giving rise not only to such scattered product ions as SiCl(2)OH(+) and SiCl(2)H(+), but also to SiCl(2)CD(3) (+) and SiCl(2)D(+). This result demonstrates that selective, multi-step reactions can be performed at a surface through low-energy ionic collisions. Such processes are potentially useful for the construction of novel surfaces from a monolayer substrate and for chemical patterning of surfaces with functional groups.

Silylation of an OH-terminated self-assembled monolayer surface through low-energy collisions of ions: a novel route to synthesis and patterning of surfaces.

The value of low-energy ionic collisions for selective surface analysis is shown by the fact that reactive scattering allows differentiation of isomeric chemisorbates. Reactions of Cr•+ and Cr-containing cations at chlorobenzyl mercaptan (CBM) monolayers on Au surfaces show different reaction products, depending on the position of chlorine substitution in the phenyl ring. The chlorine atom abstraction product, CrCl+, is observed at 45-eV collision in 4-CBM and is completely absent in 2-CBM at the same energy. The sensitivity of reactive ion scattering to the isomeric adsorbate is further demonstrated by the fact that the peak corresponding to Cr•+ addition and Cl loss via Cl−C bond cleavage, C7H6SCr+, is large in the monolayer formed from 4-CBM, but weak in the 2-CBM monolayer, suggesting that the chlorine is below the first layer of atoms in the latter case. The inverse intensity relationship applies for the dehydrohalogenation product C7H5Cr+ where the Cl atom at the ortho position in 2-CBM facilitates ...

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and Chris Evans

Papers

Reactive ion scattering from surfaces bearing isomeric chlorinated adsorbates