Low-energy deposition of neutral ${\mathrm{P}\mathrm{d}}_{N}$ clusters ($N=2--7$ and $13$) on a MgO(001) surface F center (FC) was studied by spin-density-functional molecular dynamics simulations. The incident clusters are steered by an attractive ``funnel'' created by the FC, resulting in adsorption of the cluster, with one of its atoms bonded atop of the FC. The deposited ${\mathrm{P}\mathrm{d}}_{2}\mathrm{\text{\ensuremath{-}}}{\mathrm{P}\mathrm{d}}_{6}$ clusters retain their gas-phase structures, while for $Ng6$ surface-commensurate isomers are energetically more favorable. Adsorbed clusters with $Ng3$ are found to remain magnetic at the surface.

Supported magnetic nanoclusters: Soft-landing of Pd clusters on a MgO surface

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.

Soft and reactive landing of mass-selected ions onto surfaces has become a topic of substantial interest due to its promising potential for the highly controlled preparation of materials. For example, there are possible applications in the production of peptide and protein microarrays for use in high-throughput screening, protein separation and conformational enrichment of peptides, redox protein characterization, thin-film production, and the preparation of catalysts through deposition of clusters and organometallic complexes. Soft landing overcomes many of the limitations associated with conventional thin-film production techniques and offers unprecedented selectivity and specificity of preparation of deposited species. This review discusses the fundamental aspects of soft and reactive landing of mass-selected ions on surfaces that pertain to applications of these techniques in biomaterials, molecular electronics, catalysis, and interfacial chemistry.

Soft landing of complex molecules on surfaces.

Structures, energetics and spectroscopy of gas phase transition metal ion–benzene complexes

Gas-phase synthesized vanadium−benzene 1:2 (VBz2) sandwich clusters were size-selectively deposited onto bare gold and long-chain n-alkanethiolate [−S−(CH2)n-1−CH3; n = 16, 18, and 22] self-assembl...

Soft-landing isolation of vanadium-benzene sandwich clusters on a room-temperature substrate using n-alkanethiolate self-assembled monolayer matrixes.

V(benzene)2 sandwich cluster cations produced in the gas phase 
were size-selectively deposited onto a self-assembled monolayer of 
n-hexadecanethiols (HDT-SAM) chemisorbed on a Au(111) surface as well as onto 
a bare Au(111) surface. The thermal chemistry of the neutralized clusters on 
each substrate was studied with temperature programmed desorption (TPD). 
From the analyses of the threshold in the TPD, the desorption 
activation energies of the clusters deposited were determined to be 64.4 
±12.8 kJ/mol for the Au(111) and 130 ±10 kJ/mol for the HDT-SAM. 
The remarkably large desorption activation energy from the SAM suggests that 
the deposited clusters are incorporated into the SAM matrix and 
firmly trapped inside the alkyl chains of the SAM.

E. Okada

Papers

Trapping of V(benzene)2 sandwich clusters in a n-alkanethiol self-assembled monolayer matrix