A washboard with moment of inertia model of gas-surface scattering

Atom-surface scattering in the classical multiphonon regime

Many experiments that utilize beams of incident atoms colliding with surfaces as a probe of surface properties are carried out at large energies, high temperatures and with large mass atoms. Under these conditions the scattering process does not exhibit quantum mechanical properties such as diffraction or single-phonon excitation, but rather can be treated with classical physics. This is a review of work carried out by the authors over a span of several years to develop theoretical frameworks using classical physics for describing the scattering interactions of atom with surfaces.

Mechanistic details of energy transfer and soft landing in ala2-H(+) collisions with a F-SAM surface

Previous chemical dynamics simulations (Phys. Chem. Chem. Phys., 2014, 16, 23769-23778) were analyzed to delineate atomistic details for collision of N-protonated dialanine (ala2-H(+)) with a C8 perfluorinated self-assembled monolayer (F-SAM) surface. Initial collision energies Ei of 5-70 eV and incident angles θi of 0° and 45°, with the surface normal, were considered. Four trajectory types were identified: (1) direct scattering; (2) temporary sticking/physisorption on top of the surface; (3) temporary penetration of the surface with additional physisorption on the surface; and (4) trapping on/in the surface, by physisorption or surface penetration, when the trajectory is terminated. Direct scattering increases from 12 to 100% as Ei is increased from 5 to 70 eV. For the direct scattering at 70 eV, at least one ala2-H(+) heavy atom penetrated the surface for all of the trajectories. For ∼33% of the trajectories all eleven of the ala2-H(+) heavy atoms penetrated the F-SAM at the time of deepest penetration. The importance of trapping decreased with increase in Ei, decreasing from 84 to 0% with Ei increase from 5 to 70 eV at θi = 0°. Somewhat surprisingly, the collisional energy transfers to the F-SAM surface and ala2-H(+) are overall insensitive to the trajectory type. The energy transfer to ala2-H(+) is primarily to vibration, with the transfer to rotation ∼10% or less. Adsorption and then trapping of ala2-H(+) is primarily a multi-step process, and the following five trapping mechanisms were identified: (i) physisorption-penetration-physisorption (phys-pen-phys); (ii) penetration-physisorption-penetration (pen-phys-pen); (iii) penetration-physisorption (pen-phys); (iv) physisorption-penetration (phys-pen); and (v) only physisorption (phys). For Ei = 5 eV, the pen-phys-pen, pen-phys, phys-pen, and phys trapping mechanisms have similar probabilities. For 13.5 eV, the phys-pen mechanism, important at 5 eV, is unimportant. The radius of gyration of ala2-H(+) was calculated once it is trapped on/in the F-SAM surface and trapping decreases the ion's compactness, in part by breaking hydrogen bonds. The ala2-H(+) + F-SAM simulations are compared with the penetration and trapping dynamics found in previous simulations of projectile + organic surface collisions.

Atomic and Molecular Collisions at Liquid Surfaces

The gas-liquid interface remains one of the least explored, but nevertheless most practically important, environments in which molecular collisions take place. These molecular-level processes underlie many bulk phenomena of fundamental and applied interest, spanning evaporation, respiration, multiphase catalysis, and atmospheric chemistry. We review here the research that has, during the past decade or so, been unraveling the molecular-level mechanisms of inelastic and reactive collisions at the gas-liquid interface. Armed with the knowledge that such collisions with the outer layers of the interfacial region can be unambiguously distinguished, we show that the scattering of gas-phase projectiles is a promising new tool for the interrogation of liquid surfaces with extreme surface sensitivity. Especially for reactive scattering, this method also offers absolute chemical selectivity for the groups that react to produce a specific observed product.

Soft- and reactive landing of ions onto surfaces: Concepts and applications

Soft- and reactive landing of mass-selected ions is gaining attention as a promising approach for the precisely-controlled preparation of materials on surfaces that are not amenable to deposition using conventional methods. A broad range of ionization sources and mass filters are available that make ion soft-landing a versatile tool for surface modification using beams of hyperthermal (<100 eV) ions. The ability to select the mass-to-charge ratio of the ion, its kinetic energy and charge state, along with precise control of the size, shape, and position of the ion beam on the deposition target distinguishes ion soft landing from other surface modification techniques. Soft- and reactive landing have been used to prepare interfaces for practical applications as well as precisely-defined model surfaces for fundamental investigations in chemistry, physics, and materials science. For instance, soft- and reactive landing have been applied to study the surface chemistry of ions isolated in the gas-phase, prepare arrays of proteins for high-throughput biological screening, produce novel carbon-based and polymer materials, enrich the secondary structure of peptides and the chirality of organic molecules, immobilize electrochemically-active proteins and organometallics on electrodes, create thin films of complex molecules, and immobilize catalytically active organometallics as well as ligated metal clusters. In addition, soft landing has enabled investigation of the size-dependent behavior of bare metal clusters in the critical subnanometer size regime where chemical and physical properties do not scale predictably with size. The morphology, aggregation, and immobilization of larger bare metal nanoparticles, which are directly relevant to the design of catalysts as well as improved memory and electronic devices, have also been studied using ion soft landing. This review article begins in section 1 with a brief introduction to the existing applications of ion soft- and reactive landing. Section 2 provides an overview of the ionization sources and mass filters that have been used to date for soft landing of mass-selected ions. A discussion of the competing processes that occur during ion deposition as well as the types of ions and surfaces that have been investigated follows in section 3. Section 4 discusses the physical phenomena that occur during and after ion soft landing, including retention and reduction of ionic charge along with factors that impact the efficiency of ion deposition. The influence of soft landing on the secondary structure and biological activity of complex ions is addressed in section 5. Lastly, an overview of the structure and mobility as well as the catalytic, optical, magnetic, and redox properties of bare ionic clusters and nanoparticles deposited onto surfaces is presented in section 6.

Kinematics and dynamics of atomic-beam scattering on liquid and self-assembled monolayer surfaces

We have conducted investigations of the energy transfer dynamics of atomic oxygen and argon scattering from hydrocarbon and fluorocarbon surfaces. In light of these results, we appraise the applicability and value of a kinematic scattering model, which views a gas-surface interaction as a gas-phase-like collision between an incident atom or molecule and a localized region of the surface with an effective mass. We have applied this model to interpret the effective surface mass and energy transfer when atoms strike two different surfaces under identical bombardment conditions. To this end, we have collected new data, and we have re-examined existing data sets from both molecular-beam experiments and molecular dynamics simulations. We seek to identify trends that could lead to a robust general understanding of energy transfer processes induced by collisions of gas-phase species with liquid and semi-solid surfaces.

Exploring the importance of quantum effects in nucleation: the archetypical Ne(n) case

The effect of quantum mechanics (QM) on the details of the nucleation process is explored employing Ne clusters as test cases due to their semi-quantal nature. In particular, we investigate the impact of quantum mechanics on both condensation and dissociation rates in the framework of the microcanonical ensemble. Using both classical trajectories and two semi-quantal approaches (zero point averaged dynamics, ZPAD, and Gaussian-based time dependent Hartree, G-TDH) to model cluster and collision dynamics, we simulate the dissociation and monomer capture for Ne(8) as a function of the cluster internal energy, impact parameter and collision speed. The results for the capture probability P(s)(b) as a function of the impact parameter suggest that classical trajectories always underestimate capture probabilities with respect to ZPAD, albeit at most by 15%-20% in the cases we studied. They also do so in some important situations when using G-TDH. More interestingly, dissociation rates k(diss) are grossly overestimated by classical mechanics, at least by one order of magnitude. We interpret both behaviours as mainly due to the reduced amount of kinetic energy available to a quantum cluster for a chosen total internal energy. We also find that the decrease in monomer dissociation energy due to zero point energy effects plays a key role in defining dissociation rates. In fact, semi-quantal and classical results for k(diss) seem to follow a common "corresponding states" behaviour when the proper definition of internal and dissociation energies are used in a transition state model estimation of the evaporation rate constants.

Importance of shattering fragmentation in the surface-induced dissociation of protonated octaglycine

A QM + MM direct chemical dynamics simulation was performed to study collisions of protonated octaglycine, gly(8)-H(+), with the diamond {111} surface at an initial collision energy E(i) of 100 eV and incident angle theta(i) of 0 degrees and 45 degrees. The semiempirical model AM1 was used for the gly(8)-H(+) intramolecular potential, so that its fragmentation could be studied. Shattering dominates gly(8)-H(+) fragmentation at theta(i) = 0 degrees, with 78% of the ions dissociating in this way. At theta(i) = 45 degrees shattering is much less important. For theta(i) = 0 degrees there are 304 different pathways, many related by their backbone cleavage patterns. For the theta(i) = 0 degrees fragmentations, 59% resulted from both a-x and b-y cleavages, while for theta(i) = 45 degrees 70% of the fragmentations occurred with only a-x cleavage. For theta(i) = 0 degrees, the average percentage energy transfers to the internal degrees of freedom of the ion and the surface, and the energy remaining in ion translation are 45%, 26%, and 29%. For 45 degrees these percentages are 26%, 12%, and 62%. The percentage energy-transfer to DeltaE(int) for theta(i) = 0 degrees is larger than that reported in previous experiments for collisions of des-Arg(1)-bradykinin with a diamond surface at the same theta(i). This difference is discussed in terms of differences between the model diamond surface used in the simulations and the diamond surface prepared for the experiments.

Classical theory for the in-plane scattering of atoms from corrugated surfaces: application to the Ar-Ag(111) system

A classical Wigner in-plane atom surface scattering perturbation theory within the generalized Langevin equation formalism is proposed and discussed with applications to the Ar-Ag(111) system. The theory generalizes the well-known formula of Brako as well as the "washboard model." Explicit expressions are derived for the joint angular and final momentum distributions, joint final energy, and angular distributions as well as average energy losses to the surface. The theory provides insight into the intertwining between the energy loss and angular dependence of the scattering. At low energies the energy loss in the horizontal direction is expected to be large, leading to a shift of the maximum of the angular distribution to subspecular angles, while at high energies the energy loss in the vertical direction dominates, leading to a superspecular maximum in the angular distribution. The same effect underlies the negative slope of the average final (relative) energy versus scattering angle at low energies which becomes positive at high energies. The theory also predicts that the full width at half maximum of the angular distribution varies as the square root of the temperature. We show how the theory provides insight into the experimental results for scattering of Ar from the Ag(111) surface.

Classical Wigner theory of gas surface scattering

The scattering of atoms from surfaces is studied within the classical Wigner formalism. A new analytical expression is derived for the angular distribution and its surface temperature dependence. The expression is valid in the limit of weak coupling between the vertical motion with respect to the surface and the horizontal motion of the atom along the periodic surface. The surface temperature dependence is obtained in the limit of weak coupling between the horizontal atomic motion and the surface phonons. The resulting expression, which takes into account the surface corrugation, leads to an almost symmetric double peaked angular distribution, with peaks at the rainbow angles. The analytic expression agrees with model numerical computations. It provides a good qualitative description for the experimentally measured angular distribution of Ne and Ar scattered from a Cu surface.

Collision-induced annealing of octanethiol self-assembled monolayers by high-kinetic-energy xenon atoms

Collisions with high-energy xenon atoms (1.3 eV) induce structural changes in octanethiol self-assembled monolayers on Au(111). These changes are characterized at the molecular scale using an in situ scanning tunneling microscope. Gas-surface collisions induce three types of structural transformations: domain boundary annealing, vacancy island diffusion, and phase changes. Collision-induced changes that occur tend to increase order and create more stable structures on the surface. We propose a mechanism where monolayer transformations are driven by large amounts of vibrational energy localized in the alkanethiol molecules. Because we monitor incremental changes over small regions of the surface, we can obtain structural information about octanethiol monolayers that cannot be observed directly in scanning tunneling microscopy images.

Quantum-state resolved reaction dynamics at the gas-liquid interface: Direct absorption detection of HF(v,J) product from F(P2)+Squalane

Exothermic reactive scattering of F atoms at the gas-liquid interface of a liquid hydrocarbon (squalane) surface has been studied under single collision conditions by shot noise limited high-resolution infrared absorption on the nascent HF(v,J) product. The nascent HF(v,J) vibrational distributions are inverted, indicating insufficient time for complete vibrational energy transfer into the surface liquid. The HF(v=2,J) rotational distributions are well fit with a two temperature Boltzmann analysis, with a near room temperature component (T(TD) approximately equal to 290 K) and a second much hotter scattering component (T(HDS) approximately equal to 1040 K). These data provide quantum state level support for microscopic branching in the atom abstraction dynamics corresponding to escape of nascent HF from the liquid surface on time scales both slow and fast with respect to rotational relaxation.

Structural changes of an octanethiol monolayer via hyperthermal rare-gas collisions

In situ scanning tunneling microscopy is used to measure the effect of hyperthermal rare-gas bombardment on octanethiol self-assembled monolayers. Close-packed monolayers remain largely unchanged, even after repeated collisions with 0.4 eV argon and 1.3 eV xenon atoms. In contrast, gas-surface collisions do induce structural changes in the octanethiol film near defects, domain boundaries, and disordered regions, with relatively larger changes observed for xenon-atom bombardment.

Ab initio and analytic intermolecular potentials for Ar-CF4

Ab initio calculations at the CCSD(T) level of theory were performed to characterize the Ar + CF4 intermolecular potential. Potential energy curves were calculated with the aug-cc-pVTZ basis set, and with and without a correction for basis set superposition error (BSSE). Additional calculations were performed with other correlation consistent basis sets to extrapolate the Ar-CF4 potential energy minimum to the complete basis set (CBS) limit. Both the size of the basis set and BSSE have substantial effects on the Ar + CF4 potential. Calculations with the aug-cc-pVTZ basis set, and with a BSSE correction, appear to give a good representation of the BSSE corrected potential at the CBS limit. In addition, MP2 theory is found to give potential energies in very good agreement with those determined by the much higher level CCSD(T) theory. Two model analytic potential energy functions were determined for Ar + CF4. One is a fit to the aug-cc-pVTZ calculations with a BSSE correction. The second was derived by fitting an average BSSE corrected potential, which is an average of the CCSD(T)/aug-cc-pVTZ potentials with and without a BSSE correction. These analytic functions are written as a sum of two-body potentials and excellent fits to the ab initio potentials are obtained by representing each two-body interaction as a Buckingham potential.

Dynamics of HCl Collisions with Hydroxyl- and Methyl-Terminated Self-Assembled Monolayers

Molecular beam scattering techniques are used to explore the energy exchange and thermal accommodation efficiencies of HCl in collisions with long-chain OH- and CH(3)-terminated self-assembled monolayers (SAMs) on gold. Upon colliding with the nonpolar methyl-terminated SAM, HCl (E(i) = 85 kJ/mol) is found to transfer the majority, 83%, of its translational energy to the surface. The extensive energy loss for HCl helps to bring the molecules into thermal equilibrium with the monolayer. Specifically, 72% of the HCl approaches thermal equilibrium prior to desorption. For the molecules that do not thermally accommodate, but scatter after an impulsive collision with the surface, the final translational energy is observed to be directly proportional to the surface temperature as the thermal surface energy and gas translational energy exchange during the collision. For the OH-terminated SAM, the impulsively scattered HCl escapes from the surface with slightly more average energy. The rigid nature of the OH-terminated SAM is due to the extended intra-monolayer hydrogen-bonding network that restricts some of the low-energy modes of the surface. However, despite the rigid nature of this system, the extent of thermal accommodation for HCl on these two surfaces is remarkably similar. It appears that the potential energy well between the impinging HCl and the polar surface groups is sufficient enough to trap HCl molecules that would otherwise scatter impulsively from this rigid SAM.

Rainbow scattering of CO and N2 from LiF(001)

The angular intensity distributions of CO and N(2) molecules scattered from a LiF(001) surface have been measured as functions of surface temperature, incident translational energy, and incident azimuthal direction affecting surface corrugation at a high resolution. Although both molecules have the same molecular mass and linear structure, only the CO molecule shows a rainbow feature in its scattering pattern, while the N(2) molecule shows a single peak distribution. From the comparisons of the obtained results with the calculated predictions based on the newly developed classical theory of the ellipsoid-washboard model, the differences in scattering distribution are attributed to the effects of molecular anisotropy and center-of-mass position. With an increase in the extent of the molecular anisotropy such as that of N(2) and CO as compared with rare-gas atoms, the summation of several scattering distributions depending on molecular orientation results in smearing the rainbow scattering on the corrugated surface. This smearing effect, however, attenuates when center-of-mass position deviates from the molecular center, as that for CO.

Packing density and structure effects on energy-transfer dynamics in argon collisions with organic monolayers

A combined experimental and molecular-dynamics simulation study has been used to investigate energy-transfer dynamics of argon atoms when they collide with n-alkanethiols adsorbed to gold and silver substrates. These surfaces provide the opportunity to explore how surface structure and packing density of alkane chains affect energy transfer in gas-surface collisions while maintaining the chemical nature of the surface. The chains pack standing up with 12 degrees and 30 degrees tilt angles relative to the surface normal and number densities of 18.9 and 21.5 A(2)molecule on the silver and gold substrates, respectively. For 7-kJmol argon scattering, the two surfaces behave equivalently, fully thermalizing all impinging argon atoms. In contrast, these self-assembled monolayers (SAMs) are not equally efficient at absorbing the excess translational energy from high-energy, 35 and 80 kJmol, argon collisions. When high-energy argon atoms are scattered from a SAM on silver, the fraction of atoms that reach thermal equilibrium with the surface and the average energy transferred to the surface are lower than for analogous SAMs on gold. In the case of argon atoms with 80 kJmol of translational energy scattering from long-chain SAMs, 60% and 45% of the atoms detected have reached thermal equilibrium with the monolayers on gold and silver surfaces, respectively. The differences in the scattering characteristics are attributed to excitation efficiencies of different types of surface modes. The high packing density of alkyl chains on silver restricts certain low-energy degrees of freedom from absorbing energy as efficiently as the lower-density monolayers. In addition, molecular-dynamics simulations reveal that the extent to which argon penetrates into the monolayer is related to packing density. For argon atoms with 80-kJmol incident energy, we find 16% and 7% of the atoms penetrate below the terminal methyl groups of C(10) SAMs on gold and silver, respectively.

Effect of the Ar–Ni(s) potential on the cross section for Ar+CH4/Ni{111} collision-induced desorption and the need for a more accurate CH4/Ni{111} potential

In a previous paper [L. Sun, P. de Sainte Claire, O. Meroueh, and W. L Hase, J. Chem. Phys. 114, 535 (2001)], a classical trajectory simulation was reported of CH(4) desorption from Ni{111} by Ar-atom collisions. At an incident angle theta(i) of 60 degrees (with respect to the surface normal), the calculated collision-induced desorption (CID) cross sections are in excellent agreement with experiment. However, for smaller incident angles the calculated cross sections are larger than the experimental values and for normal collisions, theta(i)=0 degrees , the calculated cross sections are approximately a factor of 2 larger. This trajectory study used an analytic function for the Ar+Ni(s) intermolecular potential which gives an Ar-Ni{111} potential energy minimum which is an order of magnitude too deep. In the work reported here, the previous trajectory study is repeated with an Ar+Ni(s) analytic intermolecular potential which gives an accurate Ar-Ni{111} potential energy minimum and also has a different surface corrugation than the previous potential. Though there are significant differences between the two Ar+Ni(s) analytic potentials, they have no important effects on the CID dynamics and the cross sections reported here are nearly identical to the previous values. Zero-point energy motions of the surface and the CH(4)-Ni(s) intermolecular modes are considered in the simulation and they are found to have a negligible effect on the CID cross sections. Calculations of the intermolecular potential between CH(4) and a Ni atom, at various levels of theory, suggest that there are substantial approximations in the ab initio calculation used to develop the CH(4)+Ni{111} potential. The implication is that the differences between the trajectory and experimental CID cross sections may arise from an inaccurate CH(4)+Ni{111} potential used in the trajectory simulation.

Crossed beams and theoretical studies of the dynamics of hyperthermal collisions between Ar and ethane

Crossed molecular beams experiments and classical trajectory calculations have been used to study the dynamics of Ar+ethane collisions at hyperthermal collision energies. Experimental time-of-flight and angular distributions of ethane molecules that scatter into the backward hemisphere (with respect to their original direction in the center-of-mass frame) have been collected. Translational energy distributions, derived from the time-of-flight distributions, reveal that a substantial fraction of the collisions transfer abnormally large amounts of energy to internal excitation of ethane. The flux of the scattered ethane molecules increased only slightly from directly backward scattering to sideways scattering. Theoretical calculations show angular and translational energy distributions which are in reasonable agreement with the experimental results. These calculations have been used to examine the microscopic mechanism for large energy transfer collisions ("supercollisions"). Collinear ("head-on") or perpendicular ("side-on") approaches of Ar to the C-C axis of ethane do not promote energy transfer as much as bent approaches, and collisions in which the H atom is "sandwiched" in a bent Ar...H-C configuration lead to the largest energy transfer. The sensitivity of collisional energy transfer to the intramolecular potential energy of ethane has also been examined.