Experimental and theoretical studies of the effect of mass on the dynamics of gas/organic-surface energy transfer

The effect of mass on gas/organic-surface energy transfer is explored via investigation of the scattering dynamics of rare gases (Ne, Ar, and Kr) from regular (CH3-terminated) and omega-fluorinated (CF3-terminated) alkanethiol self-assembled monolayers (SAMs) at 60 kJmol collision energy. Molecular-beam scattering experiments carried out in ultrahigh vacuum and molecular-dynamics simulations based on high-accuracy potentials are used to obtain the rare-gases' translational-energy distributions after collision with the SAMs. Simulations indicate that mass is the most important factor in determining the changes in the energy exchange dynamics for Ne, Ar, and Kr collisions on CH3- and CF3-terminated SAMs at 60 kJmol collision energy. Other factors, such as changes in the gas-surface potential and intrasurface interactions, play only a minor role in determining the differential dynamics behavior for the systems studied.     

Interactions of He atoms with Xe plated graphite: unified treatment of scattering and adsorbate dynamics based on method of coupled channels

Interactions of He atoms with surfaces have been in the focus of surface scientists for more than seventy years. Depending on the types of the studied problems, and on the algorithms available for their assessment, the accuracy of model calculations aiming at the interpretations of the various aspects of He-atom scattering and dynamics in the adsorbed phase have greatly varied during that period. In this article, we describe the application of a numerical algorithm based on the coupled-channel method that proves very accurate in the calculations of eigen-states and eigen-energies of He atoms interacting with strongly corrugated surfaces. The algorithm is applied to compute diffraction spectra characteristic of thermal energy He-atom scattering from a monolayer of Xe atoms adsorbed on (0 0 0 1) surface of graphite, and the thermodynamical quantities describing a quasi-two-dimensional gas of He atoms adsorbed on the same surface. The usefulness of the developed approach is illustrated in comparisons of the theoretical results with the available experimental data.     

Potential energy calculations for argon and methane adsorbed on MgO(001) substrate

A semi-empirical potential model is used to calculate the interaction energy of a rare gas atom or a methane molecule adsorbed on a MgO substrate with square symmetry. The potential surfaces are drawn and compared with the results obtained on the hexagonal (0001) face of graphite. MgO appears as a corrugated surface for both argon and methane whereas graphite is a nearly perfect planar surface. The calculated holding and corrugation energies are in agreement with experimental data.     

Optimal covering of C(60) fullerene by rare gases

Putative global energy minima of clusters formed by the adsorption of rare gases on a C(60) fullerene molecule, C(60)X(N) (X=Ne, Ar, Kr, Xe; N ≤ 70), are found using basin-hopping global optimization in an empirical potential energy surface. The association energies per rare gas atom as a function of N present two noticeable minima for Ne and Ar and just one for Kr and Xe. The minimum with the smallest N is the deepest one and corresponds to an optimal packing monolayer structure; the other one gives a monolayer with maximum packing. For Kr and Xe, optimal and maximum packing structures coincide. By using an isotropic average form of the X-C(60) interaction, we have established the relevance of the C(60) surface corrugation on the cluster structures. Quantum effects are relevant for Ne clusters. The adsorption of these rare gases on C(60) follows patterns that differ significantly from the ones found recently for He by means of experimental and theoretical methods.     

Intramolecular penning ionization in organic molecule-rare gas clusters

A supersonic beam of organic molecules (benzonitrile (Bn), toluene (Tol), benzene (Bz) and pyridine (Pyr)) seeded into Ne or Ar was ionized by VUV radiation from the Berlin electron storage ring (BESSY). Photoionization efficiency curves were measured for Bn, Bn·Ne, Bn·Ar, Tol, Tol·Ar, Tol·Ne, Bz, Pyr, Pyr-Ar, and Pyr-Ne. Resonance peaks in these spectra at the positions of the atomic rare gas resonance lines confirm our previous measurements on Bn seeded into Ar and Kr, showing that the excitation energy of the rare gas atom is transferred to the attached organic molecule within a heterogeneous cluster, leading to ionization of the molecule and fragmentation of the cluster.     

Quantum and classical study of surface characterization by three-dimensional helium atom scattering

Exact time-dependent wavepacket calculations of helium atom scattering from model symmetric, chiral, and hexagonal surfaces are presented and compared with their classical counterparts. Analysis of the momentum distribution of the scattered wavepacket provides a convenient method to obtain the resulting energy and angle resolved scattering distributions. The classical distributions are characterized by standard rainbow scattering from corrugated surfaces. It is shown that the classical results are closely related to their quantum counterparts and capture the qualitative features appearing therein. Both the quantum and classical distributions are capable of distinguishing between the structures of the three surfaces.     

Potential energy surfaces for the benzene–rare gas systems

A harmonic expansion representation of the intermolecular interaction has been exploited to obtain the potential energy surface (PES) for the C₆H₆–He, –Ne, –Ar, –Kr and –Xe systems in an analytical form. Basic data employed are binding energy, equilibrium distance and long-range attraction predicted by a semi-empirical method for selected configurations of the complexes. For those favorable cases where additional information are available the proposed PESs exhibit features in good agreement with those derived from spectroscopy and scattering experiments and/or ab initio calculations. The availability of realistic PESs expressed in an analytical form opens new perspectives of calculations in molecular dynamics and spectroscopic simulations where the benzene molecule and rare gas atoms are involved.     

The effect of kinematic parameters on inelastic scattering of glyoxal

The effect of kinematic parameters (relative velocity v(rel), relative momentum p(rel), and relative energy E(rel)) on the rotational and rovibrational inelastic scatterings of 0(0)K(0)S(1) trans-glyoxal has been investigated by colliding glyoxal seeded in He or Ar with target gases D2, He, or Ne at different scattering angles in crossed supersonic beams. The inelastic spectra for target gases He and D2 acquired with two different sets of kinematic parameters revealed no significant differences. This result shows that kinematic factors have the major influence in the inelastic scattering channel competition whereas the intermolecular potential energy surface plays only a secondary role. The well-defined exponential dependence of relative cross sections on exchanged angular momentum identifies angular momentum as the dominant kinematic factor in collision-induced rotationally and rovibrationally inelastic scatterings. This is supported by the behavior of the relative inelastic cross sections data in a "slope-p(rel)" representation. In this form, the data show a trend nearly independent of the target gas identity. Representations involving E(rel) and v(rel) show trends specific to the target gas.     

A stochastic, local mode study of neon-liquid surface collision dynamics

Equations of motion for a fast, light rare gas atom passing over a liquid surface are derived and used to infer the dynamics of neon collisions with squalane and perfluorinated polyether surfaces from experimental data. The equations incorporate the local mode model of a liquid surface via a stochastic process and explicitly account for impulsive collisional energy loss to the surface. The equations predict angular distributions for scattering of neon that are in good quantitative agreement with experimental data. Our key dynamical conclusions are that experimental angular distributions derive mainly from local mode surface topography rather than from structural features of individual surface molecules, and that the available data for these systems can be accounted for almost exclusively by single collisions between neon atoms and the liquid surface.     

Rotational spectrum and inversion motions in the neon-dimethyl sulfide complex

The rotational spectra of the (20)Ne and (22)Ne isotopomers of the Ne-dimethyl sulfide (DMS) rare gas dimer have been measured by Fourier transform microwave spectroscopy. MP2/6-311++G(2d,2p) calculations, and the experimental spectroscopic data, suggest a structure of C(s) symmetry in which the Ne atom lies above the heavy atom plane of the DMS (in the sigma(v) plane which bisects the CSC angle). Experimental rotational constants are consistent with a S...Ne distance of 3.943(6) Angstroms and a (cm...S...Ne) angle of 63.2(6) degrees (where cm is the center of mass of DMS). A motion of the Ne atom from one side of the DMS to the other gives rise to inversion splittings of around 3 MHz in the c-type transitions. An ab initio potential energy surface calculation has allowed examination of several possible tunneling pathways, and suggests a barrier of between 20 and 40 cm(-1) for the inversion motion, depending on the tunneling pathway taken by the Ne. Dipole moment measurements are consistent with both the experimental and ab initio structures.     

Dissociative ionization of neon clusters Ne(n), n=3 to 14: a realistic multisurface dynamical study

The molecular dynamics with quantum transitions (MDQT) method is applied to study the fragmentation dynamics of neon clusters following vertical ionization of neutral clusters with 3 to 14 atoms. The motion of the neon atoms is treated classically, while transitions between the adiabatic electronic states of the ionic clusters are treated quantum mechanically. The potential energy surfaces are described by the diatomics-in-molecules model in a minimal basis set consisting of the effective 2p orbitals on each neon atom for the missing electron. The fragmentation mechanism is found to be rather explosive, with a large number of events where several atoms simultaneously dissociate. This is in contrast with evaporative atom by atom fragmentation. The dynamics are highly nonadiabatic, especially at shorter times and for the larger clusters. Initial excitation of the neutral clusters does not affect the fragmentation pattern. The influence of spin-orbit coupling is also examined and found to be small, except for the smaller size systems for which the proportion of the Ne+ fragment is increased up to 43%. From the methodological point of view, most of the usual momentum adjustment methods at hopping events are shown to induce nonconservation of the total nuclear angular momentum because of the nonzero electronic to rotation coupling in these systems. A new method for separating out this coupling and enforcing the conservation of the total nuclear momentum is proposed. It is applied here to the MDQT method of Tully but it is very general and can be applied to other surface hopping methods.     

Angle and bond-length dependent C6 coefficients for H2 interacting with H,Li, Be and rare gas atoms

Summary Accurate new C₆ dispersion energy coefficients, and their dependence on the diatom orientation and bond length, are calculated for molecular hydrogen interacting with an atom of H, Li, Be, He, Ne, Ar, Kr or Xe. They are generated from accurateab initio pseudo dipole oscillator strength distributions (DOSD) for H₂, H, He and Be, and reliable semiempirical ones for Li, Ne, Ar, Kr and Xe. Compact power series expansions for the diatom bond-length dependence of these coefficients, suitable for incorporation into representations of full potential energy surfaces for these systems, are determined and assessed.     

A simple model of hydrogen bonding with particular application to trends in hydrogen‐bonded dimers

A simple model has been proposed to explain trends in the computed interaction energy, bond length changes, frequency shifts and infrared intensities for the chlorofluoromethanes CF_nCl_mH, FH and FArH on complexation with the isoelectronic diatomics BF, CO, N₂ and the rare gas atoms Kr, Ar, Ne to form a series of linear or nearly linear hydrogen‐bonded complexes. The dipole moment derivative of the proton donor (with respect to the stretching coordinate) and the chemical hardness of the hydrogen‐bonded atom of the proton acceptor are identified as two useful parameters for rationalizing the changes in some of the molecular properties of the proton donor when the hydrogen bond is formed. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Interfacial energy exchange and reaction dynamics in collisions of gases on model organic surfaces

Molecular beam scattering experiments and molecular dynamics simulations have been combined to develop an atomic-level understanding of energy transfer, accommodation, and reactions during collisions between gases and model organic surfaces. The work highlighted in this progress report has been motivated by the scientific importance of understanding fundamental interfacial chemical reactions and the relevance of reactions on organic surfaces to many areas of environmental chemistry. The experimental investigations have been accomplished by molecular beam scattering from ω-functionalized self-assembled monolayers (SAMs) on gold. Molecular beams provide a source of reactant molecules with precisely characterized collision energy and flux; SAMs afford control over the order, structure, and chemical nature of the surface. The details of molecular motion that affect energy exchange and scattering have been elucidated through classical-trajectory simulations of the experimental data using potential energy surfaces derived from ab initio calculations. Our investigations began by employing rare-gas scattering to explore how alkanethiol chain length and packing density, terminal group relative mass, orientation, and chemical functionality influence energy transfer and accommodation at organic surfaces. Subsequent studies of small molecule scattering dynamics provided insight into the influence of internal energy, molecular orientation, and gas–surface attractive forces in interfacial energy exchange. Building on the understanding of scattering dynamics in non-reactive systems, our work has recently explored the reaction probabilities and mechanisms for O₃ and atomic fluorine in collisions with a variety of functionalized SAM surfaces. Together, this body of work has helped construct a more comprehensive understanding of reaction dynamics at organic surfaces.     

Calculation of inelastic helium atom scattering from H2/NaCl(001)

The one-phonon inelastic low energy helium atom scattering theory is adapted to cases where the target monolayer is a p(1 × 1) commensurate square lattice. Experimental data for para-H(2)/NaCl(001) are re-analyzed and the relative intensities of energy loss peaks in the range 6 to 9 meV are determined. The case of the H(2)/NaCl(001) monolayer for 26 meV scattering energy is computationally challenging and difficult because it has a much more corrugated surface than those in the previous applications for triangular lattices. This requires a large number of coupled channels for convergence in the wave-packet-scattering calculation and a long series of Fourier amplitudes to represent the helium-target potential energy surface. A modified series is constructed in which a truncated Fourier expansion of the potential is constrained to give the exact value of the potential at some key points and which mimics the potential with fewer Fourier amplitudes. The shear horizontal phonon mode is again accessed by the helium scattering for small misalignment of the scattering plane relative to symmetry axes of the monolayer. For 1° misalignment, the calculated intensity of the longitudinal acoustic phonon mode frequently is higher than that of the shear horizontal phonon mode in contrast to what was found at scattering energies near 10 meV for triangular lattices of Ar, Kr, and Xe on Pt(111).     

Inelastic scattering dynamics of Ar from a perfluorinated self-assembled monolayer surface

Dynamics of Ar atom collisions with a perfluorinated alkanethiol self-assembled monolayer (F-SAM) surface on gold were investigated by classical trajectory simulations and atomic beam scattering techniques. Both explicit-atom (EA) and united-atom (UA) models were used to represent the F-SAM surface; in the UA model, the CF3 and CF2 units are represented as single pseudoatoms. Additionally the nonbonded interactions in both models are different. The simulations show the three limiting mechanisms expected for collisions of rare gas atoms (or small molecules) with SAMs, that is, direct scattering, physisorption, and penetration. Surface penetration results in a translational energy distribution, P(Ef), that can be approximately fit to the Boltzmann for thermal desorption, suggesting that surface accommodation is attained to a large extent. Fluorination of the alkanethiol monolayer leads to less energy transfer in Ar collisions. This results from a denser and stiffer surface structure in comparison with that of the alkanethiol SAM, which introduces constraints for conformational changes which play a significant role in the energy-transfer process. The trajectory simulations predict P(Ef) distributions in quite good agreement with those observed in the experiments. The results obtained with the EA and UA models are in reasonably good agreement, although there are some differences.