Stereodynamic Effects in the Adsorption of Ethylene onto a Metal Surface

Setting benchmarks for modelling gas-surface interactions using coherent control of rotational orientation states

The coherent evolution of a molecular quantum state during a molecule-surface collision is a detailed descriptor of the interaction potential which was so far inaccessible to measurements. Here we use a magnetically controlled molecular beam technique to study the collision of rotationally oriented ground state hydrogen molecules with a lithium fluoride surface. The coherent control nature of the technique allows us to measure the changes in the complex amplitudes of the rotational projection quantum states, and express them using a scattering matrix formalism. The quantum state-to-state transition probabilities we extract reveal a strong dependency of the molecule-surface interaction on the rotational orientation of the molecules, and a remarkably high probability of the collision flipping the rotational orientation. The scattering matrix we obtain from the experimental data delivers an ultra-sensitive benchmark for theory to reproduce, guiding the development of accurate theoretical models for the interaction of H₂ with a solid surface.

A fundamental and predictive understanding of molecule-surface interactions is challenging to obtain. Here the authors report an experimental technique allowing direct measurement of the scattering matrix, which reports on the coherent evolution of quantum states of a molecule scattering from a surface.

A Method for Obtaining Liquid-Solid Adsorption Rates from Molecular Dynamics Simulations: Applied to Methanol on Pt(111) in H2O

Adsorption is an important step in heterogeneous catalysis as it predetermines how many reactant molecules can participate in a surface reaction per unit time. While the rate of adsorption processes is well studied in gas-solid adsorption in both theory and experiment, such rates are still not well studied for liquid-solid adsorption. This is partly because the ever-changing configurations of liquid-phase solvent molecules impede the ability to study a molecule approaching a surface from a liquid phase by either experiment or theory. In this work, we develop a method using molecular dynamics (MD) simulations to study the rate of adsorption in liquid-solid adsorption processes. Specifically, we use MD to model the diffusion of a methanol molecule in aqueous solvent and its adsorption to a Pt(111) surface. We find that by approximating the solute motion as following the same displacement rates as a random walk model, the adsorbed and non-adsorbed states of the methanol molecule near the Pt(111) surface can be discerned and quantified. In particular, this methodology enables extracting a sticking coefficient and a macroscopically relatable adsorption rate. This method can be applied to arbitrary types of reactants and surfaces, as well as different liquid environments, thus providing a general tool for predicting quantitative adsorption rates of liquid-solid adsorption systems.

Stereodynamical Effects by Anisotropic Intermolecular Forces

Electric and magnetic field gradients, arising from sufficiently strong anisotropic intermolecular forces, tend to induce molecular polarization which can often modify substantially the results of molecular collisions, especially at low rotational temperatures and low collision energies. The knowledge of these phenomena, today still not fully understood, is of general relevance for the control of the stereo-dynamics of elementary chemical-physical processes, involving neutral and ionic species under a variety of conditions. This paper reports on results obtained by combining information from scattering, spectroscopic and reactivity experiments, within a collaboration between the research groups in Perugia and Trento. We addressed particular attention to the reactions of small atomic ions with polar neutrals for their relevance in several environments, including interstellar medium, planetary atmospheres, and laboratory plasmas. In the case of ion-molecule reactions, alignment/orientation is a general phenomenon due to the electric field generated by the charged particle. Such phenomenon originates critical stereo-dynamic effects that can either suppress (when the orientation drives the collision complex into non-reactive or less reactive configurations), or enhance the reactivity (when orientation confines reagents in the most appropriate configuration for reaction). The associated rate coefficients show the propensity to follow an Arrhenius and a non-Arrhenius behavior, respectively.

Surface chemical reactions induced by well-controlled molecular beams: translational energy and molecular orientation control

I review our recent studies of chemical reactions on single-crystalline Cu and Si surfaces induced by hyperthermal oxygen molecular beams and by oriented molecular beams, respectively. Studies of oxide formation on Cu induced by hyperthermal molecular beams suggest that the translational energy of the incident molecules plays a significant role. The use of hyperthermal molecular beams enables us to open up new chemical reaction paths, and to develop new methods for the fabrication of thin films. Oriented molecular beams also demonstrate the possibility for controlling surface chemical reactions by varying the orientation of the incident molecules. The steric effects found on Si surfaces hint at new ways of achieving material fabrication on Si surfaces. Controlling the initial conditions of incoming molecules is a powerful tool for creating new materials on surfaces with well-controlled chemical reactions.

Selective production of reactive and nonreactive oxygen atoms on Pd(001) by rotationally aligned oxygen molecules

Sticking together: The occupation of different sites by oxygen atoms that are produced by the dissociation of O(2) on Pd(100) is determined by the initial rotational alignment of the parent molecules. The atom locations are characterized by different chemical reactivities in the reaction with CO to form CO(2) (see picture), which are followed by synchrotron radiation (SR) experiments with a supersonic molecular beam (SMB).

Stereodynamics in dissociative adsorption of NO on Si(111)

We report results of our study on the surface-temperature dependence of the steric effect in the dissociative adsorption of NO on Si(111)-(7x7). Data presented here show that, at an incident energy of 58 meV, the reactive sticking probability for the N-end collision is larger than that for the O-end collision. Furthermore, this steric preference is quite sensitive to the surface temperature and the surface coverage. This study shows that the transient surface trapping into a shallow precursor well plays a key role in the stereodynamics of the dissociative adsorption at the low energy region.

Observed molecular alignment in gaseous streams and possible chiral effects in vortices and in surface scattering

Extensive work in this laboratory has been devoted to the study of intermolecular interactions from scattering experiments, in order to provide ingredients for modelling forces acting in systems involving hydrocarbons, the components of atmospheres, and water. Our detection of aligned oxygen in gaseous streams and further evidence on simple molecules has been extended to benzene and various hydrocarbons. Chiral effects can be seen in the differential scattering of oriented molecules, in particular from surfaces. It is pointed out that it may be of pre-biotical interest that we focus on possible mechanisms for chiral bio-stereochemistry of oriented reactants, for example when flowing in atmospheres of rotating bodies, specifically the planet earth, as well as in vortex motions of celestial objects. Molecular dynamics simulations and experimental verifications are in progress.

Stereodynamic Effects in the Adsorption of Propylene Molecules on Ag(001)

We report on the experimental evidence of the role of rotational alignment of the gas-phase molecules in the interaction of propylene with Ag(001). Molecular alignment has been controlled by a velocity selection of the impinging molecules, flying in a supersonic seeded molecular beam. The experimental findings indicate that at low surface coverage the sticking probability is independent of molecular alignment, while when coverage exceeds few percent of a monolayer, molecules impinging rotating parallel to the surface (helicopter-like configuration) achieve a higher chance to be trapped than those which impinge rotating perpendicularly (cartwheels). The sudden appearance of a large stereodynamic effect suggests that the adsorption proceeds via a mobile precursor state and is tentatively correlated with a change in the configuration of the added propylene molecules, which adsorb tilted rather than flat at the surface.

Dynamical steric effect in the decomposition of methyl chloride on a silicon surface

We report results of a study on the incident energy and the surface-temperature dependence of the steric effects in the dissociative adsorption of CH3Cl on a Si{100} surface. Data presented here show that the initial sticking probability for the Cl-end collision is larger at an incident energy of 120 meV than that in the CH3-end collision. Furthermore, this steric preference is quite sensitive to the kinetic energy and the rotational state of CH3Cl and the surface temperature. This study shows that the nonequilibrium surface trapping plays a key role in the initial step of the decomposition of CH3Cl on Si{100}.

Dynamics of propene adsorption on Ag(001)

The interaction of propene with Ag(001) is investigated by high resolution electron energy loss spectroscopy and supersonic molecular beam methods under ultra high vacuum conditions. Propene adsorbs molecularly at 110 K and desorbs intact leaving a clean surface after annealing to 160 K. Two adsorption sites, characterized by slightly different vibrational modes, exist. The low frequency species is observed already at low coverage for molecules impinging at strongly hyperthermal energies while at lower translational energy it appears only at high coverage. The initial sticking probability S(0) decreases with increasing translational energy, as appropriate for nonactivated adsorption systems. The angle and energy dependence of S(0) indicate that scaling is intermediate between total and normal energy. From the coverage dependence of the sticking probability we infer that both a nonthermal intrinsic and a thermal extrinsic precursor exist.