Gas Surface Chemical Reactions Research Articles

Spiers Memorial Lecture: New directions in molecular scattering.

The field of molecular scattering is reviewed as it pertains to gas-gas as well as gas-surface chemical reaction dynamics. We emphasize the importance of collaboration of experiment and theory, from which new directions of research are being pursued on increasingly complex problems. We review both experimental and theoretical advances that provide the modern toolbox available to molecular-scattering studies. We distinguish between two classes of work. The first involves simple systems and uses experiment to validate theory so that from the validated theory, one may learn far more than could ever be measured in the laboratory. The second class involves problems of great complexity that would be difficult or impossible to understand without a partnership of experiment and theory. Key topics covered in this review include crossed-beams reactive scattering and scattering at extremely low energies, where quantum effects dominate. They also include scattering from surfaces, reactive scattering and kinetics at surfaces, and scattering work done at liquid surfaces. The review closes with thoughts on future promising directions of research.

Numerical Simulation of Plasma Plume With Gas–Surface Interaction and Chemical Reaction Coupling Effect Based on DSMC-PIC Parallel Algorithm

The spatio-temporal evolution of plasma plumes is accompanied by a series of complex chemical reactions and gas–surface interactions that crucially affect the working characteristics and performance of plasma devices. The direct simulation Monte Carlo (DSMC) and particle-in-cell (PIC) hybrid parallel algorithm is often used to numerically simulate such phenomena. This article discusses the gas–surface interaction models of neutral molecules as elucidated by the parallel DSMC-PIC program. These models include the Maxwell model, the Cercignani–Lampis–Lord model, the complete diffuse reflection (CDR) gas–surface reflection model, and coupling with H combined reactions at the wall. Meanwhile, we consider the ion gas–surface interaction models, which include the CDR model and coupling of the four wall chemical reactions of ions. In addition, the dissociation reactions of H2 and recombination reactions of H are considered in the flow field. For various combinations of models, the simulations reveal the structural characteristics, number density, and coupling effects of gas–surface reflections, gas–surface chemical reactions, chemical reactions in the flow field and electric field, and spatio-temporal evolution of H2, H, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$X$ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{H}_{2}^{+}$ </tex-math></inline-formula> , and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{H}^{+}$ </tex-math></inline-formula> , which are shown by the structural characteristics of the number-density contours for different model combinations and analyzed in detail. The gas–surface interaction critically influences the number-density distribution near the wall and in the main flow. It also strongly affects the number-density distribution of light neutral molecules but not heavy neutral molecules and ions and produces the wall stacking effect. Apart from the stacking area, it has an indirect influence on the number-density distribution. The chemical reactions of ions on the wall affect the number-density distribution of all particles in the flow field and decisively affect the ions near the wall. Acceleration by the electric field intensifies the ion diffusion. The effect of coupling of gas–surface interaction and acceleration by the electric field is obvious. Via the number density and temperature, the chemical reactions in the flow field strongly affect the distribution of neutral molecules at the inlet; further, from the inlet, this effect weakens. Neutral particles and ions interact mainly through collisions.

Molecular Dynamics Study of Hydrogen Dissociation on Pd Surfaces using Reactive Force Fields

Developing a widely-used reactive force field is meaningful to explore the fundamental reaction mechanism on gas-surface chemical reaction dynamics due to its very high computational efficiency. We here present a study of hydrogen and its deuterated molecules dissociation on Pd surfaces based on a full-dimensional potential energy surface (PES) constructed by using a simple second moment approximation reactive force field (SMA RFF). Although the descriptions of the adsorbate-substrate interaction contain only the dissociation reaction of H2/Pd(111) system, a good transferability of SMA potential energy surface (PES) is shown to investigate the hydrogen dissociation on Pd(100). Our simulation results show that, the dissociation probabilities of H2 and its deuterated molecules on Pd(111) and Pd(100) surfaces keep non-monotonous variations with respect to the incident energy Ei, which is in good agreement with the previous ab initio molecular dynamics. Furthermore, for the oriented molecules, the dissociation probabilities of the oriented H2 (D2 and T2) molecule have the same orientation dependence behavior as those oriented HD (HT and DT) molecules.

Gas−Surface Chemical Reactions at High Collision Energies?

Most gas-surface chemical reactions occur via reaction of adsorbed species to form a thermal-energy ( approximately kT) product; however, some instances exist where an energetic projectile directly reacts with an adsorbate in a single-collision event to form a hyperthermal product (with a kinetic energy of a few eV). Here we show for the first time that 30-300 eV F(+) bombardment of fluorinated Ag and Si surfaces produces "ultrafast" F(2)(-) products with exit energies of up to 90 eV via a multistep direct-reaction mechanism. Experiments conclusively show that the projectile F atom ends up in the fast molecular product despite the fact that the impact energy is far greater than typical bond energies.

Dynamics of the formation of HD from D(H) atoms colliding with [formula omitted]: a model study of an Eley-Rideal reaction

It has long been postulated that gas-surface chemical reactions can occur upon a single gas-surface encounter, by an Eley-Rideal (ER) mechanism. In this paper, we describe results for the reaction of D(H) atoms with H(D) Cu(111) , which clearly confirmed that such direct gas-surface reactions indeed occur. We present angular and velocity distributions for the molecular products, as well as nascent internal-state distributions. We show that the angular distributions are asymmetric about the surface normal, providing proof of the direct nature of the reactions. We also show that the HD product has a very high degree of translational, vibrational, and rotational excitation, accounting for essentially all of the ∼ 2.3 eV available energy. Theses energies are in the order E trans > E vib > E rot . Strong isotope effects are observed, particularly in the angular and vibrational state distribution, where results for H-on-D are clearly different than those for D-on-H. We briefly compare our findings with the results of recent calculations.

The role of gas-phase in the laser etching of Cu by CCl4

In the laser etching of surfaces, the gas-surface chemical reaction used may be spontaneous or induced by a laser-gas interaction. We put into evidence a third mechanism allowing to use gas molecules that interact directly neither with the surface nor with the laser but that contains reactive atoms.

Distinguishing the Direct and Indirect Products of a Gas-Surface Reaction

It has long been postulated that gas-surface chemical reactions can occur by means of two distinct mechanisms: direct reaction on a single gas-surface encounter or reaction between two adsorbed species. It is shown here that these mechanisms have distinct dynamical signatures, as illustrated by the reaction of hydrogen with chlorine on gold(111). The direct reaction product leaves the surface with a high kinetic energy in a narrow angular distribution that displays a "memory" of the direction and energy of the incident hydrogen atom. The indirect reaction product has a near-thermal energy distribution and an angular distribution that is close to that of a cosine function.

CO2 laser deposition of diamond thin films on electronic materials

Laser-induced pyrolysis has been utilized to create gas-surface chemical reactions necessary for diamond deposition on electronic materials. A 1200 W CO2 gas laser has been used as an energy source for depositing diamond thin films from a gas mixture of CH4 and H2 in a chemical vapour deposition chamber. The substrate temperature was about 500°C. The laser beam energy was largely absorbed by the gases that lead to their excitation and decomposition on contact with the nearby hot substrate. Raman spectroscopy and scanning electron microscope analysis revealed high quality, fine crystalline diamond structures.

Ion- and electron-assisted gas-surface chemistry—An important effect in plasma etching

The extent to which gas-surface chemical reactions can be enhanced by energetic radiation (primarily ions and electrons) incident on the surface is described. Emphasis is placed on chemical systems which lead to volatile reaction products. In particular, the reactions of Si, SiO2, and Si3N4 with XeF2, F2, and Cl2 are examined experimentally. Possible mechanisms for the radiation-induced enhancement are discussed and some technological implications of this process in plasma etching technology and lithography are considered.