C. B. Mullins

Molecularly chemisorbed intermediates to oxygen adsorption on Pt(111): A molecular beam and electron energy-loss spectroscopy study

High translational energy adsorption of oxygen on the (111) surface of platinum was examined with electron energy loss spectroscopy (EELS) and molecular beam techniques. EEL spectra indicate that over an incident energy range of 0.2–1.37 eV and on a Pt(111) surface held at 77 K, oxygen adsorbs in an associative chemisorbed state—yielding to the dissociated state only after sufficient substrate heating. Simple direct dissociation appears negligible for all incident kinetic energies studied. At near-zero surface coverages, exclusive population of the peroxolike molecular precursor is observed for adsorption at these high translational energies, while both superoxolike and peroxolike forms are detected for low energy adsorption (0.055 eV). This peculiarity represents evidence that translational energy is effective in differentially populating reaction intermediates and provides better quantification of potential energy barriers to dissociation. We estimate the activation barrier for dissociation from the per...

Kinetics and dynamics of the dissociative chemisorption of oxygen on Ir(111)

The initial dissociative chemisorption probability, S0, of O2 on Ir(111) has been investigated with molecular beam techniques and electron energy loss spectroscopy (EELS). The adsorption process on the clean surface occurs by distinct dynamical mechanisms. At incident kinetic energies, Ei, of 0.1 eV and below, the dissociative chemisorption probability decreases with increasing kinetic energy, indicating the dominance of a trapping-mediated mechanism. A decrease in the value of S0 with increasing surface temperature, Ts, is also characteristic of this regime. This temperature dependence reflects the participation of a physically adsorbed state and molecularly chemisorbed state in the dissociation scheme. Additionally, the dependence of S0 on incident angle, θi, in the low kinetic energy regime exhibits near normal energy scaling. At high kinetic energy (Ei>0.1 eV), the initial dissociative chemisorption probability rises with increasing Ei indicating that translational energy is effective in surmounting a...

Oxygen adsorption on Si(100)-2×1 via trapping-mediated and direct mechanisms

We present the results from a molecular beam study of the initial adsorption probability (S0) of O2 on Si(100)-2×1 as a function of surface temperature, incident kinetic energy and angle. The data show two distinct kinetic energy regimes with opposite temperature and energy dependencies, and correspond to two different adsorption mechanisms. For low incident kinetic energies, a trapping-mediated mechanism is dominant, exhibiting a strong increase in S0 with decreasing surface temperature and kinetic energy. Also, adsorption at low kinetic energies is independent of incident angle, indicating total energy scaling. Data in this range are well-described by a simple precursor model, which gives a difference in activation barrier heights of (Ed−Ec)=28 meV, and a ratio of preexponentials νd/νc=24.2. Trapping probabilities can also be estimated from the model, and show a strong falloff with increasing energy, as would be expected. At high incident kinetic energies, a strong increase in S0 with kinetic energy ind...

The nucleation rate of crystalline ice in amorphous solid water

The kinetics of crystalline ice nucleation and growth in nonporous, molecular beam deposited amorphous solid water (ASW) films are investigated at temperatures near 140 K. We implement an experimental methodology and corresponding model of crystallization kinetics to decouple growth from nucleation and quantify the temperature dependence and absolute rates of both processes. Nucleation rates are found to increase from approximately 3x10(13) m(-3) s(-1) at 134 K to approximately 2x10(17) m(-3) s(-1) at 142 K, corresponding to an Arrhenius activation energy of 168 kJ/mol. Over the same temperature range, the growth velocity increases from approximately 0.4 to approximately 4 A s(-1), also exhibiting Arrhenius behavior with an activation energy of 47 kJ/mol. These nucleation rates are up to ten orders of magnitude larger than in liquid water near 235 K, while growth velocities are approximately 10(9) times smaller. Crystalline ice nucleation kinetics determined in this study differ significantly from those reported previously for porous, background vapor deposited ASW, suggesting the nucleation mechanism is dependent upon film morphology.

Thickness dependent crystallization kinetics of sub-micron amorphous solid water films

The kinetics of isothermal crystallization at the free surface of dense, 150–1050 bilayer (BL) (∼55–385 nm) thick amorphous solid water (ASW) films is investigated experimentally, and a model accounting for the observed thickness dependence is proposed. We find that as the ASW film thickness is increased above 150 BL, surface crystallization accelerates, rapidly at first and then more slowly until essentially size-independent kinetics are attained by 1050 BL. The potential origin of this thickness dependence is elucidated by a geometrical model of surface crystallization that we formulated using mechanistic information deduced from available experimental data. This simple mean-field model predicts that as film thickness is reduced below some critical value, the number of grains contributing to surface transformation progressively decreases, forcing each grain to convert a larger surface area and thus slowing crystallization. Good agreement between experimental data and the theory is realized using only tw...

EXPERIMENTAL STUDY OF CO OXIDATION BY AN ATOMIC OXYGEN BEAM ON PT(111), IR(111), AND RU(001)

Impinging O-atoms react with adsorbed CO on Pt(111), Ir(111), and Ru(001), to form CO2 at surface temperatures as low as 77 K. The initial reaction probability is measured on these three surfaces using reflectivity techniques and is much lower on Pt(111) than previously supposed. The reaction probability is measured as a function of surface temperature, incident O-atom flux, kinetic energy, and angle. Interestingly, a significant dependence on incident angle is observed on all surfaces (the reaction probability is ∼2.5 times greater at normal incidence than at glancing angles), and a kinetic energy effect is noted at the higher incident angles studied. Also, surface temperature is shown to have an effect on the reaction probability in measurements performed on Pt(111) and Ir(111) at normal incidence.

Trapping-mediated chemisorption of disilane on Si(100)-2×1

Disilane adsorption probabilities have been measured on Si(100)-2×1 over a wide range of incident kinetic energies, incident angles, and surface temperatures using supersonic molecular beam techniques. The trapping-mediated chemisorption mechanism is shown to be the dominant adsorption pathway under the conditions investigated. The first step in such a mechanism, namely trapping into the physical adsorption well, has been studied directly via measurements at a surface temperature of 77 K. As expected, the trapping probability drops with increasing kinetic energy, but nearly 50% of incident molecules trap at 1 eV incident energy, indicating that trapping is quite efficient over a wide range of translational energies. Chemisorption probability values measured at higher surface temperatures are fit to a simple trapping-mediated chemisorption model that can be used to predict adsorption probabilities over a wide range of conditions. Measurements of the chemisorption probability at 500 K are independent of inc...

Trapping dynamics of ethane on Si(100)-(2×1): Molecular beam experiments and molecular dynamics simulations

The trapping probability, or physical adsorption probability, of ethane on a clean Si(100)-(2×1) surface has been measured as a function of the incident translational energy and incident polar angle of the molecule at a surface temperature of 65 K. At all incident angles the trapping probability decreases as the translational energy of the incoming ethane molecule is increased from 0.05 to 1.3 eV. As the incident polar angle, with respect to the surface normal, is increased, the trapping probability decreases. This decrease in trapping probability with increasing polar angle contradicts the idea of normal energy scaling and has been seen in very few cases. Classical molecular dynamics calculations have been employed to study the cause of this unusual angular dependence. This simulation predicts trapping probabilities in good agreement with the experimental data. Analysis of the computed trajectories indicates that the initial site of impact within the unit cell, as well as energy exchange on initial impact with the surface, is important in determining the fate of an incident molecule. Normal momentum of the incident molecule is dissipated during the first impact much more efficiently than is parallel momentum. The simulations also indicate that the observed angular dependence can be explained in terms of parallel momentum accommodation. Large amounts of parallel momentum remaining after initial impact may be converted to normal momentum on subsequent impacts, causing molecules to scatter from the surface. Therefore, molecules that impact the surface at glancing angles and high translational kinetic energies are more likely to scatter from the surface than those at normal incidence or with lower translational kinetic energy.

Adsorption mechanisms of translationally-energetic O2 and N2: direct dissociation versus direct molecular chemisorption

Abstract A direct dissociation mechanism has been traditionally assigned to molecular beam data that exhibit an increase in the initial adsorption probability with increasing kinetic energy. Yet, recent experiments of nitrogen and oxygen adsorption provide support for an alternative high kinetic energy pathway in which incident energy assists in surmounting barriers to molecular chemisorption on a surface as the first step to dissociation. Moreover, systems for which the experimental evidence supports such a mechanism also demonstrate that molecularly chemisorbed intermediates can be spectroscopically observed at low temperatures and coverages from exposure to a gas in thermal equilibrium at room temperature. Likewise, such observations have not been measured for systems which are consistent with direct dissociation. A consideration of this trend regarding the existence of molecularly chemisorbed states and the implications for the dominant, dissociative chemisorption pathway at high kinetic energy is presented for a number of gas surface systems.

Angular dependence of the dynamic displacement of O2 from Pt(111) by atomic oxygen

Impingement of 16O-atoms on an 18O2 covered Pt(111) surface at 77 K induces the evolution of gas-phase 18O2 and 16O18O in a ratio of ∼4:1 independent of atomic flux or kinetic energy. The total initial probability of desorption of molecular oxygen induced by impingement of atomic oxygen is measured as a function of incident O-atom flux, kinetic energy, and angle. Interestingly, the probability is found to depend on incident angle with values ranging from 0.25±0.02 for a 60° angle of incidence to 0.42±0.02 for normal incidence.