Ion and velocity map imaging for surface dynamics and kinetics.

We describe a new instrument that uses ion imaging to study molecular beam-surface scattering and surface desorption kinetics, allowing independent determination of both residence times on the surface and scattering velocities of desorbing molecules. This instrument thus provides the capability to derive true kinetic traces, i.e., product flux versus residence time, and allows dramatically accelerated data acquisition compared to previous molecular beam kinetics methods. The experiment exploits non-resonant multiphoton ionization in the near-IR using a powerful 150-fs laser pulse, making detection more general than previous experiments using resonance enhanced multiphoton ionization. We demonstrate the capabilities of the new instrument by examining the desorption kinetics of CO on Pd(111) and Pt(111) and obtain both pre-exponential factors and activation energies of desorption. We also show that the new approach is compatible with velocity map imaging.

Single-field slice-imaging with a movable repeller: photodissociation of N2O from a hot nozzle.

We present a new photo-fragment imaging spectrometer, which employs a movable repeller in a single field imaging geometry. This innovation offers two principal advantages. First, the optimal fields for velocity mapping can easily be achieved even using a large molecular beam diameter (5 mm); the velocity resolution (better than 1%) is sufficient to easily resolve photo-electron recoil in (2 + 1) resonant enhanced multiphoton ionization of N2 photoproducts from N2O or from molecular beam cooled N2. Second, rapid changes between spatial imaging, velocity mapping, and slice imaging are straightforward. We demonstrate this technique's utility in a re-investigation of the photodissociation of N2O. Using a hot nozzle, we observe slice images that strongly depend on nozzle temperature. Our data indicate that in our hot nozzle expansion, only pure bending vibrations--(0, v2, 0)--are populated, as vibrational excitation in pure stretching or bend-stretch combination modes are quenched via collisional near-resonant V-V energy transfer to the nearly degenerate bending states. We derive vibrationally state resolved absolute absorption cross-sections for (0, v2 ≤ 7, 0). These results agree well with previous work at lower values of v2, both experimental and theoretical. The dissociation energy of N2O with respect to the O((1)D) + N2¹Σ(g)⁺ asymptote was determined to be 3.65 ± 0.02 eV.

Six-dimensional dynamics study of reactive and non reactive scattering of H(2) from Cu(111) using a chemically accurate potential energy surface.

We have studied the interaction of H(2) on Cu(111) using quasi-classical and quantum dynamics, and a chemically accurate six-dimensional potential energy surface (PES). The PES was computed using the specific reaction parameter (SRP) approach to density functional theory (DFT), in an implementation adapted to molecules interacting with metal surfaces. To perform this study we have applied the Born-Oppenheimer static surface (BOSS) approximation, i.e., we used both the Born-Oppenheimer (BO) and the static surface (SS) approximations. We show that our theoretical approach accurately describes experiments on dissociative adsorption, the effect of molecular vibrational and rotational motion on dissociative (associative) adsorption (desorption), and rotational excitation upon scattering. More specifically, dynamics calculations on reactive scattering of H(2) reproduce reaction probabilities measured in molecular beam experiments, effective barrier heights describing the dependence of reaction on the initial rovibrational state, and data on rotationally inelastic scattering with chemical accuracy (i.e., within 1 kcal mol(-1) approximately 4.2 kJ mol(-1)). These processes are not affected much by surface motion, either because they were measured using a low surface temperature, T(s), or because the computed observable is independent of T(s). However, we show that to account for the dependence of molecular orientation on a reaction the inclusion of surface motion is required. We have also found that vibrational excitation is poorly described within the BOSS approximation, suggesting a breakdown of this approximation.

Chemically accurate simulation of a prototypical surface reaction: H2 dissociation on Cu(111).

Methods for accurately computing the interaction of molecules with metal surfaces are critical to understanding and thereby improving heterogeneous catalysis. We introduce an implementation of the specific reaction parameter (SRP) approach to density functional theory (DFT) that carries the method forward from a semiquantitative to a quantitative description of the molecule-surface interaction. Dynamics calculations on reactive scattering of hydrogen from the copper (111) surface using an SRP-DFT potential energy surface reproduce data on the dissociative adsorption probability as a function of incidence energy and reactant state and data on rotationally inelastic scattering with chemical accuracy (within approximately 4.2 kilojoules per mole).

Inverse velocity dependence of vibrationally promoted electron emission from a metal surface.

All previous experimental and theoretical studies of molecular interactions at metal surfaces show that electronically nonadiabatic influences increase with molecular velocity. We report the observation of a nonadiabatic electronic effect that follows the opposite trend: The probability of electron emission from a low-work function surface--Au(111) capped by half a monolayer of Cs--increases as the velocity of the incident NO molecule decreases during collisions with highly vibrationally excited NO(X(2)pi((1/2)), V = 18; V is the vibrational quantum number of NO), reaching 0.1 at the lowest velocity studied. We show that these results are consistent with a vibrational autodetachment mechanism, whereby electron emission is possible only beyond a certain critical distance from the surface. This outcome implies that important energy-dissipation pathways involving nonadiabatic electronic excitations and, furthermore, not captured by present theoretical methods may influence reaction rates at surfaces.

The work function of submonolayer cesium-covered gold: a photoelectron spectroscopy study.

Using visible and x-ray photoelectron spectroscopy, we measured the work function of a Au(111) surface at a well-defined submonolayer coverage of Cs. For a Cs coverage producing a photoemission maximum with a He-Ne laser, the work function is 1.61+/-0.08 eV, consistent with previous assumptions used to analyze vibrationally promoted electron emission. A discussion of possible Cs layer structures is also presented.

Observation of a change of vibrational excitation mechanism with surface temperature: HCl collisions with Au(111).

We have measured the vibrational excitation probability (Pv) of HCl incident on a Au(111) surface at kinetic energies (Ei) of 0.59 eV to 1.37 eV and surface temperatures (Ts) of 273 K to 1073 K. For all energies, the slope of the Pv as a function of Ts exhibits a sharp increase above Ts approximately 800 K. We show this change in slope and the threshold behavior of Pv to be consistent with a change in excitation mechanism from an electronically adiabatic mechanical mechanism to an electronically nonadiabatic mechanism involving excited electron-hole pairs.

Vibrational promotion of electron transfer.

By using laser methods to prepare specific quantum states of gas-phase nitric oxide molecules, we examined the role of vibrational motion in electron transfer to a molecule from a metal surface free from the complicating influence of solvation effects. The signature of the electron transfer process is a highly efficient multiquantum vibrational relaxation event, where the nitrogen oxide loses hundreds of kilojoules per mole of energy on a subpicosecond time scale. These results cannot be explained simply on the basis of Franck-Condon factors. The large-amplitude vibrational motion associated with molecules in high vibrational states strongly modulates the energetic driving force of the electron transfer reaction. These results show the importance of molecular vibration in promoting electron transfer reactions, a class of chemistry important to molecular electronics devices, solar energy conversion, and many biological processes.

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.

UHV application of spring-loaded Teflon seals.

Large, bakeable, rotatable ultrahigh vacuum seals capable of operation at 10(-11) Torr are described. They employ doubly differentially pumped commercially available spring-loaded Teflon seals. A static version is very convenient for quick access as it requires nearly negligible sealing force other than atmospheric force.