Using ion imaging to measure velocity distributions in surface scattering experiments.

We present a new implementation of ion imaging for the study of surface scattering processes. The technique uses a combination of spatial ion imaging with laser slicing and delayed pulsed extraction. The scattering velocities of interest are parallel to the imaging plane, allowing speed and angular distributions to be extracted from a single image. The first results of direct scattering of N-2 from a clean, single-crystal Au(111) surface are reported, and the speed resolution is shown to be competitive with current state-of-the-art time-of-flight methods for velocity measurements while providing simultaneous measurements of in-plane angular distributions

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

Rotationally resolved vacuum ultraviolet resonance-enhanced multiphoton ionization (VUV REMPI) of acetylene via the G̃ Rydberg state.

We present a 1 + 1′ resonance-enhanced multiphoton ionization (REMPI) scheme for acetylene via the linear G̃ 4sσ 1Πu Rydberg state, offering partial rotational resolution and the possibility to detect excitation in both the cis- and trans-bending modes. The resonant transition to the G̃ state is driven by a vacuum ultraviolet (VUV) photon, generated by resonant four-wave mixing (FWM) in krypton. Ionization from the short-lived G̃ state then occurs quickly, driven by the high intensity of the residual light from the FWM process. We have observed nine bands in the region between 79 200 cm–1 and 80 500 cm–1 in C2H2 and C2D2. We compare our results with published spectra in this region and suggest alternative assignments for some of the Renner–Teller split bands. Similar REMPI schemes should be applicable to other small molecules with picosecond lifetime Rydberg states

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, v 2, 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, v 2 ≤ 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(1D) + N2 Σg+1 asymptote was determined to be 3.65 ± 0.02 eV

Probing the effect of surface strain on CO binding to Pd thin films.

We report measurements to investigate the effects of mechanical strain on the binding energy of carbon monoxide (CO) on the (111) surface of a 16 nm thin film of palladium (Pd) grown on rutile titanium dioxide (r-TiO2). The lattice mismatch between Pd and the r-TiO2 leads to a tensile mechanical in-plane stress in the Pd layer of approximately 0.38 GPa. We observe an increase of (40 ± 10) kJ mol–1 in the CO binding energy for the 16 nm sample compared to a bulk Pd(111) crystal, which is in qualitative agreement with expectations based on the d-band model

The kinetics of elementary thermal reactions in heterogeneous catalysis.

The kinetics of elementary reactions is fundamental to our understanding of catalysis. Just as microkinetic models of atmospheric chemistry provided the predictive power that led to the Montreal Protocol reversing loss of stratospheric ozone, pursuing a microkinetic approach to heterogeneous catalysis has tremendous potential for societal impact. However, the development of this approach for catalysis faces great challenges. Methods for measuring rate constants are quite limited, and the present predictive theoretical methods remain largely unvalidated. Here, we present a short Perspective on recent experimental advances in the measurement of rates of elementary reactions at surfaces that rely on a stroboscopic pump-probe concept for neutral matter. We present the principles behind successful measurement methods and discuss a recent implementation of those principles. The topic is discussed within the context of a specific but highly typical surface reaction, CO oxidation on Pt, which, despite more than 40 years of study, was only clarified after experiments with velocity-resolved kinetics became possible. This deceptively simple reaction illustrates fundamental lessons concerning the coverage dependence of activation energies, the nature of reaction mechanisms involving multiple reaction sites, the validity of transition-state theory to describe reaction rates at surfaces and the dramatic changes in reaction mechanism that are possible when studying reactions at low temperatures

Velocity-resolved kinetics of site-specific carbon monoxide oxidation on platinum surfaces.

Catalysts are widely used to increase reaction rates. They function by stabilizing the transition state of the reaction at their active site, where the atomic arrangement ensures favourable interactions 1 . However, mechanistic understanding is often limited when catalysts possess multiple active sites-such as sites associated with either the step edges or the close-packed terraces of inorganic nanoparticles2-4-with distinct activities that cannot be measured simultaneously. An example is the oxidation of carbon monoxide over platinum surfaces, one of the oldest and best studied heterogeneous reactions. In 1824, this reaction was recognized to be crucial for the function of the Davy safety lamp, and today it is used to optimize combustion, hydrogen production and fuel-cell operation5,6. The carbon dioxide products are formed in a bimodal kinetic energy distribution7-13; however, despite extensive study 5 , it remains unclear whether this reflects the involvement of more than one reaction mechanism occurring at multiple active sites12,13. Here we show that the reaction rates at different active sites can be measured simultaneously, using molecular beams to controllably introduce reactants and slice ion imaging14,15 to map the velocity vectors of the product molecules, which reflect the symmetry and the orientation of the active site 16 . We use this velocity-resolved kinetics approach to map the oxidation rates of carbon monoxide at step edges and terrace sites on platinum surfaces, and find that the reaction proceeds through two distinct channels11-13: it is dominated at low temperatures by the more active step sites, and at high temperatures by the more abundant terrace sites. We expect our approach to be applicable to a wide range of heterogeneous reactions and to provide improved mechanistic understanding of the contribution of different active sites, which should be useful in the design of improved catalysts