Vibrational activation in direct and precursor-mediated chemisorption of SiH4 on Si(100)

State-resolved studies of CO2 sticking to CO2 ice

Internal vibrations may affect the adsorption, scattering, and reactions of molecules impinging onto a surface. The energy of the ν 3 antisymmetric stretch vibration of CO2 slightly exceeds the desorption energy of CO2 bound to CO2 ice. We use supersonic molecular beam techniques and rovibrationally state-resolved excitation to determine whether this vibration affects condensation of gas phase CO2 to its ice. We detect sticking and CO2 ice formation using RAIRS and quantify the sticking probability using the King and Wells method with modulation of the vibrational excitation and Fourier transform based detection. We find that the influence of this vibration on the structure of the formed ice and on the sticking probability is negligible under our conditions. Based on our detection limit, we quantify the weighted average sticking probability at approximately 0.9 and the difference between the state-resolved and weighted average sticking probability as below 0.03%.

Fundamental mechanisms for molecular energy conversion and chemical reactions at surfaces

The dream of theoretical surface chemistry is to predict the outcome of reactions in order to find the ideal catalyst for a certain application. Having a working ab initio theory in hand would not only enable these predictions but also provide insights into the mechanisms of surface reactions. The development of theoretical models can be assisted by experimental studies providing benchmark data. Though for some reactions a quantitative agreement between experimental observations and theoretical calculations has been achieved, theoretical surface chemistry is in general still far away from gaining predictive power. Here we review recent experimental developments towards the understanding of surface reactions. It is demonstrated how quantum-state resolved scattering experiments on reactive and nonreactive systems can be used to test front-running theoretical approaches. Two challenges for describing dynamics at surfaces are addressed: nonadiabaticity in diatomic molecule surface scattering and the increasing system size when observing and describing the dynamics of polyatomic molecules at surfaces. Finally recent experimental studies on reactive systems are presented. It is shown how elementary steps in a complex surface reaction can be revealed experimentally.

Quantum state-resolved gas/surface reaction dynamics probed by reflection absorption infrared spectroscopy

We report the design and characterization of a new molecular-beam/surface-science apparatus for quantum state-resolved studies of gas/surface reaction dynamics combining optical state-specific reactant preparation in a molecular beam by rapid adiabatic passage with detection of surface-bound reaction products by reflection absorption infrared spectroscopy (RAIRS). RAIRS is a non-invasive infrared spectroscopic detection technique that enables online monitoring of the buildup of reaction products on the target surface during reactant deposition by a molecular beam. The product uptake rate obtained by calibrated RAIRS detection yields the coverage dependent state-resolved reaction probability S(θ). Furthermore, the infrared absorption spectra of the adsorbed products obtained by the RAIRS technique provide structural information, which help to identify nascent reaction products, investigate reaction pathways, and determine branching ratios for different pathways of a chemisorption reaction. Measurements of the dissociative chemisorption of methane on Pt(111) with this new apparatus are presented to illustrate the utility of RAIRS detection for highly detailed studies of chemical reactions at the gas/surface interface.

An effusive molecular beam technique for studies of polyatomic gas-surface reactivity and energy transfer

An effusive molecular beam technique is described to measure alkane dissociative sticking coefficients, S(T(g), T(s); ϑ), on metal surfaces for which the impinging gas temperature, T(g), and surface temperature, T(s), can be independently varied, along with the angle of incidence, ϑ, of the impinging gas. Effusive beam experiments with T(g) = T(s) = T allow for determination of angle-resolved dissociative sticking coefficients, S(T; ϑ), which when averaged over the cos (ϑ)/π angular distribution appropriate to the impinging flux from a thermal ambient gas yield the thermal dissociative sticking coefficient, S(T). Nonequilibrium S(T(g), T(s); ϑ) measurements for which T(g) ≠ T(s) provide additional opportunities to characterize the transition state and gas-surface energy transfer at reactive energies. A resistively heated effusive molecular beam doser controls the T(g) of the impinging gas striking the surface. The flux of molecules striking the surface from the effusive beam is determined from knowledge of the dosing geometry, chamber pressure, and pumping speed. Separate experiments with a calibrated leak serve to fix the chamber pumping speed. Postdosing Auger electron spectroscopy is used to measure the carbon of the alkyl radical reaction product that is deposited on the surface as a result of alkane dissociative sticking. As implemented in a typical ultrahigh vacuum chamber for surface analysis, the technique has provided access to a dynamic range of roughly 6 orders of magnitude in the initial dissociative sticking coefficient for small alkanes on Pt(111).