Quantum state resolved molecular beam reflectivity measurements: CH4 dissociation on Pt(111)

The King and Wells molecular beam reflectivity method has been used for a quantum state resolved study of the dissociative chemisorption of CH4 on Pt(111) at several surface temperatures. Initial sticking coefficients were measured for incident CH4 prepared both with a single quantum of antisymmetric stretch vibration by infrared laser pumping, and without laser excitation. Vibrational excitation of the mode is observed to be less efficient than incident translational energy in promoting the dissociation reaction with a vibrational efficacy = 0.65. The initial state resolved sticking coefficient was found to be independent of the surface temperature over the 50kJ/mol to 120kJ/mol translational energy range studied here. However, the surface temperature dependence of the King and Wells data reveals the migration of adsorbed carbon formed by CH4 dissociation on the Pt(111) surface leading to the growth of carbon particles

High resolution stimulated raman spectroscopy

The high resolution stimulated Raman spectrometer at OSU was modified to improve its sensitivity and to extend its range of applications to low Raman shifts and to the spectroscopy of solids and liquids. As part of the characterization of the spectrometer, optical Stark effects on rotational and vibrational transitions of N₂ were investigated and the pure rotational spectrum of C₄N₂ was recorded for the first time. Stimulated Raman spectroscopy (SRS) was used for the first time for high resolution Raman spectroscopy of cold molecular liquids and crystals. Accurate measurements of the linewidth and the Raman shift for the very narrow vibrational transition in condensed nitrogen (liquid, solid-β, and solid-α phases) were made between 110 and 15 K. These measurements establish the temperature dependence of the N₂ vibrational frequency and linewidth for the condensed phases and show the effect of the phase transitions on the vibrational Raman spectrum. This data was used in the analysis of SRS spectra of large N₂ clusters formed in free jet expansions which were observed for the first time with the SRS technique. By probing at different points along the axis of the expansion changes in the vibrational Raman spectrum are observed which signify the formation of liquid aggregates, followed by strong supercooling and subsequent freezing to the solid beta phase. High spatial resolution of the SRS probing results in good time resolution for the observation of the extremely fast condensation and freezing phase transitions. Aggregate temperatures are determined by using the temperature dependence of the vibrational frequency measured in the static samples and the assumption that the observed clusters are large enough to be treated as bulk material. That this assumption is valid is shown by several independent size estimates which indicate that the means cluster diameter is in the range of 50 - 500 nm. Monomer temperatures are determined from the rotational Q-branch structure and the extent of aggregation is found to be about 10 % from the relative Raman intensity of monomer and cluster spectra. The results show that SRS offers unique diagnostic capabilities for the study of condensation processes in free jet expansions

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

International audienceWe 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

The sticking probability of D2O-water on ice: Isotope effects and the influence of vibrational excitation

International audienceThe present study measures the sticking probability of heavy water (D2O) on H2O- and on D2O-ice and probes the influence of selective OD-stretch excitation on D2O sticking on these ices. Molecular beam techniques are combined with infrared laser excitation to allow for precise control of incident angle, translational energy, and vibrational state of the incident molecules. For a translational energy of 69 kJ/mol and large incident angles (θ ≥ 45°), the sticking probability of D2O on H2O-ice was found to be 1% lower than on D2O-ice. OD-stretch excitation by IR laser pumping of the incident D2O molecules produces no detectable change of the D2O sticking probability (<10−3). The results are compared with other gas/surface systems for which the effect of vibrational excitation on trapping has been probed experimentally

CHD3 Dissociation on the Kinked Pt(210) Surface: A Comparison of Experiment and Theory

To be able to simulate activated heterogeneously catalyzed reactions on the edge and corner sites of nanoparticles, a method for calculating accurate activation barriers for the reactions is required. We have recently demonstrated that a semiempirical specific reaction parameter (SRP) density functional developed to describe CHD3 dissociation on a flat Ni(111) surface is transferable to describing the same reaction on a stepped Pt(211) surface. In the current work, we compare initial sticking coefficients measured using the King and Wells beam reflectivity technique and calculated from ab initio molecular dynamics trajectories using the same SRP functional for CHD3 dissociation on a kinked Pt(210) surface at a temperature of 650 K. The calculated sticking coefficients overestimate those determined experimentally, with an average energy shift between the two curves of 13.6 kJ/mol, which is over a factor of 3 times higher than the 4.2 kJ/mol limit that defines chemical accuracy. This suggests the SRP functional predicts an activation barrier that is too low for the dissociation on the least coordinated kink atom, which is the site of the lowest energy transition state and where most of the dissociation occurs in the calculations

Dynamics of methane dissociation on transition metals

One of the many contributions of Harold Winters to surface science was his pioneering ultrahigh vacuum study on the kinetics of the technologically important dissociation of CH4 on transition metals in the 1970s. He observed a dramatic activation of the dissociation with surface temperature alone and a huge isotope effect and suggested a simple dynamical model to rationalize his results. Since that time, our general understanding of the dynamics of gas-surface dissociations has exploded due to experimental advances (e.g., molecular beam and eigenstate resolved studies) and theoretical advances (quantum or classical dynamics on ab initio potential energy surfaces). This review tries to highlight how our understanding of the dynamics of CH4 dissociation on transition metals has matured since Harold’s pioneering experiments and original model

Steric Effects in the Chemisorption of Vibrationally Excited Methane on Ni(100)

Newly available, powerful infrared laser sources enable the preparation of intense molecular beams of quantum-state prepared and aligned molecules for gas/surface reaction dynamics experiments. We present a stereodynamics study of the chemisorption of vibrationally excited methane on the (100) surface of nickel. Using linearly polarized infrared excitation of the C-H stretch modes of two methane isotopologues [CH4(n3) and CD3H(n1)], we aligned methane’s angular momentum and vibrational transition dipole moment in the laboratory frame. An increase in methane reactivity of as much as 60% is observed when the laser polarization is parallel rather than normal to the surface. The dependence of the alignment effect on the rotational branch used for excitation indicates that alignment of the vibrational transition dipole moment of methane is responsible for the steric effect. Potential explanations for the steric effect in terms of an alignment-dependent reaction barrier height or electronically nonadiabatic effects are discussed

Incident Angle Dependence of CHD3Dissociation on the Stepped Pt(211) Surface

The dissociation of methane on transition metal surfaces is not only of fundamental interest but also of industrial importance as it represents a rate-controlling step in the steam-reforming reaction used commercially to produce hydrogen. Recently, a specific reaction parameter functional (SRP32- vdW) has been developed, which describes the dissociative chemisorption of CHD3 at normal incidence on Ni(111), Pt(111), and Pt(211) within chemical accuracy (4.2 kJ/mol). Here, we further test the validity of this functional by comparing the initial sticking coefficients (S0), obtained from ab-initio molecular dynamics calculations run using this functional, with those measured with the King and Wells method at different angles of incidence for CHD3 dissociation on Pt(211). The two sets of data are in good agreement, demonstrating that the SRP32-vdW functional also accurately describes CHD3 dissociation at off-normal angles of incidence. When the direction of incidence is perpendicular to the step edges, an asymmetry is seen in the reactivity with respect to the surface normal, with S0 being higher when the molecule is directed toward the (100) step rather than the (111) terrace. Although there is a small shadowing effect, the trends in S0 can be attributed to different activation barriers for different surface sites, which in turn is related to the generalized co-ordination numbers of the surface atom to which the dissociating molecule is adsorbed in the transition state. Consequently, most reactivity is seen on the least co-ordinated step atoms at all angles of incidence