Faraday Discussions最新文献

The rotational excitation of a singly deuterated water molecule (HDO) by a heavy atom (Ne) and a light diatomic molecule (H₂) is investigated theoretically and experimentally in the near-threshold regime. Crossed-molecular-beam measurements with a variable crossing angle are compared to close-coupling calculations based on high-accuracy potential energy surfaces. The two lowest rotational transitions, 0₀₀ → 1₀₁ and 0₀₀ → 1₁₁, are probed in detail and a good agreement between theory and experiment is observed for both transitions in the case of HDO + Ne, where scattering resonances are however blurred out experimentally. In the case of HDO + H₂, the predicted theoretical overlapping resonances are faithfully reproduced by experiment for the 0₀₀ → 1₁₁ transition, while the calculated strong signal for the 0₀₀ → 1₀₁ transition is not detected. Future work is needed to reconcile this discrepancy.

The specific geometry of a molecule can have a pronounced influence on its chemical reactivity. However, experimental data on reactions of individual molecular isomers are still sparse because they are often difficult to separate and frequently interconvert into one another under ambient conditions. Here, we employ a novel crossed-beam experiment featuring an electrostatically controlled molecular beam combined with a source for radicals and metastables to spatially separate the cis and trans stereoisomers as well as individual rotational states of 1,2-dibromoethene and study their specific reactivities in the chemi-ionisation reaction with excited neon atoms. The experiments reveal pronounced isomeric and rotational specificities in the rates and product branching ratios of the reaction. The present study underlines the importance and combined role of molecular geometry and of rotational motion in the dynamics of chemi-ionisation reactions.

Mirrors for atoms and molecules are essential tools for matter-wave optics with neutral particles. Their realization has required either a clean and atomically smooth crystal surface, sophisticated tailored electromagnetic fields, nanofabrication, or particle cooling because of the inherently short de Broglie wavelengths and strong interactions of atoms with surfaces. Here, we demonstrate reflection of He atoms from inexpensive, readily available, and robust gratings designed for light waves. Using different types of blazed gratings with different periods, we study how microscopic and macroscopic grating properties affect the mirror performance. A holographic grating with 417 nm period shows reflectivity up to 47% for He atoms, demonstrating that commercial gratings can serve as mirrors for thermal energy atoms and molecules. We also observe reflection of He₂ and He₃ which implies that the grating might also function as a mirror for other breakable particles that, under typical conditions, do not scatter nondestructively from a solid surface such as, e.g., metastable atoms or antihydrogen atoms.

We investigate the collisions of different alcohol molecules with hydrated nitric acid clusters using a molecular beam experiment and molecular dynamics simulations. The uptake cross sections σ_p for the molecules evaluated from the experiment are in excellent agreement with the simulations. This suggests that (i) the nontrivial assumptions implemented in the evaluation procedure of the experimental data are valid, and (ii) the simulations describe correctly the major processes in the molecule–cluster collisions. We observe that σ_p decreases with the increasing alkyl chain length of the alcohol, and also with the branching of the molecules that have the same mass but different structures. These systematic trends can be rationalized based on the accessibility of the hydrophilic OH group, which decreases with the increasing chain length and steric hindrance. The observed trends and their interpretation differ significantly from the simple model of hard-sphere collisions. The obtained data shall be beneficial not only for the fundamental understanding of the molecule–cluster collisions, but also in the modelling of atmospheric new-particle formation and aerosol growth.

We extend the use of our recently developed Near-Ambient Pressure Velocity Map Imaging (NAP-VMI) technique to study the kinetics and dynamics of catalytic reactions in the pressure gap. As an example, we show that NAP-VMI combined with molecular beam surface scattering allows the direct measurement of time- and velocity-resolved kinetics of the scattering and oxidation of CO on the Pd(110) surface with oxygen pressures at the surface up to 1 × 10⁻⁵ mbar, where different metastable surface structures form. Our results show that the c(2 × 4) oxide structure formed at low O₂ pressure is highly active for CO oxidation. The velocity distribution of the CO₂ products shows the presence of two reaction channels, which we attribute to reactions starting from two distinct but rapidly interconverting CO binding sites. The effective CO oxidation reaction activation energy is E_r = (1.0 ± 0.13) eV. The CO₂ production is suppressed at higher O₂ pressure due to the number of antiphase domain boundaries increasing, and the missing row sites are filled by O-atoms at O₂ pressures approaching 1 × 10⁻⁶ mbar. Filling of these sites by O-atoms reduces the CO surface lifetime, meaning the surface oxide is inactive for CO oxidation. We briefly outline further developments planned for the NAP-VMI and its application to other types of experiments.

Density functional theory (DFT) at the generalized gradient approximation (GGA) level is often considered the best compromise between feasibility and accuracy for reactions of molecules on metal surfaces. Recent work, however, strongly suggests that density functionals (DFs) based on GGA exchange are not able to describe molecule–metal surface reactions for which the work function of the metal surface minus the electron affinity of the molecule is less than 7 eV. Systems for which this is true exhibit an increased charge transfer from the metal to the molecule at the transition state, increasing the delocalisation of the electron density. This enlarged delocalisation can cause GGA-DFT to underestimate energy values relative to the gas-phase and thus underestimate the barrier height, similar to what has been observed for several gas-phase reactions. An example of such a molecule–metal surface system is O₂ + Al(111). Following a similar strategy as for gas-phase reactions, previous work showed results of increased accuracy when using a screened hybrid DF for O₂ + Al(111). However, even screened hybrid DFs are computationally expensive to use for metal surfaces. To resolve this, we test a non-self-consistent field (NSCF) screened hybrid DF approach. This approach computes screened hybrid DFT energies based on self-consistent-field (SCF) GGA electronic densities. Here, we explore the accuracy of the NSCF screened hybrid DF approach by implementing the NSCF HSE03-1/3x@RPBE DF for O₂ + Al(111). We compute and analyse molecular beam sticking probabilities as well as a set of sticking probabilities for rotationally aligned O₂. Our results show that the NSCF approach results in reaction probability curves that reproduce SCF results with near-chemical accuracy, suggesting that the NSCF approach can be used advantageously for exploratory purposes. An analysis of the potential energy surface and the barriers gives insight into the cause of the disagreement between the SCF and NSCF reaction probabilities and into the changes needed in theoretical modelling to further improve the description of the O₂ + Al(111) system. Finally, the hole model yields fair agreement with dynamics results for the reaction probability curve, but results in an increased slope of the reaction probability curve compared to the molecular dynamics, with a shift to lower or higher energies depending on whether the vibrational energy of the molecule is included in the initial energy of the molecule or not.

Electron-induced dissociation of a fluorocarbon adsorbate CF₃ (ad) at 4.6 K is shown by Scanning Tunnelling Microscopy (STM) to form directed energetic F-atom ‘projectiles’ on Cu(110). The outcome of a collision between these directed projectiles and stationary co-adsorbed allyl ‘target’ molecules was found through STM to give rotational excitation of the target allyl, clockwise or anti-clockwise, depending on the chosen collision geometry. Molecular dynamics computation linked the collisional excitation of the allyl target to an ‘abortive chemical reaction’, in which the approach of the F-projectile stretched an H–C bond lifting the allyl above the surface, facilitating isomerization from ‘Across’ to ‘Along’ a Cu row.

Ion–molecule reactions are an essential contributor to the chemistry of a diverse range of environments. While a great deal of work has been done to understand the fundamental mechanisms driving these reactions, there is still much more to discover. Here, we expand upon prior studies on ion–molecule reactions involving two isomers of C₃H₄, allene (H₂C₃H₂) and propyne (H₃C₃H). Specifically, we probe the previously observed isomeric dependent reactivity of these molecules by reacting them with two ions with nearly identical ionization potentials, Xe⁺ and O₂⁺. Our goal is to determine if the isomer-dependent reaction mechanisms previously observed are universal for C₃H₄ or if they depend on the ion character as well. Through the combination of experimental measurements and theoretical calculations, we found that both isomeric structure and identity of the ion contribute to the propensity of a reaction complex forming or for only long-range charge transfer to occur.