Molecular Beam Scattering Research Articles

Evaporation and Molecular Beam Scattering from a Flat Liquid Jet.

An experimental setup for molecular beam scattering from flat liquid sheets has been developed, with the goal of studying reactions at gas-liquid interfaces for volatile liquids. Specifically, a crossed molecular beam instrument that can measure angular and translational energy distributions of scattered products has been adapted for liquid jet scattering. A microfluidic chip is used to create a stable flat liquid sheet inside vacuum from which scattering occurs, and both evaporation and scattering from this sheet are characterized using a rotatable mass spectrometer that can measure product time-of-flight distributions. This article describes the instrument and reports on the first measurements of evaporation of dodecane and Ne from a Ne-doped dodecane flat jet, as well as scattering of Ne from a flat jet of pure dodecane.

Molecular Beam Scattering Experiments as a Sensitive Probe of the Interaction in Bromine-Noble Gas Complexes.

This paper reports for the first time molecular beam experiments for the scattering of He, Ne, and Ar by the Br2 molecule, with the aim of probing in detail the intermolecular interaction. Measurements have been performed under the experimental condition to resolve the glory pattern, a quantum interference effect observable in the collision velocity dependence of the integral cross section. We analyzed the experimental data with a reliable potential model defined as a combination of an anisotropic van der Waals component with the additional contribution due to charge transfer and polar flattening effects related to the formation of an intermolecular halogen bond. The model involves few parameters, whose values are related to fundamental physical properties of the interacting partners, and it allows an internally consistent comparison of the stability of the gas-phase adducts formed by Br2 moiety with different noble gases as well as homologous complexes with the Cl2 molecule. The same model appears to be also easily generalized to describe the interaction of diatomic halogen molecules in the excited B(3Π) electronic state where the halogen bond contribution tends to vanish and more anisotropic van der Waals components dominate the structure of the complexes with noble gases.

Note: A versatile mass spectrometer chamber for molecular beam and temperature programmed desorption experiments.

A dual purpose mass spectrometer chamber capable of performing molecular beam scattering (MBS) and temperature programmed desorption (TPD) is detailed. Two simple features of this design allow it to perform these techniques. First, the diameter of entrance aperture to the mass spectrometer can be varied to maximize signal for TPD or to maximize angular resolution for MBS. Second, the mass spectrometer chamber can be radially translated so that it can be positioned close to the sample to maximize signal or far from the sample to maximize angular resolution. The performance of this system is described and compares well with systems designed for only one of these techniques.

Interaction of O2 with CH4, CF4, and CCl4 by Molecular Beam Scattering Experiments and Theoretical Calculations.

Gas phase collisions of O2 by CH4, CF4, and CCl4 have been investigated with the molecular beam technique by measuring both the integral cross section value, Q, and its dependence on the collision velocity, v. The adopted experimental conditions have been appropriate to resolve the oscillating "glory" pattern, a quantum interference effect controlled by the features of the intermolecular interaction, for all the three case studies. The analysis of the Q(v) data, performed by adopting a suitable representation of the intermolecular potential function, provided the basic features of the anisotropic potential energy surfaces at intermediate and large separation distances and information on the relative role of the physically relevant types of contributions to the global interaction. The present work demonstrates that while O2-CH4 and O2-CF4 are basically bound through the balance between size (Pauli) repulsion and dispersion attraction, an appreaciable intermolecular bond stabilization by charge transfer is operative in O2-CCl4. Ab initio calculations of the strength of the interaction, coupled with detailed analysis of electronic charge displacement promoted by the formation of the dimer, fully rationalizes the experimental findings. This investigation indicates that the interactions of O2, when averaged over its relative orientations, are similar to that of a noble gas (Ng), specifically Ar. We also show that the binding energy in the basic configurations of the prototypical Ng-CF4,CCl4 systems [ Cappelletti , D. ; Chem. Eur. J. 2015 , 21 , 6234 - 6240 ] can be reconstructed by using the interactions in Ng-F and Ng-Cl systems, previously characterized by molecular beam scattering experiments of state-selected halogen atom beams. This information is fundamental to approach the modeling of the weak intermolecular halogen bond. On the basis of the electronic polarizability, this also confirms [ Aquilanti , V. ; Angew. Chem., Int. Ed. 2005 , 44 , 2356 - 2360 ] that O2 can be taken as a proper reference partner for simulating the behavior of some basic noncovalent components of the interactions involving water. Present results are of fundamental relevance to build up the force field controlling the hydrophobic behavior of prototypical apolar CX4 (X = H, F, Cl) molecules.

Interactions of Hydrogen and Carbon Monoxide on Pd–Au Bimetallic Surfaces

Pd–Au bimetallic catalysts have shown potential applications in numerous heterogeneous reactions in which hydrogen and CO act as reactants, intermediates, or products. A fundamental understanding of the interplay between coadsorbed H and CO on the Pd–Au surface is necessary for improving the understanding of catalytic performance. In this study, the interactions of hydrogen and CO with Pd/Au(111) model surfaces were investigated by temperature-programmed desorption (TPD) and molecular beam scattering (MBS) experiments, carried out under ultra-high-vacuum conditions. Our results reveal that CO adsorbs competitively on the hydrogen-precovered Pd–Au surface, causing surface H adatoms to diffuse away from stronger-binding sites (e.g., Pd(111)-like islands) to weaker-binding sites (e.g., Pd–Au alloy sites and subsurface), as evidenced by a shift of the H2 desorption feature to lower temperatures in TPD measurements. Additionally, evolution of H2 was observed when a CO molecular beam was impinged onto the H-pre...

Intermolecular Interaction in the NH3–H2 and H2O–H2 Complexes by Molecular Beam Scattering Experiments: The Role of Charge Transfer

New molecular beam scattering experiments are reported for the ammonia-hydrogen system recording with unprecedented resolution "glory" quantum interferences in the total cross sections. Direct comparison with the analogous water-hydrogen complex, investigated under the same experimental conditions, highlights relevant differences in the intermolecular interaction affecting the observables. Analysis of the electronic charge displacement accompanying formation of both complexes, calculated using very accurate ab initio methods, helps to rationalize the experimental findings and unveils the selective and crucial role of charge transfer in driving water interactions and formation of a weak hydrogen bond.

Nature and Stability of Weak Halogen Bonds in the Gas Phase: Molecular Beam Scattering Experiments and Ab Initio Charge Displacement Calculations

Molecular beam scattering experiments are presented for the first time in order to characterize the nature and strength of the intermolecular interaction of the gas phase H2O–CF4 and H2O–CCl4 weakl...

Adsorption Dynamics of CO on Silica-Supported Cu Clusters: A Molecular Beam Scattering Study

The adsorption kinetics and dynamics of CO on silica-supported Cu clusters were studied with thermal desorption spectroscopy and molecular beam scattering. In addition, the electronic properties of the Cu clusters and sample cleanliness were characterized with X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Physical vapor deposition was used to deposit the clusters; according to XPS, the Cu clusters remain metallic. The CO impact energy, surface temperature, and Cu coverage dependence of the adsorption dynamics can be discussed in the framework of the capture zone model. The XPS/AES and thermal desorption spectroscopy (TDS) data are consistent with a standard growth mode of Cu on silica: a nucleation phase is followed by a cluster growth phase until thick and rather smooth Cu films are formed.

Charge-Transfer Energy in the Water−Hydrogen Molecular Aggregate Revealed by Molecular-Beam Scattering Experiments, Charge Displacement Analysis, and ab Initio Calculations

Integral cross-section measurements for the system water-H(2) in molecular-beam scattering experiments are reported. Their analysis demonstrates that the average attractive component of the water-H(2) intermolecular potential in the well region is about 30% stronger than dispersion and induction forces would imply. An extensive and detailed theoretical analysis of the electron charge displacement accompanying the interaction, over several crucial sections of the potential energy surface (PES), shows that water-H(2) interaction is accompanied by charge transfer (CT) and that the observed stabilization energy correlates quantitatively with CT magnitude at all distances. Based on the experimentally determined potential and the calculated CT, a general theoretical model is devised which reproduces very accurately PES sections obtained at the CCSD(T) level with large basis sets. The energy stabilization associated with CT is calculated to be 2.5 eV per electron transferred. Thus, CT is shown to be a significant, strongly stereospecific component of the interaction, with water functioning as electron donor or acceptor in different orientations. The general relevance of these findings for water's chemistry is discussed.

Adsorption Dynamics of CO on Silica Supported Gold Clusters: Cluster Size Effects in Molecular Beam Scattering Experiments

We report on particle size effects in the adsorption dynamics (gas-surface energy transfer) of CO, studied by molecular beam scattering. The initial adsorption probability shows a maximum as a function of Au exposure, χAu, i.e., cluster size. This reactivity maximum appears to coincide with the enhancement of catalytic activity seen for the CO oxidation reaction for small clusters.

Molecular-Beam Scattering Experiments and Theoretical Calculations Probing Charge Transfer in Weakly Bound Complexes of Water

We describe and analyze in depth a series of molecular beam scattering experiments, first reported by Aquilanti et al. (Angew. Chemie Int. Ed. 2005, 44, 2356.), proving that a measurable bond stabilization component beyond the van-der-Waals forces is present in the prototypal hydrophobic interaction of water with the noble gases (Ng). The experimental integral cross-section data, exhibiting a fully resolved "glory" interference pattern in the velocity dependence, are here quantitatively analyzed and characterized employing a recently proposed model potential. The stabilization component of the water-Ng bond has recently been attributed, through very accurate theoretical calculations and an unambiguous, model-free analysis of the electron density displacement, to a net electron transfer taking place from Ng to H(2)O. We review the theoretical analysis and discuss additional computational results, comparing them to experiment, that clarify the effect of charge transfer on the interaction energies.

Unraveling the Dynamics of the C(3P,D) + C2H2 Reactions by the Crossed Molecular Beam Scattering Technique

A detailed investigation of the dynamics of the reactions of ground- and excited-state carbon atoms, C(3P) and C(1D), with acetylene is reported over a wide collision energy range (3.6-49.1 kJ mol-1) using the crossed molecular beam (CMB) scattering technique with electron ionization mass spectrometric detection and time-of-flight (TOF) analysis. We have exploited the capability of (a) generating continuous intense supersonic beams of C(3P, 1D), (b) crossing the two reactant beams at different intersection angles (45, 90, and 135 degrees ) to attain a wide range of collision energies, and (c) tuning the energy of the ionizing electrons to low values (soft ionization) to suppress interferences from dissociative ionization processes. From angular and TOF distribution measurements of products at m/z=37 and 36, the primary reaction products of the C(3P) and C(1D) reactions with C2H2 have been identified to be cyclic (c)-C3H + H, linear (l)-C3H + H, and C3 + H2. From the data analysis, product angular and translational energy distributions in the center-of-mass (CM) system for both the linear and cyclic C3H isomers as well as the C3 product from C(3P) and for l/c-C3H and C3 from C(1D) have been derived as a function of collision energy from 3.6 to 49.1 kJ mol-1. The cyclic/linear C3H ratio and the C3/(C3 + c/l-C3H) branching ratios for the C(3P) reaction have been determined as a function of collision energy. The present findings have been compared with those from previous CMB studies using pulsed beams; here, a marked contrast is noted in the CM angular distributions for both C3H- and C3-forming channels from C(3P) and their trend with collision energy. Consequently, the interpretation of the reaction dynamics derived in the present work contradicts that previously proposed from the pulsed CMB studies. The results have been discussed in the light of the available theoretical information on the relevant triplet and singlet C3H2 ab initio potential energy surfaces (PESs). In particular, the branching ratios for the C(3P) + C2H2 reaction have been compared with the available theoretical predictions (approximate quantum scattering calculations and quasiclassical trajectory calculations on ab initio triplet PESs and, very recent, statistical calculations on ab initio triplet PESs as well as on ab initio triplet/singlet PESs including nonadiabatic effects, that is, intersystem crossing). While the experimental branching ratios have been corroborated by the statistical predictions, strong disagreement has been found with the results of the dynamical calculations. The astrophysical implications of the present results have been noted.

Hans Pauly

Hans Pauly, a highly respected, internationally known leader in the development of molecular-beam scattering studies of intermolecular forces, died of a heart attack on 13 March 2004 at his home in Göttingen, Germany. He had been a founding director of the Max Planck Institut für Strömungsforschung (Max Planck Institute for Fluid Dynamics; renamed the Max Planck Institute for Dynamics and Self-Organization in 2004) in Göttingen and was instrumental in transforming that institute into a world-class center for molecular-beam research.Born in Bonn on 12 November 1928, Pauly grew up in Cologne and Bonn. He entered the University of Bonn in 1949, a few years after its reopening following World War II. He devoted his diplom project, which he carried out under the direction of Wolfgang Paul, the newly appointed professor of experimental physics who would later share the 1989 Nobel Prize in Physics, to the construction of a special ion source for the just-developed quadrupole mass filter. Subsequently, he was fascinated by the vision of Paul’s colleague, Rudolf Jaeckel, who wanted to explore atomic collision processes in vacuum diffusion pumps to improve the pumps’ performance.Working largely independently, Pauly realized the basic importance of intermolecular potentials. To investigate them properly, he constructed in 1955 the first prototype of a modern molecular-beam apparatus, which in itself was a remarkable accomplishment considering the still-adverse postwar conditions. His successful measurements of the velocity dependence of integral atom–atom scattering cross sections confirmed the v −2/5 law predicted by H. S. W. Massey and C. B. O. Mohr in 1933. Pauly’s findings thereby provided the first direct experimental confirmation of the R −6 radial dependence of the fundamentally important long-range attractive potential predicted in the 1930s by Fritz London and others. In his 1958 doctoral dissertation, entitled “Streuversuche an Molekularstrahlen” (“Molecular Beam Scattering Experiments”), Pauly developed and anticipated many aspects of modern quantum mechanical approximations for calculating atomic collision cross sections.His research and the parallel independent molecular-beam experiments on chemical reactions in the US, triggered by the breakthrough experiment of Ellison Taylor and Sheldon Datz in 1955, were responsible for the ensuing rapid developments in molecular-beam scattering experiments in the early 1960s. His work also stimulated the research of US scientists Richard Bernstein, Datz, Ned Greene and his colleague John Ross, Dudley Herschbach, and Erhard Rothe and colleagues and inspired related studies in Europe largely by the German groups of Dieter Beck, Otto Osberghaus and Christof Schlier in Freiburg, and Hans-Gerhard Bennewitz and one of us (Toennies) in Bonn. It was the spectacular successes of those groups that ultimately convinced the Max Planck Society to reorganize the MPI Strömungsforschung in 1969 to make molecular-beam scattering research a central part of that institute.In the newly reorganized institute, Pauly and his colleagues continued their precise measurements of scattering cross sections. They systematically investigated elastic integral and small-angle differential cross sections and resolved glory undulations and identical particle oscillations and resonances, first for alkali atom projectiles and then for nonalkali systems with all kinds of targets. To extend the collision energy range to more than 1000 eV, an effective charge-exchange source was developed. Similar experiments were carried out for measuring large-angle differential cross sections and completely resolved the supernumerary rainbow oscillations with superimposed, closely spaced interference oscillations. Through a newly developed, highly efficient scheme, the measured cross sections could be inverted to obtain precise experimental potential curves for the first time.Those and related experiments stimulated calculations by quantum chemists throughout the world. Subsequent investigations at the institute were devoted to understanding the interactions of electronically excited atoms, either short lived (laser excitation) or long lived (metastables). Careful execution and extraordinary precision coupled with elegant theoretical modeling were hallmarks of the Göttingen research. Pauly retired from the institute in 1996.A quiet, gentle, and modest person with the proverbial humor of a Rhinelander, Pauly occasionally enjoyed a good joke. As a colleague and group leader, he was open for discussions and would freely share his deep insights and vast experience. His students and coworkers are deeply grateful to him for supporting them generously, while simultaneously giving them sufficient freedom for their own scientific development.Pauly will long be remembered for his many reviews, some with one of us (Toennies), but especially for his authoritative two-volume monograph Atom, Molecule and Cluster Beams (Springer, 2000). He is missed not only by his wife and two children, but also by the molecular-beam community. We have lost a great scientist, an inspiring teacher, and a loyal colleague. Hans Pauly PPT|High resolution© 2005 American Institute of Physics.

Photochemical Kinetics: Reaction Orders and Analogies with Molecular Beam Scattering and Cavity Ring-Down Experiments

Despite its fundamental importance, photochemical kinetics is not often treated in much detail in physical chemistry courses and concepts often remain unclear. One topic of recent debate concerns reaction orders of photochemical reactions. In this article, we emphasize that a photochemical reaction system is composed of several elementary steps, each of which has a defined molecularity and reaction order. The elementary, primary absorption step can be considered a bimolecular reaction. Depending on the experimental conditions, the apparent total reaction order of the mechanism may have different values, but will still be defined in most cases. Possible conceptual difficulties may be avoided by realizing two analogies between experiments involving light and kinetic experiments not involving light: a standard absorption measurement has an analogy with a molecular beam scattering experiment, and cavity ring-down spectroscopy has an analogy with a conventional static reactor experiment.

Computer simulation bythe quantum mechanical time-dependent wavepacket method, especiallyfor atom/molecule-solid-surface interaction

Thermal energy atomic scattering (TEAS) on solid surfaces formsthe basis of a useful experimental method for obtaining information about the structure, disorder and phonon spectra of the solid surface. Theprobe particles (usually He atoms) can spend a relatively longtime near the solid surface in the interaction region. The dynamicsof the interaction processes cannot be investigated directly atpresent. However, an appropriate physical model of the interactionfitted to the intensity distribution of the scattering may besuitable for use in investigating the interaction processestheoretically, by computer simulation. The present work emphasizesthis computer simulation method. For an actual computersimulation, the detector region TEAS experimental method must be appropriate. Moreover a theoretical model for the TEAS(e.g. a one-particle quantum mechanical wavepacket model governedby the time-dependent Schrödinger equation (TDSE)) and a numericalmethod for solving the TDSE are necessary.The state functions of the consecutive time steps provide enoughinformation to produce an `animation' displaying the dynamics of theinteraction processes. An `animation' is a series of snapshots ofe.g. the probability density function (PDF), taken in rapid succession.The sequence of these snapshots provides a `movie' of the PDF timeevolution.Applications, physical models, numerical solution procedures andsimulation techniques relating to the time-dependentwavepacket (TDWP) method are overviewed in the presentcontribution - especially for the case of TEAS and molecular beamscattering. Several relevant applications of the TDWPmethod - in the case of TEAS - are discussed (to scattering onordered, stepped and adsorbed surfaces, the intensity distributionas a function of transfer width, resonant adsorption and trapping,classical and quantum chaos, scattering from vibrating surfaces).Preliminary results on the quantum chaos in TEAS are presented forthe first time. The theoretical and computational background (theTDWP and coupled channel methods) as well as applications (todiffraction probability, sticking probability, dissociativeadsorption, the steering effect, inelastic channels) ofsix-dimensional molecule/surface dynamics calculations aredescribed.

Orientation of benzene in supersonic expansions, probed by IR-laser absorption and by molecular beam scattering.

This work represents the first experimental demonstration that planar molecules tend to travel as a "frisbee" when a gaseous mixture with lighter carriers expands into a vacuum, the orientation being due to collisions. The molecule is benzene, the prototype of aromatic chemistry. The demonstration is via two complementary experiments: interrogating benzene by IR-laser light and controlling its orientation by selective scattering on rare gas targets. The results cast new light on the microscopic mechanisms of collisional alignment and suggest a useful way to produce intense beams of aligned molecules, permitting studies of steric effects in gas-phase processes and in surface catalysis.

Interaction of methanol with lanthanum oxide surfaces and LaOx/Cu(111) interfaces. Part I. Adsorption and thermal decomposition

The adsorption of methanol on ordered-epitaxial layers of lanthanum oxide, possessing a La2O3(001)-like structure and grown on a Cu(111) substrate, has been studied using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), molecular beam scattering (MBS), temperature-programmed desorption (TPD) and Fourier-transform reflection/absorption infrared spectroscopy (FT-RAIRS). Methanol adsorbs dissociatively at 300 K, with a high sticking probability, to yield surface methoxy species. The methoxy species on the (001)-type terraces of the oxide films are stable to temperatures in excess of 500 K, but then decompose to yield almost exclusively H2 and CO, with only trace quantities of methanol and formaldehyde being simultaneously desorbed. IR spectra also indicate the formation of some formate species on well-oxidised surfaces, and at high coverages the adsorbed methoxy is seen to partially convert upon mild heating to a new species which has characteristics of methyl formate. When regions of the Cu(111) surface are exposed, a greater proportion of the methoxy is converted to formaldehyde.

Inelastic Collision Processes of Methane and Ethane Molecules at a Pt(111) Surface Studied by Molecular Beam Scattering Techniques

Despite distinct differences in molecular structures and chemical bonds of various alkanes, the inelastic collision processes of CH4 and C2H6 on a Pt(111) surface have been found to be very similar. Both processes can be explained qualitatively by a simple classical binary collision model. Angular intensity distributions of reflected molecules of both species indicate that direct inelastic collision is primarily governed by the molecular mass, the incident kinetic energy of the molecule and the surface temperature of the Pt(111). Other factors such as the molecular structure and chemical bond play minor roles in the process. However, the degree of inelastic collision, that is, the energy transfer rate per collision, increases with the molecular mass. Helium atom scattering has revealed that the dissociation process on the surface is identical for both molecules depending on the incident energy and the surface temperature, as methyl moiety, ethylidyne moiety and complete decomposition into carbon and hydrogen atoms, successively. At elevated surface temperatures, however, thermally assisted hydrogen tunneling appears to dominate ethane dissociation.

Molecular Beam Scattering of Nitrogen Molecules in Supersonic Seeded Beams: A Probe of Rotational Alignment

Measurements of total integral cross sections for scattering of nitrogen molecules by Xe atoms in the glory collision energy range (40−600 meV) are reported under two different experimental conditions, using either a rotationally “hot” (most probable levels J = 8, 9) effusive beam of nitrogen (obtaining information on the isotropic component of the interaction potential) or rotationally “cold” N2 seeded beams emerging from supersonic expansions (obtaining quantitative information on rotational alignment of molecular nitrogen). The scattering results presented in this paper allow us to establish that the recently reported phenomenon of the strong correlation between the degree of collision-induced alignment and the speed of the molecules within a supersonic seeded velocity distribution, as previously observed for the first time in molecular oxygen, also occurs for the case of nitrogen molecules. Alignment parameters are reported for both the ortho and para forms of nitrogen, which are cooled down in the seeded supersonic expansion process to their lowest rotational levels J = 0, 1, 2.

Molecular Beam Scattering and Desorption Dynamics. Dynamics of Photo-Induced Desorption and Dissociation.

Molecular Beam Scattering and Desorption Dynamics. Dynamics of Photo-Induced Desorption and Dissociation.