Temperature dependent stereodynamics in surface scattering measured through subtle changes in the molecular wave function.

A magnetically manipulated molecular beam technique is used to change the rotational orientation of H2 molecules which collide with a stepped Cu(511) surface and explore how the polarisation dependence of molecules scattering into the specular channel changes as a function of surface temperature. At all temperatures, H2 molecules that are rotating like cartwheels are more likely to be scattered into the specular channel than those that are rotating like helicopters. Furthermore, the scattered molecules are more likely to be rotating like cartwheels, regardless of their state before the collision. Increasing the temperature of the Cu(511) surface causes the polarisation effects to become stronger, with the scattering becoming more selective for H2 with cartwheel like rotation. Therefore, scattering a molecular beam of H2 from a Cu(511) surface and taking the molecules scattered into the specular channel provides a method to create a rotationally polarised beam of H2, where the polarisation can be tuned by changing the surface temperature. In contrast, the rotational orientation dependence observed for specular scattering from a flat Cu(111) surface is independent of surface temperature within the same temperature range.

Measuring surface phonons using molecular spin-echo.

A new method to measure surface phonons with a molecular beam is presented. The method extends the principles of 3He spin-echo spectroscopy, to the more complex case of a molecular beam exchanging energy with the surface. Measurements are presented for inelastic scattering of D2 from a Cu(111) surface. Similarly to helium spin-echo, experiments can be performed along optimal tilted projections making it possible to resolve energy peaks with a high energy resolution which is not restricted by the spread of energies of the incident beam. Two analysis methods for these molecular spin echo experiments are presented. A classical approach, analogous to that used for helium spin-echo, explains the most dominant excitation peaks measured, whereas a semi-classical approach allows us to identify smaller peaks which are related to the complexity of the multiple spin-rotation states which exist for molecules.

Multiple echoes in beam spin-echo spectroscopy and their effect on measurements of ultra-fast dynamics.

Helium (3He) spin-echo is a powerful experimental technique used to probe ultra-fast atomic scale surface dynamics. The analysis of these measurements is typically performed assuming there is only a single spin-echo condition, expected to produce a constant signal for pure elastic scattering, a monotonically decaying signal for quasi-elastic scattering and oscillations from inelastic scattering events. In the present work, we show that there are in fact four spin-echoes which must be correctly accounted for, and that even in the case of elastic scattering these additional echoes lead to oscillations which could mistakenly be interpreted as being due to inelastic scattering. We demonstrate that it is possible to accurately simulate the experimental data by propagating the3He through the measured magnetic field profile of the apparatus and considering the geometry of the machine, allowing the effect of these additional echoes to be disentangled from inelastic scattering events in future3He spin-echo measurements.

Stopping molecular rotation using coherent ultra-low-energy magnetic manipulations.

Rotational motion lies at the heart of intermolecular, molecule-surface chemistry and cold molecule science, motivating the development of methods to excite and de-excite rotations. Existing schemes involve perturbing the molecules with photons or electrons which supply or remove energy comparable to the rotational level spacing. Here, we study the possibility of de-exciting the molecular rotation of a D2 molecule, from J = 2 to the non-rotating J = 0 state, without using an energy-matched perturbation. We show that passing the beam through a 1 m long magnetic field, which splits the rotational projection states by only 10-12 eV, can change the probability that a molecule-surface collision will stop a molecule from rotating and lose rotational energy which is 9 orders larger than that of the magnetic manipulation. Calculations confirm that different rotational orientations have different de-excitation probabilities but underestimate rotational flips (∆mJ[Formula: see text]0), highlighting the importance of the results as a sensitive benchmark for further developing theoretical models of molecule-surface interactions.

Material properties particularly suited to be measured with helium scattering: selected examples from 2D materials, van der Waals heterostructures, glassy materials, catalytic substrates, topological insulators and superconducting radio frequency materials.

Helium Atom Scattering (HAS) and Helium Spin-Echo scattering (HeSE), together helium scattering, are well established, but non-commercial surface science techniques. They are characterised by the beam inertness and very low beam energy (<0.1 eV) which allows essentially all materials and adsorbates, including fragile and/or insulating materials and light adsorbates such as hydrogen to be investigated on the atomic scale. At present there only exist an estimated less than 15 helium and helium spin-echo scattering instruments in total, spread across the world. This means that up till now the techniques have not been readily available for a broad scientific community. Efforts are ongoing to change this by establishing a central helium scattering facility, possibly in connection with a neutron or synchrotron facility. In this context it is important to clarify what information can be obtained from helium scattering that cannot be obtained with other surface science techniques. Here we present a non-exclusive overview of a range of material properties particularly suited to be measured with helium scattering: (i) high precision, direct measurements of bending rigidity and substrate coupling strength of a range of 2D materials and van der Waals heterostructures as a function of temperature, (ii) direct measurements of the electron-phonon coupling constant λ exclusively in the low energy range (<0.1 eV, tuneable) for 2D materials and van der Waals heterostructures (iii) direct measurements of the surface boson peak in glassy materials, (iv) aspects of polymer chain surface dynamics under nano-confinement (v) certain aspects of nanoscale surface topography, (vi) central properties of surface dynamics and surface diffusion of adsorbates (HeSE) and (vii) two specific science case examples - topological insulators and superconducting radio frequency materials, illustrating how combined HAS and HeSE are necessary to understand the properties of quantum materials. The paper finishes with (viii) examples of molecular surface scattering experiments and other atom surface scattering experiments which can be performed using HAS and HeSE instruments.

Molecular spin echoes; multiple magnetic coherences in molecule surface scattering experiments.

In this paper we demonstrate that a molecular beam of hydrogen molecules can be magnetically manipulated to produce multiple coherences in the molecular interference pattern. Unlike spin 1/2 magnetic beam experiments, i.e., neutron and helium spin echo, the nuclear and rotational magnetic moments in a molecule are strongly coupled. We show experimentally and theoretically that this coupling leads to multiple magnetic field conditions under which the magnetic moment of molecules travelling with different speeds can be coherently refocussed. We also demonstrate that these multiple coherence signals are extremely sensitive to the scattering event, opening up new possibilities for measuring molecule-surface interactions.

Setting benchmarks for modelling gas-surface interactions using coherent control of rotational orientation states.

The coherent evolution of a molecular quantum state during a molecule-surface collision is a detailed descriptor of the interaction potential which was so far inaccessible to measurements. Here we use a magnetically controlled molecular beam technique to study the collision of rotationally oriented ground state hydrogen molecules with a lithium fluoride surface. The coherent control nature of the technique allows us to measure the changes in the complex amplitudes of the rotational projection quantum states, and express them using a scattering matrix formalism. The quantum state-to-state transition probabilities we extract reveal a strong dependency of the molecule-surface interaction on the rotational orientation of the molecules, and a remarkably high probability of the collision flipping the rotational orientation. The scattering matrix we obtain from the experimental data delivers an ultra-sensitive benchmark for theory to reproduce, guiding the development of accurate theoretical models for the interaction of H2 with a solid surface.

Quantum State-Resolved Characterization of a Magnetically Focused Beam of ortho-H2O.

Magnetic focusing of a molecular beam formed from a rotationally cooled supersonic jet of H2O seeded in argon is shown to yield water vapor highly enriched in the ortho-H2O nuclear spin isomer (NSI). Rotationally resolved resonance-enhanced multiphoton ionization time-of-flight mass spectrometry demonstrates that this methodology enables the preparation of a beam of water molecules enriched to >98% in the ortho-H2O NSI, that is, having an ortho-to-para ratio in excess of 50:1. The flux and quantum-state purity achieved through the methodology described herein could enable heterogeneous chemistry applications including the preparation of nuclear spin-polarized water adlayers, making nuclear magnetic resonance investigations amenable to surface science studies, as well as laboratory astrophysics investigations of NSI interconversion mechanisms and rates in ice and at its surface.

Ice Nucleation on a Corrugated Surface.

Heterogeneous ice nucleation is a key process in many environmental and technical fields and is of particular importance in modeling atmospheric behavior and the Earth's climate. Despite an improved understanding of how water binds at solid surfaces, no clear picture has emerged to describe how 3D ice grows from the first water layer, nor what makes a particular surface efficient at nucleating bulk ice. This study reports how water at a corrugated, hydrophilic/hydrophobic surface restructures from a complex 2D network, optimized to match the solid surface, to grow into a continuous ice film. Unlike the water networks formed on plane surfaces, the corrugated Cu(511) surface stabilizes a buckled hexagonal wetting layer containing both hydrogen acceptor and donor sites. First layer water is able to relax into an "icelike" arrangement as further water is deposited, creating an array of donor and acceptor sites with the correct spacing and corrugation to stabilize second layer ice and allow continued commensurate multilayer ice growth. Comparison to previous studies of flat surfaces indicates nanoscale corrugation strongly favors ice nucleation, implying surface corrugation will be an important aspect of the surface morphology on other natural or engineered surfaces.

A general method for controlling and resolving rotational orientation of molecules in molecule-surface collisions.

The outcome of molecule-surface collisions can be modified by pre-aligning the molecule; however, experiments accomplishing this are rare because of the difficulty of preparing molecules in aligned quantum states. Here we present a general solution to this problem based on magnetic manipulation of the rotational magnetic moment of the incident molecule. We apply the technique to the scattering of H2 from flat and stepped copper surfaces. We demonstrate control of the molecule's initial quantum state, allowing a direct comparison of differences in the stereodynamic scattering from the two surfaces. Our results show that a stepped surface exhibits a much larger dependence of the corrugation of the interaction on the alignment of the molecule than the low-index surface. We also demonstrate an extension of the technique that transforms the set-up into an interferometer, which is sensitive to molecular quantum states both before and after the scattering event.

Confinement Effects on the Nuclear Spin Isomer Conversion of H2O.

The mechanism for interconversion between the nuclear spin isomers (NSI) of H2O remains shrouded in uncertainties. The temperature dependence displayed by NSI interconversion rates for H2O isolated in an argon matrix provides evidence that confinement effects are responsible for the dramatic increase in their kinetics with respect to the gas phase, providing new pathways for o-H2O↔p-H2O conversion in endohedral compounds. This reveals intramolecular aspects of the interconversion mechanism which may improve methodologies for the separation and storage of NSI en route to applications ranging from magnetic resonance spectroscopy and imaging to interpretations of spin temperatures in the interstellar medium.

Mass Transport in Surface Diffusion of van der Waals Bonded Systems: Boosted by Rotations?

Mass transport at a surface is a key factor in heterogeneous catalysis. The rate is determined by excitation across a translational barrier and depends on the energy landscape and the coupling to the thermal bath of the surface. Here we use helium spin-echo spectroscopy to track the microscopic motion of benzene adsorbed on Cu(001) at low coverage (θ ∼ 0.07 ML). Specifically, our combined experimental and computational data determine both the absolute rate and mechanism of the molecular motion. The observed rate is significantly higher by a factor of 3.0 ± 0.1 than is possible in a conventional, point-particle model and can be understood only by including additional molecular (rotational) coordinates. We argue that the effect can be described as an entropic contribution that enhances the population of molecules in the transition state. The process is generally relevant to molecular systems and illustrates the importance of the pre-exponential factor alongside the activation barrier in studies of surface kinetics.

Probing the non-pairwise interactions between CO molecules moving on a Cu(111) surface.

The coverage dependent dynamics of CO on a Cu(111) surface are studied on an atomic scale using helium spin-echo spectroscopy. CO molecules occupy top sites preferentially, but also visit intermediate bridge sites in their motion along the reaction coordinate. We observe an increase in hopping rate as the CO coverage grows; however, the motion remains uncorrelated up to at least 0.10 monolayers (ML). From the temperature dependence of the diffusion rate, we find an effective barrier of 98 ± 5 meV for diffusion. Thermal motion is modelled with Langevin molecular dynamics, using a potential energy surface having adsorption sites at top and bridge positions and the experimental data are well represented by an adiabatic barrier for hopping of 123 meV. The sites are not degenerate and the rate changes observed with coverage are modelled successfully by changing the shape of the adiabatic potential energy surface in the region of the transition state without modifying the energy barrier. The results demonstrate that sufficient detail exists in the experimental data to provide information on the principal adsorption sites as well as the energy landscape in the region of the transition state.

Observation of uncorrelated microscopic motion in a strongly interacting adsorbate system.

Modeling of intermolecular forces is a central theme in the physical sciences. The prototypical heterogeneous catalysis system, CO/Pt(111), is an extensively studied example where strong pairwise repulsive forces between the CO molecules have been used to explain the observed structure and dynamics. No direct measurements of these forces were available; yet, they offered a natural way of explaining various macroscopic observations assuming a separable adsorbate-substrate interaction and pairwise adsorbate-adsorbate interactions. In the present study, we measure intermolecular forces by following CO motion on a microscopic scale. The uncorrelated dynamics we observe throughout the coverage range of the measurements excludes the existence of the strong pairwise forces previously suggested. The increase in the rate of uncorrelated motion is explained by a nonlocal modification of the adsorbate-substrate interaction, reflecting a many-body system that cannot be described by the standard separable interaction approach.

Miniature self-contained intravascular magnetic resonance (IVMI) probe for clinical applications.

A miniature (1.73 mm in diameter) NMR probe, which contains a magnet and a radiofrequency (RF) coil, is presented. This probe is integrated at the tip of a standard catheter and can be inserted into the human coronary arteries, creating local magnetic fields needed to obtain the NMR signal from the blood vessel walls, without the need for external magnet or RF coils. The basic theory governing the probe performance in terms of signal-to-noise-ratio and contrast parameters is presented, along with measured results from test samples. The NMR signal can be analyzed to obtain tissue contrast parameters such as T1, T2 and the diffusion coefficient, which may be used to detect lipid-rich vulnerable plaques in the coronary arteries.

Ultrahigh-resolution spin-echo measurement of surface potential energy landscapes.

We demonstrate two approaches that use the recently developed helium spin-echo technique to measure surface potential energy landscapes. For helium-lithium fluoride (100), we use the selective adsorption phenomenon to obtain the complete experimental band structure of atoms in a corrugated surface potential. For carbon monoxide-copper (001), we measure the diffusion-induced energy broadening in the scattered helium beam and extract properties of the adsorbate-substrate potential. The measurements are made possible by the resolution of our new spectrometer, which improves on existing resolution by three orders of magnitude. We show that it is possible to produce benchmark energy landscapes to assist evaluation and development of first-principles theory in the problematic van der Waals/weak chemisorption regime.