Abstract When hydrogen molecules collide with a surface, they can either scatter away from the surface or undergo a dissociation reaction leaving two hydrogen atoms adsorbed on the surface. The relative probabilities of these two potential outcomes could depend on the rotational orientation of the impinging molecules, however, due to the lack of steric control techniques for ground state hydrogen, direct measurements of this dependency were previously inaccessible to experiments. Here, we demonstrate that magnetic field manipulation can be used to control the rotational orientation of the impinging H2 and change the balance between reactive and scattering collision events. Analysing the controlled collision experiments reveals that molecules approaching the surface in a helicopter orientation have a higher probability to react and dissociate, whereas those which approach in a cartwheel orientation are more likely to scatter.
Abstract When hydrogen molecules collide with a surface, they can either scatter away from the surface or undergo a dissociation reaction leaving two hydrogen atoms adsorbed on the surface. The relative probabilities of these two potential outcomes could depend on the rotational orientation of the impinging molecules, however, due to the lack of steric control techniques for ground state hydrogen, direct measurements of this dependency were previously inaccessible to experiments. Here, we demonstrate that magnetic field manipulation can be used to control the rotational orientation of the impinging H2 and change the balance between reactive and scattering collision events. Analysing the controlled collision experiments reveals that molecules approaching the surface in a helicopter orientation have a higher probability to react and dissociate, whereas those which approach in a cartwheel orientation are more likely to scatter.
Molecular beam techniques contribute crucial knowledge for a wide range of research fields and applications[1, 2]. The goal of this PhD thesis is to contribute to the development of the new magnetic molec-ular interferometry technique and use it to study fundamentally im-portant molecule-surface systems in their ground state. The thesis describes both surface scattering experiments and the development of new instrumentation and experimental methods which make this technique more powerful and insightful. The development, character-isation and testing of a new direct beam line, capable of magnetically analysing the molecular beam without scattering from a surface, is presented. This new addition to the instrument enables gathering important information about the composition and properties of the molecular beam and its response to magnetic fields. This type of infor-mation will be particularly crucial when extending the interferometry technique to molecules where the magnetic Hamiltonian is not well known. Scattering experiments of hydrogen beams from graphene, copper and salt surfaces were performed, gaining unique quantum in-formation about the scattering process. Using magnetic manipulation, we can characterise the different quantum rotational projection states of the molecules before and after scattering from a surface and detect subtle changes in the quantum state during the scattering event.Both the scattering and the direct beam experiments presented in the thesis, were analysed by solving the magnetic evolution quantum mechanically. The results show that the scattering of hydrogen from a lithium-fluoride (001) surface into different diffraction channels, is highly dependent on the rotational projection states, i.e. the orien-tation of the molecular rotation of the incident molecules. In con-trast, the specular scattering of flat copper, Cu(111), and graphene surfaces shows no obvious change in the rotational state populations upon scattering. Further helium and deuterium beams experiments are presented, enhancing our understanding of the molecular magnetic interferometer and validating our analysis methods.
The amount of information that can be obtained from a scattering experiment depends upon the precision with which the quantum states are defined in the incoming channel. By precisely defining the incoming states and measuring the outgoing states in a scattering experiment, we set up the boundary condition for experimentally solving the Schrödinger equation. In this Perspective we discuss cold inelastic scattering experiments using the most theoretically tractable H 2 and its isotopologues as the target. We prepare the target in a precisely defined rovibrational ( v, j, m ) quantum state using a special coherent optical technique called the Stark-induced adiabatic Raman passage (SARP). v and j represent the quantum numbers of the vibrational and rotational energy levels, and m refers to the projection of the rotational angular momentum vector j on a suitable quantization axis in the laboratory frame. Selection of the m quantum numbers defines the alignment of the molecular frame, which is necessary to probe the anisotropic interactions. For us to achieve the collision temperature in the range of a few degrees Kelvin, we co-expand the colliding partners in a mixed supersonic beam that is collimated to define a direction for the collision velocity. When the bond axis is aligned with respect to a well-defined collision velocity, SARP achieves stereodynamic control at the quantum scale. Through various examples of rotationally inelastic cold scattering experiments, we show how SARP coherently controls the dynamics of anisotropic interactions by preparing quantum superpositions of the orientational m states within a single rovibrational ( v, j ) energy state. A partial wave analysis, which has been developed for the cold scattering experiments, shows dominance of a resonant orbital that leaves its mark in the scattering angular distribution. These highly controlled cold collision experiments at the single partial wave limit allow the most direct comparison with the results of theoretical computations, necessary for accurate modeling of the molecular interaction potential.
This article reviews the current state of the art in the field of cold and ultracold molecules and demonstrates that chemical reactions, inelastic collisions and dissociation of molecules at subkelvin temperatures can be manipulated with external electric or magnetic fields. The creation of ultracold molecules may allow for spectroscopy measurements with extremely high precision and tests of fundamental symmetries of nature, quantum computation with molecules as qubits, and controlled chemistry. The probability of chemical reactions and collisional energy transfer can be very large at temperatures near zero kelvin. The collision energy of ultracold atoms and molecules is much smaller than perturbations due to interactions with external electric or magnetic fields available in the laboratory. External fields may therefore be used to induce dissociation of weakly bound molecules, stimulate forbidden electronic transitions, suppress the effect of centrifugal barriers in outgoing reaction channels or tune Feshbach resonances that enhance chemical reactivity.
Abstract 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 a J=2 to the non-rotating J=0 state, without using an energy-matched perturbation. We show that a magnetic field which splits the rotational projection states by only pico 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 the origin of the control scheme, but also underestimate rotational flips (Δm_J≠0), highlighting the importance of the results as a sensitive benchmark for further developing theoretical models of molecule-surface interactions.
In this thesis I describe how we achieve control over the quantum state of a single H2+ hydrogen molecular ion. Scientific progress is based on the comparison of theories with experimental measurements. For the simplest molecular quantum systems, such as the hydrogen molecular ions H2+ or HD+, the theoretical predictions of quantum electrodynamics are very accurate. In order to test the theory or to extract fundamental constants from the combination of theory and experiments, the experiments must also achieve high precision. In the recent past, hydrogen molecular ions were mainly measured in ensembles. For example, measurements exist both on molecular beams and with dissociative detection in ion traps. However, the spectroscopy of single ions is one of the most precise measurement methods in physics, which was demonstrated by the spectroscopy of a single ytterbium ion in an ion trap. To perform spectroscopy of a single ion in this way, it must have a suitable energy structure for laser cooling, state preparation and fluorescence detection. This is not the case for many ions, including H2+. Quantum logic spectroscopy (QLS) is a technique that makes it possible to perform measurements on an ion without such an energy structure. The ion to be measured (spectroscopy ion) is trapped in an ion trap with a second ion (helper ion), which has the properties described in the previous paragraph. The shared modes of motion between the two ions allow cooling, preparation and detection of the spectroscopy ion via the helper ion. This was demonstrated with an aluminum spectroscopy ion co-trapped with a magnesium helper ion. QLS can also be used for molecules. This has been shown with MgH+, CaH+ and N2+. We implement QLS to enable spectroscopy of a single H2+ ion. For this we trap H2+ together with a 9Be+ ion. We cool the joint motion of the ions to their ground state. We combine buffer gas cooling with a laser and microwave pulse based state preparation sequence to prepare the H2+ molecule in a pure quantum state. This allows us to perform high contrast QLS on a single H2+ ion. Furthermore, we can change and control the quantum state in the hyperfine structure using microwave pulses. We demonstrate the power of our technique by performing microwave spectroscopy on a magnetic field insensitive hyperfine transition. We use QLS to read out the result of the measurement. Within half an hour, we can achieve a statistical uncertainty of 2 Hz. This paves the way for future high-precision measurements on H2+. The techniques described here could potentially be applied to other light molecular ions.
In sufficiently large magnetic fields, the weakly bound levels of H2 acquire finite lifetimes owing to hyperfine predissociation and become manifest as scattering resonances in spin-polarized atomic hydrogen (H↓). In this work we investigate in detail the contribution to the reaction kinetics in H↓ due to the resonance, denoted [Formula: see text], arising from the predissociation of the (v,J) = (14,4) level of H2. To characterize the formation and disintegration of [Formula: see text], we calculate the predissociation as a function of both the applied magnetic field and the dissociation energy, D, of the level. Recombination occurs when [Formula: see text] is stabilized in collisions that cause transitions to lower molecular levels. For low H↓, densities, collisions with atoms of the saturated 4He vapour dominate; we calculate the cross section in the distorted wave Born approximation and also use a coupled-channels approach. At higher densities, stabilization occurs in collisions with H↓ as well; the effect of proton exchange between the atom and the molecule is derived. Previous experiments, analyzed neglecting proton exchange, are reanalyzed including this; the dissociation energy D is not as well determined by the data as was previously thought. Prospects for future experiments are discussed.