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.
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.
This thesis describes the design and construction of a novel molecule-surface scattering apparatus. The setup combines a Stark decelerator with a state-of- the-art surface scattering chamber, and allows background-gas free scattering of packets of velocity-controlled polar molecules in selected quantum states from well-defined surfaces with unprecedented energy resolution. Results for scattering of CO (a3Π) in different vibrational quantum states from a clean Au(111) surface are presented. CO is co-expanded with Xe or Ar to yield a supersonic molecular beam which produces fast, internally cold molecules. Excitation of CO to the long-lived electronically excited a3Π1 state makes it accessible to manipulation with electric fields due to the state’s large dipole moment. The complete removal of both carrier gas and remaining ground- state CO is achieved by focusing and deflecting the metastable CO with a switched electrostatic hexapole state selector. The 131-stage Stark decelerator is used to tune the velocity of a part of the metastable CO pulse, with an initial velocity of 360 m/s, in the range from (512 ± 4) m/s – (50 ± 4) m/s, corresponding to an incident translational energy between (307 ± 5) cm-1 and (3.0 ± 0.5) cm-1, respectively.This range can be extended up to (1300 ± 25) m/s, corresponding to an energy of(1980 ± 80) cm-1, by simple modifications to the setup. Prior to collision, the molecules can be laser- prepared in different vibrational and rotational quantum states. State- selective detection of the scattering products makes it possible to determine the quantum state, the scattering angle, and the kinetic energy of the scattered molecules. Therefore, detailed information about the scattering process can be obtained. The design of the machine allows for flexible laser access near the surface, such that a variety of different preparation and detection schemes can be employed. Ultra-high vacuum conditions in the scattering chamber are achieved by baking the entire apparatus, including the decelerator and the hexapole. Because of the carrier gas removal and a total of three differential pumping stages, no pressure rise is detected in the scattering chamber due to the incident molecular beam. Scattering experiments of CO (a3Π) on Au(111) have been performed. The scattering process leads to electron emission because the metastable excitation energy exceeds the work function of the surface. The electron emission yield, i.e. the number of electrons emitted per incident metastable particle, has been measured to be 0.13 ± 0.05. A laser ionization-depletion method is used which makes it possible to directly compare the intensity of the metastable CO ionization signal to the intensity decrease of the electron emission signal. It was also shown that vibrationally excited metastable CO leads to a higher electron emission yield. By preparing different vibrational distributions of CO in the a3Π electronic state via Franck-Condon pumping, the electron emission yield was determined for groups of vibrational levels. The first three vibrationally excited levels have an increased electron emission yield which on average is a factor of 1.5 larger than that of the vibrational ground state. Averaged over the vibrational levels with quantum number v = 4, 5, 6 , this factor is approximately 2.6 . In the range between 93 m/s and 512 m/s, the electron emission was observed to be independent of the velocity of the incident molecules. When the surface temperature is raised from 250 K to 900 K, the intensity of the electron emission signal increases by 27 %. These results are not in agreement with the models currently used to describe the de-excitation of electronically excited molecules at metal surfaces. This demonstrates the need for more theoretical work to incorporate the role of vibration into the description of the de-excitation mechanism.
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.
Since external magnetic fields were first employed to deflect paramagnetic atoms in 1921, a range of magnetic field-based methods have been introduced to state-selectively manipulate paramagnetic species. These methods include magnetic guides, which selectively filter paramagnetic species from all other components of a beam, and magnetic traps, where paramagnetic species can be spatially confined for extended periods of time. However, many of these techniques were developed for atomic-rather than molecular-paramagnetic species. It has proven challenging to apply some of these experimental methods developed for atoms to paramagnetic molecules. Thanks to the emergence of new experimental approaches and new combinations of existing techniques, the past decade has seen significant progress toward the manipulation and control of paramagnetic molecules. This review identifies the key methods that have been implemented for the state-selective manipulation of paramagnetic molecules-discussing the motivation, state of the art, and future prospects of the field. Key applications include the ability to control chemical interactions, undertake precise spectroscopic measurements, and challenge our understanding of chemical reactivity at a fundamental level.
This thesis considers (1) novel manifestations and applications of quantum interference in complex systems and (2) development of new approaches to study complex quantum systems. First, I examine a general model of particles with long-range hopping amplitudes. For at least 30 years, it has been widely accepted that these particles do not undergo Anderson localization in 3D lattices. We show that these particles do undergo Anderson localization in 3D lattices if their hopping amplitudes are isotropic. In contrast, particles with anisotropic long-range hopping appear to follow the widely held expectations. We show these results by demonstrating that cooperative shielding extends to 3D cubic lattices with isotropic long-range hopping, but not with anisotropic long-range hopping and by computing the scaling behaviour of participation ratios and energy level statistics. Secondly, I develop a fully quantum mechanical model of molecular surface spin-echo experiments, which study surface properties and dynamics by scattering molecules off the sample surface. This model, based on the transfer matrix method, incorporates molecular hyperfine degrees of freedom, allows for the efficient calculation of the experimental signal given a molecule-surface scattering matrix, and permits us to begin addressing the inverse scattering problem. This fully quantum model is required to properly describe these experiments as the semi-classical methods used to describe experiments using helium-3 atoms do not take magnetic-field induced momentum changes into account. We apply our method to the case of ortho-hydrogen and then apply Bayesian optimization to determine the molecule-surface scattering matrix elements from a calculated signal, for a scattering matrix defined by three parameters. Our work sets the stage for Bayesian optimization to solve the inverse scattering problem for these experiments. Finally, I propose using Bayesian model calibration to improve the convergence of Monte Carlo calculations in regions where the sign problem or critical slowing down are an issue. Specifically, Bayesian model calibration would correct poorly converged Monte Carlo calculations with the information from a small number of well-converged Monte Carlo calculations. As a simple proof of principle demonstration, we apply Bayesian model calibration to a diagrammatic Monte Carlo calculation of the scattering length of a spherical potential barrier.
Research of cold and ultracold molecules is currently a burgeoning field in experimental and theoretical physics. New experimental techniques, involving an increasingly large set of molecule types under high levels of control, are currently opening up new avenues of research with a vast array of potential applications. From understanding the role of quantum mechanics in molecular scattering and cold chemistry, to testing the fundamental symmetries of nature and realizing quantum computing with dipolar molecular qubits, experiments are accessing regimes not dreamed of even a few years ago. Theoretical interest and computing capabilities are also at an all time high, spurred on by the possibility of creating ultracold dipolar gases as tunable realizations of strongly interacting quantum Hamiltonians, creating exotic phases of matter, and the investigation of controlling molecular interactions with applied electromagnetic fields.\nLess than a decade ago, cold molecule experiments had seemingly reached a technological plateau, being capable of creating moderate densities of 106-107 molecules/cm3 at temperatures of 10-100 mK. With many applications requiring colder temperatures and higher densities, the field was ripe for new advances. Today, via a plethora of methods such as direct molecular laser cooling, electro-optical cooling, magneto- and photo-association, and new molecular beam deceleration techniques, the field is just beginning to have the tools capable of producing truly interesting systems for study.\nThis work will discuss a couple of major steps taken in the direction of achieving scientific goals using cold molecules. The first experimental advancement discussed will be the development of a co-trap environment for studying interactions and collisions between ultracold atoms and Stark decelerated cold polar molecules. In this experiment, rubidium atoms are trapped using magnetic fields, and ammonia atoms are decelerated and trapped using electric fields. The two traps are spatially overlapped in order to investigate inter-species interactions. The co-trap environment provides exceedingly long interaction times, many orders of magnitude longer than typical beam-based interaction studies. As a result, it provides extremely high sensitivity to weak interaction mechanisms. The second experimental advancement discussed will be the development and construction of a new style of Stark decelerator, capable of producing much larger densities of cold molecules. This apparatus has the potential to expand the realm of possible experiments with chemically interesting species, and provide an unprecedented amount of control over molecular beams and traps. The gains haven't come easily though, as a new class of custom high-voltage amplifiers have needed to be developed. This part of the experiment alone took approximately two years of consistent effort to bring to fruition. After many years of development, this experiment is poised to come online, finally fulfilling its potential.
We describe the state of the art in preparing, manipulating and detecting coherent molecular matter. We focus on experimental methods for handling the quantum motion of compound systems from diatomic molecules to clusters or biomolecules.Molecular quantum optics offers many challenges and innovative prospects: already the combination of two atoms into one molecule takes several well-established methods from atomic physics, such as for instance laser cooling, to their limits. The enormous internal complexity that arises when hundreds or thousands of atoms are bound in a single organic molecule, cluster or nanocrystal provides a richness that can only be tackled by combining methods from atomic physics, chemistry, cluster physics, nanotechnology and the life sciences.We review various molecular beam sources and their suitability for matter-wave experiments. We discuss numerous molecular detection schemes and give an overview over diffraction and interference experiments that have already been performed with molecules or clusters.Applications of de Broglie studies with composite systems range from fundamental tests of physics up to quantum-enhanced metrology in physical chemistry, biophysics and the surface sciences.Nanoparticle quantum optics is a growing field, which will intrigue researchers still for many years to come. This review can, therefore, only be a snapshot of a very dynamical process.