Periodic Reporting for period 4 - Rotational Waves (Controlling and resolving rotational quantum states in a molecule-surface collision: Matter-wave magnetic interference experiments with ground state molecules.)
Okres sprawozdawczy: 2023-02-01 do 2024-01-31
The interaction of molecules with surfaces is fundamental to a huge variety of research fields and applications. One example where understanding this interaction has an obvious importance for society is heterogenous catalysis, where the interaction of gas phase molecules with a solid surface is used to produce vital chemical compounds in an industrial process or expunge unwanted chemicals for environmental reasons. To design better catalysts for such processes, one needs a reliable method for modelling the interaction with the surface. Theoretical modelling methods, and in particular the popular density functional theory (DFT) approach, approximate the interactions to make the computations possible which comes on the expense of their accuracy, often in ways which are hard to predict. To be able to trust predictions made by DFT based modelling and allow the further development of advanced more accurate functionals, it is absolutely essential to benchmark the results for simple systems, where clean well-controlled experiments are possible.
In this project we developed and used new experimental methods, which allowed us to both control and measure the rotational orientation of ground state molecules as they collide with a surface. These type of experiments, which were previously impossible, create experimental benchmarks for testing the accuracy of existing molecule-surface calculation methods and guiding the development of new more accurate calculation methods in the future.
The overall objectives of the project were to; (1) develop our new magnetic molecular interferometry technique, and provide the much needed experimental benchmarks for studying the stereodynamic nature of molecule-surface interactions, (2) explore the possibility of modifying reaction rates of heterogeneous reactions by controlling the rotation projection states of the impinging molecule, and (3) Study ultra-fast dynamics on surfaces with molecular probes (perform the first molecular spin echo experiments).
In this project we developed and used new experimental methods, which allowed us to both control and measure the rotational orientation of ground state molecules as they collide with a surface. These type of experiments, which were previously impossible, create experimental benchmarks for testing the accuracy of existing molecule-surface calculation methods and guiding the development of new more accurate calculation methods in the future.
The overall objectives of the project were to; (1) develop our new magnetic molecular interferometry technique, and provide the much needed experimental benchmarks for studying the stereodynamic nature of molecule-surface interactions, (2) explore the possibility of modifying reaction rates of heterogeneous reactions by controlling the rotation projection states of the impinging molecule, and (3) Study ultra-fast dynamics on surfaces with molecular probes (perform the first molecular spin echo experiments).
Two main results, which enabled all the other achievements, were the development of the Magnetic Molecular Interferometry (MMI) setup and the development of a theoretical framework for analysing MMI measurements. The MMI setup (figure 1) which is the main experimental platform for this project has been extensively developed throughout the project. Major changes included adding a new beam line for characterising the quantum state evolution, low vibration cooling systems, and a setup for characterising surfaces using low energy electron diffraction. The theoretical framework we developed allows us to calculate what happens to a molecule as it passes through the entire setup and simulate what the signal should look like when the molecular state changes due to scattering.
We applied the developments mentioned above to study several molecule-surface systems. One example was studying collisions of hydrogen molecules with the surface of an ionic surface. The change of the quantum state of the molecule during the collision was quantified by comparing simulated and measured MMI data. Figure 2, shows the level of agreement between the theoretical and experimental interference patterns . Obtaining unique numerical values for the quantum wave function components directly from an experiment was previously impossible, it acceded our initial expectations when designing these experiments, and arguably supplies the most sensitive benchmark for calculating molecule-surface interactions.
Another example, was when we demonstrated that it is possible to control the probability of rotational de-excitation of a molecule upon colliding with a surface, by controlling the rotational orientation of the incoming molecules. The results of this study were surprising, as a tiny energy perturbation was used to control a much larger energy exchange process, but also supplied numerical values for the de-excitation probabilities, values which even state-of-the-art calculations cannot yet reproduce, driving further development of calculation methods.
Other results include discovering and studying multiple magnetic coherences in molecular scattering experiments, developing a new method to measure surface phonons (collective oscillations of the atoms of a surface) with unprecedented energy resolution and finally developing a completely new imaging scheme which is based on a magnetically manipulated atomic beams and offers new opportunities in the field of neutral beam microscopy.
We applied the developments mentioned above to study several molecule-surface systems. One example was studying collisions of hydrogen molecules with the surface of an ionic surface. The change of the quantum state of the molecule during the collision was quantified by comparing simulated and measured MMI data. Figure 2, shows the level of agreement between the theoretical and experimental interference patterns . Obtaining unique numerical values for the quantum wave function components directly from an experiment was previously impossible, it acceded our initial expectations when designing these experiments, and arguably supplies the most sensitive benchmark for calculating molecule-surface interactions.
Another example, was when we demonstrated that it is possible to control the probability of rotational de-excitation of a molecule upon colliding with a surface, by controlling the rotational orientation of the incoming molecules. The results of this study were surprising, as a tiny energy perturbation was used to control a much larger energy exchange process, but also supplied numerical values for the de-excitation probabilities, values which even state-of-the-art calculations cannot yet reproduce, driving further development of calculation methods.
Other results include discovering and studying multiple magnetic coherences in molecular scattering experiments, developing a new method to measure surface phonons (collective oscillations of the atoms of a surface) with unprecedented energy resolution and finally developing a completely new imaging scheme which is based on a magnetically manipulated atomic beams and offers new opportunities in the field of neutral beam microscopy.
The MMI developed in this project is currently the only instrument which can control the rotational orientation of a ground-state closed-shell molecule such as hydrogen.
Other existing techniques either use polarised photons to align and orient molecules, and correspondingly are applied to vibrationally and rotationally excited molecules, or use deflecting magnets which are suitable for open-shell / paramagnetic molecules but cannot be applied to closed-shell ground state molecules. The unique capabilities of the MMI apparatus mean that all of the measurements we performed with this instrument, and in particular the ones described above, reveal physical properties which were previously inaccessible to experiments and offer significant new insights.
Other existing techniques either use polarised photons to align and orient molecules, and correspondingly are applied to vibrationally and rotationally excited molecules, or use deflecting magnets which are suitable for open-shell / paramagnetic molecules but cannot be applied to closed-shell ground state molecules. The unique capabilities of the MMI apparatus mean that all of the measurements we performed with this instrument, and in particular the ones described above, reveal physical properties which were previously inaccessible to experiments and offer significant new insights.