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Controlling and resolving rotational quantum states in a molecule-surface collision: Matter-wave magnetic interference experiments with ground state molecules.

Periodic Reporting for period 2 - Rotational Waves (Controlling and resolving rotational quantum states in a molecule-surface collision: Matter-wave magnetic interference experiments with ground state molecules.)

Reporting period: 2020-02-01 to 2021-07-31

To understand the interaction between a molecule and a surface, and to able to bench mark the various new approaches for modelling surfaces currently under development, quantum state resolved experiments on various ground state molecules are needed. In this project we develop and use new experimental methods, which allow both control and measurement of the rotational orientation of a ground state molecules, previously inaccessible to experiments. Using these techniques we study the role of rotational orientation in molecule–surface collisions and reactions, for various molecule-surface systems.

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 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.

The overall objectives of the project:
Objective 1) Develop our new magnetic molecular interferometry technique, and provide the much needed experimental benchmarks for studying the stereodynamic nature of molecule-surface interactions.
Objective 2) Explore the possibility of modifying reaction rates of heterogeneous reactions by controlling the rotation projection states of the impinging molecule.
Objective 3) Study ultra-fast dynamics on surfaces with molecular probes (perform the first molecular spin echo experiments).
Providing experimental benchmarks for the stereodynamic nature of molecule-surface interactions and development of interpretation methods for molecular interference measurements: We studied the collisions of H2 molecules with a salt (LiF) surface [Nature Communications 11, 3110 (2020).]. Combining magnetic manipulation experiments with a complete methodology for interpreting the measurements, we extracted the scattering matrix from a comparison of measured and calculated oscillation patterns. Figure 1, shows the level of agreement between the theoretical and experimental patterns when using the empirically derived scattering matrix for two different diffraction conditions. The scattering matrix defines the change in the quantum state of the molecules due to the collision with the surface. Obtaining a scattering matrix from an experiment was not previously possible and is a major achievement by itself. It allows us to calculate all the state-to-state probabilities of the scattering event, and is arguably the most sensitive benchmark which currently exists to validate the accuracy of molecule-surface interaction potential calculations.

Another major step in terms of exploiting and interpreting molecular interference measurements to study molecule-surface interactions, was the discovery and explanation of multiple magnetic coherences in molecular scattering experiments. We demonstrated experimentally and theoretically that there is a dramatic increase in the number of possible coherence conditions when using a rotating molecular probe [Physical Chemistry Chemical Physics, advanced publication (2021)]. This new type of measurement presents an alternative and very sensitive way of assessing the molecule-surface interaction and serves as an important guide for future molecular spin echo experiments.

Development of the Magnetic Molecular Interferometry (MMI) setup:
The MMI setup which is the main experimental platform for this project has been extensively developed. The most dramatic change was the addition of a third straight through arm (molecular beam line) which allows us to characterise the quantum state evolution in the beam without surface scattering. Figure 2A shows a picture of the instrument, figure 2B shows a colour-plot profile of the beam reaching the detector, measured by scanning a small aperture. Figure 2c shows the measured oscillation curves (intensity versus magnetic field) obtained for different apertures, the position and shape of these apertures is superimposed on the intensity map in figure 2B. The red lines are calculations performed by the evolution code which overall show a very good agreement with the measured data. Other major improvements include changes to the sample and source cooling and adding an low energy electron diffraction option to better characterise the samples we study.

Preparation for molecular spin echo surface dynamics measurements:
We have developed a fully quantum mechanical theoretical framework for simulating a magnetically manipulated molecule-surface collision. Unlike other calculation methodologies we developed, which treated the magnetic Hamiltonian quantum mechanically and the molecular centre of mass classically, this fully quantum approach can be extended to include time dependent scattering and simulation of molecular spin echo experiments from dynamic surfaces [ Physical Review A, 101, 062703 (2020)].

Extending molecular interferometry experiments beyond H2 beams.
A first step towards extending MMI measurement to other poly atomic molecules is being able to perform efficient initial state selection. Using an instrument which combines magnetic spin selection techniques we developed with REMPI-TOF-MS, developed by the group of Prof. Ayotte in Sherbrooke, we demonstrated and characterised the selection capabilities for an ortho-water [The Journal of Physical Chemistry A, 123, 9234(2019)].
All the activities described above are beyond the state of the art, using new experimental and theoretical methodology to perform studies which were previously impossible. The results achieved so far are very encouraging and the goals of the original research proposal will be pursued as planned. The effects of the shutdown period due to Covid-19 should be mostly balanced out by the 6 months no-cost extension of the grant.

We expect to produce more stereodynamic scattering studies with H2 colliding with different surfaces resulting in valuable benchmarks for theoretical developments , perform similar experiments using D2 instead of H2, perform 3rd arm experiments on a methane and perhaps also a water molecular beam, and attempt scattering experiments with one or both of these more complex molecular beams.
We also intend to continue and pursue the two high risk activities of this grant, controlling reaction rates through rotational projection selection and measuring surface dynamics with a molecular beam probe. It is impossible to predict if these activities will succeed but we are preparing the system for these experiments and preparing appropriate theoretical modelling for analysing the surface dynamics experiments. We expect to have an empirical answer regarding the feasibility of such measurements by the end of the project.