Do single-bounce nuclear spin flips happen, and can they be measured?

Lead Research Organisation: Swansea University
Department Name: College of Science

Abstract

Atoms, and consequently molecules, possess a fundamental property called nuclear spin. In simple terms, nuclear spin can be thought of as a microscopic magnet associated with each atom. Nuclear spins play a role in many very different fields, ranging from medical applications such as magnetic resonance imaging to efficient and safe storage of liquid hydrogen rocket fuel. An important process in essentially all the applications which use nuclear spins, is the conversion of one spin state to another when the nuclear spins interact with each other and with their environment. Using the simple analogy to a magnet mentioned above, this can be considered as the microscopic magnet flipping its orientation.
In this project, we will attempt to look at a fundamental question in this field, which is whether the nuclear spin state of a molecule can change due to a single collision with a surface, and if it does, which types of surfaces and collision properties affect the probability of such a spin change. Conventional experimental techniques do not have the required sensitivity to follow such an event and the current level of theory cannot provide conclusive answers.
We will use a recently developed instrument which should be capable of sensing a spin change in a single collision. The instrument creates a beam of molecules in a well-defined nuclear spin state using various magnetic manipulations, aims them toward a solid surface, and then detects and analyses the spin state of the molecules emerging from the collision.
The results of this study are expected to contribute to our fundamental understanding of nuclear spin states, act as a benchmark for existing theoretical calculations, and eventually assist the development of more advanced theoretical models, which will be able to reliably model complex systems such as those encountered in the many scientific and industrial applications involving nuclear spins.

Publications

10 25 50

publication icon
Chadwick H (2022) Measuring surface phonons using molecular spin-echo. in Physical chemistry chemical physics : PCCP

publication icon
Chadwick H (2022) Multiple echoes in beam spin-echo spectroscopy and their effect on measurements of ultra-fast dynamics. in Journal of physics. Condensed matter : an Institute of Physics journal

 
Description The goal of this study was to study experimentally for the first time, whether the nuclear spin of an atom or molecule hitting a surface can flip in a single collision. To design and interpret such experiments we developed numerical methods, capable of simulating the expected changes in the signals due to spin flip events in three type of particle beams; 3He, H2 and D2.
Spin flip detection experiments were performed for different types of beams (3He, D2 and H2), different surfaces (clean and H-terminated Cr(110) and Chromium oxide) and different scattering conditions and surface temperatures. For all the conditions we measured the signals could be reproduced using a unity scattering matrix for the nuclear spin projection states, indicating (1) a negligible probability of spin-flips and (2) no preference for scattering as function of the nuclear spin state of the impinging particle. While this is a negative result, and it would be more exciting to be observe spin flips, the fact that our experiment has the necessary sensitivity to detect such events but doesn't, supplies a definitive experimental answer to this previously unanswerable question. These results will be published in the near future.
The modelling developments mentioned above, and the experiments we performed with the three types of particle beams, gave rise to four additional important results which were not anticipated in the original plan: The first was the identification of new coherencies in helium spin echo measurements which need to be taken into account when interpreting inelastic scattering experiments [J. Phys.: Cond. Matt. 34, 345901 (2022)]. A second, was the observation of clear stereodynamic effects in the scattering of H2 from H-terminated Cr(110). The oscillation curves we measured provide an experimental benchmark for testing future potential energy surface calculations of this system when these become available [manuscript in preparation].
Two other unexpected findings resulted from the D2 experiments we performed. One was the demonstration that the probability of a rotational de-excitation event in a molecule surface collision, can be altered by changing the rotational projection states of the impinging molecules [Nat. Comms. 13, 2287 (2022)]. The pico-eV manipulations we used to control a milli-eV energy exchange event challenges existing paradigms about energy matching and the need for ultra-cold environments in quantum control experiments. A second was the development of a new ultra-high-energy resolution scheme to measure energy exchanges between a molecule and surface phonons [PCCP, 24, 14198 (2022)]. Energy losses which can be measured with this new technique lie at the heart of surface sticking and surface based chemistry.
In summary, we succeeded in experimentally answering the question whether nuclear spins can flip in a single collision, and the answer is no, for the systems we looked at. The methods we developed can and will be used in the near future to search for spin flips in other types of magnetic surfaces, and have also lead to new insights and new methods to control and measure different types of energy exchange processes in a surface collision event .
Exploitation Route The results of this project can be taken forward in several ways:
1) Ruling out spin flips in the scattering processes we studied can be used to test the reliability of nuclear spin flip calculations and provides guidance to theoreticians working in the field.
2) The methods we developed in this project for detecting spin-flips can be immediately used for future measurements on other magnetic surfaces, significantly facilitating these follow up experiments.
3) The multiple coherencies in helium spin echo (HSE) measurements we identified alter the interpretation of inelastic HSE experiments, for example high resolution phonon studies performed with the HSE apparatus at the Cavendish laboratory, University of Cambridge.
4) Demonstrating that rotational de-excitation during a molecule-surface collision can be controlled using magnetic manipulation can be extended into other fields, for example the community of experimentalists and theoreticians which study collisions in the gas phase.
5) The quantitative results from both the D2 rotational de-excitation study and the stereodynamics of H2 scattering from Chromium are extremely sensitive benchmarks for theoreticians who calculate potential energies of molecule-surface systems.
6) The development of an experimental method capable of measuring energy exchange between molecules and surfaces with ultra-high resolution will supply unique experimental data for modelling surfaces beyond the static surface approximation.
Sectors Other

 
Title Development of a simulation to extract spin flips from molecular interferometry measurements 
Description In our group we have developed a semi-classical computer simulation which is capable of propagating the quantum states of helium-3, H2 and D2 through magnetic fields, calculating the expected signal in a molecular interferometry scattering experiment with or without spin flips during the scattering event. This computational tool is essential for analysing the scattering experiments we will perform and should be capable of identifying nuclear spin flips if they are present. A publication is being prepared describing the method and how it can be used (by us or others) to detect spin flips. 
Type Of Material Improvements to research infrastructure 
Year Produced 2022 
Provided To Others? No  
Impact An unintended use of the calculation tool we developed originally for spin flip detection experiments, was that it is also capable of analysing rotational control experiments of the multi-level J=2,I=2. The results of this application were submitted for publication and are currently under review. 
 
Description ECOSS presentation 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Oral presentation (given by Dr. Chadwick) of project results in the European conference on surface science (ECOSS), University of Luxembourg, Luxembourg.
Year(s) Of Engagement Activity 2022
URL https://ecoss2022.uni.lu
 
Description GRC presentation 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Oral presentation and poster presentation (Dr. Chadwick) at the Dynamics of Molecular Collisions and Interactions Gordon Research Conference, Stonehill College, Massachusetts, USA
Year(s) Of Engagement Activity 2022
URL https://www.grc.org/molecular-interactions-and-dynamics-conference/2022/
 
Description Lorentz Workshop 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Oral presentation of project results (Dr. Chadwick) at the Energy Dissipation at Surfaces Lorentz Workshop, Leiden, Netherlands
Year(s) Of Engagement Activity 2022
URL https://www.lorentzcenter.nl/energy-dissipation-at-interfaces-from-catalysis-to-astrochemistry-2022....
 
Description Presentation Molec 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Oral presentation (Dr. Chadwick) of project results , Molec conference series, DESY, Hamburg, Germany
Year(s) Of Engagement Activity 2022
URL https://indico.desy.de/event/32443/
 
Description SAMS meeting 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Oral presentation (Dr. Helen Chadwick) as part of the Scattering of Atoms and Molecules from Surfaces meeting, Cambridge, UK.
Year(s) Of Engagement Activity 2022
URL https://www.sams5.uk
 
Description Stereodynamics conference 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Oral presentation (Dr. Helen Chadwick ) of the project results during the Stereodynamics conference , Rethymnon, Crete.
Year(s) Of Engagement Activity 2022
URL https://stereodynamics2020.mitos.com.gr