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PhD Opportunities

Positron binding and annihilation in molecules

School of Mathematics and Physics | PHD
Funding
Unfunded
Reference Number
MAP/2022/28
Application Deadline
11 February 2022
Start Date
1 October 2022

Overview

Project supervisors: Dr G. F. Gribakin and Dr D. G. Green Contacts: g.gribakin@qub.ac.uk, d.green@qub.ac.uk State of the art and motivations The positron is the antiparticle of the electron. It was the first antimatter particle ever discovered, first theoretically by Paul Dirac in 1931, and then experimentally by Carl Anderson in 1932, both physicists awarded the Nobel Prize soon after. Positrons are also the simplest and most abundant form of antimatter. They come from B+ radioactive decays, can be generated in accelerators, and are produced in large quantities (15x10 tonnes per second!) near the centre of our Galaxy. The ability of positrons to annihilate with electrons and emit characteristic annihilation gamma rays, underpins their use in various diagnostics, from positron lifetime spectroscopy of solids to positron emission tomography (PET) in medicine. When partnered with antiprotons, positrons can form antihydrogen, currently under intense investigation at CERN.

The electrons with which positrons annihilate are usually not free but packed in atoms or molecules. The process of annihilation is strongly affected by the positron interaction with the target. For example, positrons are repelled by atomic nuclei, so they usually annihilate only with the outermost, valence electrons. On the other hand, when a positron approaches an atom or molecule, it polarises it by pulling the electron cloud towards itself. This gives rise to an attractive polarisation potential acting on the positron. Another important effect is 'hopping' of an atomic electron to the positron, temporarily forming an electron-positron 'atom' called positronium (Ps). This increases the positron-atom attraction and strongly enhances the positron annihilation probability (see, e.g., detailed calculations for noble-gas atoms [1]).

For many atoms, the attraction is so strong that it overcomes the positron-nucleus repulsion and allows the creation of positron-atom bound states (predicted in [2] and proved variationally in [3]). To date, positron binding to about ten atoms has been predicted in state-of-the-art calculations. There are also from expectations that many more atoms are capable of binding [4, 5], but there have not been any experimental conformation of this phenomenon yet [6].

What makes the problem of binding so important is the effect it has on positron annihilation in molecules [7]. When a positron collides with a molecule, it can be captured into the bound state by transferring its excess energy into molecular vibrations. This gives rise to resonances and orders-of-magnitude enhancement of the annihilation rates. Annihilation studies of resonances enabled measurements of positron binding energies for over 70 molecules (see [8] and references therein). In contrast, a significant theoretical effort towards computing positron-molecule bound states resulted in only a handful of predictions that could be compared with experiment, with the best agreement being at 25% level [9]. The reason for this was that positron-molecule attraction
and binding are due to electron-positron correlation effects, such as polarisation and virtual Ps formation, which are difficult to describe theoretically.

In 2018 a new method was proposed to attack the problem. Based on the earlier extensive studies of positron-atom interactions [1], we constructed a model positron-molecule correlation as well as positron scattering and annihilation from small molecules [12]. The relative simplicity of the method has enabled us to apply it to more difficult problems, such as the dependence of the positron binding on the molecular conformation (i.e., the shape of the molecule) for long alkane molecules with up to 16 carbon atoms [13].

Objectives & Methodology
The aim of the project is to explore positron binding to molecules and related problems by developing the new approach further. At the heart of the theory is the positron-molecule model correlation potential. While some of its parameters are based on the known molecular properties (such as the dipole polarisabilities), others are adjustable, e.g., by comparison with experimental data for positron binding. The potential also has some limitations, e.g., its long-range form is assumed to be spherical, which is an additional approximation.
Future work will involve introduction of non-spherical potentials, consideration of new molecular families and new processes, e.g., the interaction of the positron-molecule complexes with photons, or determination of the spectra of annihilation gamma rays, an area where large amounts of experimental data still await proper theoretical understanding.

On the technical side, calculations of positron binding and annihilation rates will be performed by using GAMESS [14], an advanced, free quantum-chemistry package. Its capabilities have been expanded by Dr Andrew Swann who introduced the model correlation potential for positron binding and implemented the calculations of the annihilation rate in the positron bound state. Studying new problems will require further development of the computer codes, analysis of the results, comparisons with experimental data (where available) and making predictions of new processes and properties.

Collaborations
Our group has a successful long-term collaboration with the experimentalist group of Prof. Cliff Surko (University of California, San Diego), whose famous positron trap is the workhorse of many positron experiments world over, and who pioneered measurements of positron-molecule resonant annihilation and binding energies. New links are emerging with the group of Dr. David Cassidy at UCL (London), who develop novel experiments with long-lived Rydberg-state positronium, with prospects of measuring positron binding to atoms and molecules [6].

Required skills
The candidate is expected to have good working knowledge of and interest in Quantum Mechanics. He/she will be expected to learn and master quantum-chemistry computational approaches. Running, modifying and developing computer codes will be an integral and substantial part of the project. In spite of the complexity of the codes, the underlying physics of positron-molecule binding and positron annihilation is relatively simple, and it is expected that many interesting insights into the problem will be obtained.

Further information
While focussed on the positron-molecule binding problem, the project will introduce the candidate to the range of problems concerning antimatter and its applications, as well as to methods of quantum chemistry and its computational approaches. The experience of handling and writing computer codes, that will be acquired through the work on the project, will be useful in a wide range of future careers.

For further information, please contact Dr Gleb Gribakin.


References
[1] D. G. Green, J. A. Ludlow, and G. F. Gribakin, Phys. Rev. A 90, 032712 (2014); D. G. Green and G. F. Gribakin, Phys. Rev. Lett. 114, 093201 (2015).
[2] V. A. Dzuba, V. V. Flambaum, G. F. Gribakin, and W. A. King, Phys. Rev. A 52, 4541 (1995).
[3] G. G. Ryzhikh and J. Mitroy, Phys. Rev. Lett. 79, 4124 (1997).
[4] J. Mitroy, M. W. J. Bromley and G. G. Ryzhikh, J. Phys. B 35, R81 (2002).
[5] V. A. Dzuba, V. V. Flambaum, and G. F. Gribakin, Phys. Rev. Lett. 105, 203401 (2010); V. A. Dzuba, V. V. Flambaum, G. F. Gribakin, and C. Harabati, Phys. Rev. A 86, 032503
(2012); C. Harabati, V. A. Dzuba, and V. V. Flambaum, Phys. Rev. A 89, 022517 (2014).
[6] A. R. Swann, D. B. Cassidy, A. Deller, and G. F. Gribakin, Phys. Rev. A 93, 052712 (2016).
[7] G. F. Gribakin, J. A. Young and C. M. Surko, Rev. Mod. Phys. 82, 2557 (2010).
[8] J. R. Danielson, A. C. L. and Jones, J. J. Gosselin, M. R. Natisin, and C. M. Surko, Phys. Rev. A 85, 022709 (2012).
[9] M. Tachikawa, Y. Kita, and R. J. Buenker, Phys. Chem. Chem. Phys. 13, 2701 (2011); M. Tachikawa, J. Phys. Conf. Ser. 488, 012053 (2014).
[10] A. R. Swann and G. F. Gribakin, Calculations of positron binding and annihilation in polyatomic molecules, J. Chem. Phys. 149, 244305 (2018).
[11] A. R. Swann and G. F. Gribakin, Positron binding and annihilation in alkane molecules, Phys. Rev. Lett 123, 113402 (2019).
[12] A. R. Swann and G. F. Gribakin, Model-potential calculations of positron binding, scattering, and annihilation for atoms and small molecules using a Gaussian basis, Phys. Rev. A 101, 022702 (2020).
[13] A. R. Swann and G. F. Gribakin, J. Chem. Phys. 153, Effect of molecular constitution and conformation on positron binding and annihilation in alkanes, 184311 (2020).
[14] M. W. Schmidt et al., J. Comput. Chem. 14, 1347 (1993); M. S. Gordon and M. W. Schmidt, in Theory and Applications of Computational Chemistry: the first forty years, edited by C. E. Dykstra et al. (Elsevier, Amsterdam, 2005) pp. 1167{1189.

Project Summary
Supervisor

Dr Gleb Gribakin


Mode of Study

Full-time: 3 years


Apply now Register your interest

Physics overview

The scientific research within the School of Mathematics and Physics was highly rated in the 2014 REF peer-review exercise, with 70% of research being judged as internationally excellent or world-leading. Physics at Queen's is currently joint 6th in the UK for Research Intensity and has been voted 4th in the UK for teaching satisfaction.

Physics research activity in the School is focused into five specific Research Centres; all members of academic staff belong to one of these Research Centres, listed below.

Astrophysics (PhD/MPhil)
Find out more below, or email Professor Mihalis Mathioudakis (m.mathioudakis@qub.ac.uk)

Atomistic Simulation (PhD/MPhil)
Find out more below, or email Dr Myrta Gruening (m.gruening@qub.ac.uk)

Nanostructured Media (PhD/MPhil)
Find out more below, or email Dr Amit Kumar (a.kumar@qub.ac.uk)

Plasma Physics (PhD/MPhil)
Find out more below, or email Professor Marco Borghesi (m.borghesi@qub.ac.uk)

Theoretical Atomic, Molecular and Optical Physics (PhD/MPhil)
Find out more below, or email Dr Alessandro Ferraro (a.ferraro@qub.ac.uk)

Registration is on a full-time or part-time basis, under the direction of a supervisory team appointed by the University. You will be expected to submit your thesis at the end of three years of full-time registration for PhD, or two years for MPhil (or part-time equivalent).

Physics Highlights
Career Development
  • Queen's graduates from Physics have secured employment through a number of companies such as Allstate, AquaQ Analytics, Citigroup, Deloitte, First Derivatives, PwC, Randox, Seagate, Teach First and UCAS. In addition, Belfast has been ranked as the world’s most business friendly small-medium sized city (Financial Times’ fDi Intelligence, 2018)
World Class Facilities
  • Since 2014, the School has invested over £12 million in new world-class student and staff facilities. Maths and Physics students have their own teaching centre that opened in 2016, housing brand experimental physics laboratories, two large computer rooms plus a student interaction area with a new lecture theatre and study rooms. In addition to this, Northern Ireland has the lowest student cost of living in the UK (Which? University, 2018) and is over £5000 per year cheaper for students to live in Northern Ireland compared to London
Internationally Renowned Experts
  • Queen's is joint 6th in the UK for Research Intensity for Physics and Astronomy (Complete University Guide 2021). The School has a continually growing international community of both undergraduate and postgraduate students and staff. Our research is conducted and recognised as excellent across the world. Staff are involved in cutting-edge research projects that span a multitude of fields.
Key Facts

  • Students will have access to our facilities, resources and our dedicated staff. The School of Maths & Physics is one of the largest Schools in the University. Staff are involved in cutting-edge research that spans a multitude of fields.

Course content

Research Information

Research Themes
Astrophysics (PhD/MPhil)

You’ll be involved in the search for distant supernovae and where they came from; study the asteroid and comet population in the Solar system; look for planets orbiting other stars in our Galaxy; study flares and other dynamic processes in the atmosphere of the Sun. You’ll have the opportunity to spend extensive periods at world-leading research centres such as the European Southern Observatory and NASA Goddard Space Flight Center.

At Queen’s we lead major European consortia and are supported by a multi-million pounds portfolio of research grants from a range of sources, including the UK Science and Technology Facilities Council, the Royal Society, and European Union.

Research Themes
Atomistic Simulation (PhD/MPhil)

Atomistic Simulation is the development and use of theoretical and computational methods to study structural, dynamical, and optical properties of molecules, liquids, solids and plasmas at the nanoscale. Computational experiments are used to interpret existing experimental data and to predict phenomena yet unobserved.

You’ll study problems at the interfaces between condensed matter physics, materials science, chemistry, biology, and engineering. You’ll interact with laboratory-based colleagues at Queen's and internationally, addressing fundamental and/or practical questions, and you will develop and program novel simulation methodologies to model for instance, electronic excitations, optical properties of materials, and the interaction between electric currents, heat and light.

Themes that are presently studied in the ASC include: non-linear optics in 2D materials, plasmonics, laser and ion-matter interactions, conduction in nanowires, aqueous Interfaces, nucleation, and crystallisation. Tools include time-dependent density-functional theory, many-body perturbation theory, classical molecular dynamics, Monte Carlo simulations and machine learning.

Research Themes
Centre for Nanostructured Media (PhD/MPhil)

Human history is defined by the materials we use to underpin our technology: stone, bronze, iron, silicon. As a PhD student in the Centre for Nanostructured Media, you will be playing a part in the development of materials systems which will, in some way, define our technology for the future. How can this not be exciting ? You will seek to reveal the physics of material behaviour at the boundary of current global knowledge and, at the same time, become proficient in techniques for materials growth, patterning and characterisation that are highly valued in high-tech companies and commercial research institutions, as well as in academic research settings. Our laboratories are extremely well-equipped for international-level research and our links to other research teams throughout the world in both academia and industry are strong and you should expect to travel, should you wish to, as part of your PhD experience.

Research Themes
Plasma Physics (PhD/MPhil)

Your research will involve identifying, and responding to, major contemporary issues within ionised matter physics, with major activities in laser- and electrically-produced plasmas, ultra-fast atomic and molecular physics and the interaction of ionising radiation and plasmas with matter, including biological systems. This research will employ local, national and international facilities, including some of the most powerful laser systems worldwide. You will also benefit from transferring your research findings into the industrial and medical sectors.

Research Themes
Theoretical Atomic, Molecular and Optical Physics (PhD/MPhil)

You’ll contribute to a body of work with recent major developments including strong field laser interactions with atoms and molecules, quantum information processing, quantum optics, and quantum thermodynamics, antimatter interactions with atoms and molecules, electron scattering by very complex targets such as the iron peak elements, and by Rydberg atoms, quantum many-body physics, ultra-cold atomic systems, and simulation of their features, and foundations of quantum mechanics.

Postgraduate research programmes within CNM provide experience and training in state-of-the art academic research: many of our research strands are world-leading, as evidenced by performance in REF2014. In addition, most of our postgraduate researchers are exposed to functional materials and photonics in major multinational companies.

Prof Marty Gregg - School of Mathematics and Physics
Career Prospects

Alumni Success
Many of our PhD graduates have moved into academic and research roles in Higher Education while others have progressed into jobs such as Data Scientist, Software Engineer, Financial Software Developer, IT Graduate Associate, Technology Consultant, Research Physicist, Telescope Operator and R&D Engineer.
http://www.qub.ac.uk/directorates/sgc/careers/CareersInformationbySchoolandSector/MathsandPhysics/MathsandPhysicsCareerOptions/

People teaching you

Dr Amit Kumar
Head of Research Centre - Centre for Nanostructured Media
School of Maths and Physics

Dr Connor Ballance
Head of Centre - Centre for Theoretical and Atomic Molecular Physics
School of Maths and Physics
https://www.qub.ac.uk/Research/GRI/mitchell-institute/Study/linas/

Dr Myrta Gruening
Head of Research Centre - Atomistic Simulation Centre
School of Maths and Physics

Prof Marco Borghesi
Head of Research Centre - Centre for Plasma Physics
School of Maths and Physics

Prof Mihalis Mathioudakis
Head of Research Centre - Astrophysics Research Centre
School of Maths and Physics

Learning Outcomes

Course structure

0

Assessment

Assessment processes for the Research Degree differ from taught degrees. Students will be expected to present drafts of their work at regular intervals to their supervisor who will provide written and oral feedback; a formal assessment process takes place annually.

This Annual Progress Review requires students to present their work in writing and orally to a panel of academics from within the School. Successful completion of this process will allow students to register for the next academic year.

The final assessment of the doctoral degree is both oral and written. Students will submit their thesis to an internal and external examining team who will review the written thesis before inviting the student to orally defend their work at a Viva Voce.

Feedback

Supervisors will offer feedback on draft work at regular intervals throughout the period of registration on the degree.

Facilities

Our world-class facilities support research and teaching across a diverse range of areas designed to fulfil specific activities. The School contains 4,700m2 of purpose-built laboratory space which includes the ANSIN materials research hub, the Ewald Microscopy Facility (EMF) and the Taranis laser facility. The Teaching Centre (opened in 2016) includes experimental physics laboratories, two large computer rooms and plenty of student study and interaction space. Our laboratories and equipment are looked after by a dedicated team of technicians and are used by our researchers, students and industry.

Entrance requirements

Graduate
The minimum academic requirement for admission to a research programme is normally an Upper Second Class Honours degree from a UK or ROI HE provider, or an equivalent qualification acceptable to the University. Further information can be obtained by contacting the School of Mathematics and Physics.

International Students

For information on international qualification equivalents, please check the specific information for your country.

English Language Requirements

Evidence of an IELTS* score of 6.0, with not less than 5.5 in any component, or an equivalent qualification acceptable to the University is required. *Taken within the last two years

International students wishing to apply to Queen's University Belfast (and for whom English is not their first language), must be able to demonstrate their proficiency in English in order to benefit fully from their course of study or research. Non-EEA nationals must also satisfy UK Visas and Immigration (UKVI) immigration requirements for English language for visa purposes.

For more information on English Language requirements for EEA and non-EEA nationals see: www.qub.ac.uk/EnglishLanguageReqs.

If you need to improve your English language skills before you enter this degree programme, INTO Queen's University Belfast offers a range of English language courses. These intensive and flexible courses are designed to improve your English ability for admission to this degree.

Tuition Fees

Northern Ireland (NI) 1 £4,596
Republic of Ireland (ROI) 2 £4,596
England, Scotland or Wales (GB) 1 £4,596
EU Other 3 £23,850
International £23,850

1 EU citizens in the EU Settlement Scheme, with settled or pre-settled status, are expected to be charged the NI or GB tuition fee based on where they are ordinarily resident, however this is provisional and subject to the publication of the Northern Ireland Assembly Student Fees Regulations. Students who are ROI nationals resident in GB are expected to be charged the GB fee, however this is provisional and subject to the publication of the Northern Ireland Assembly student fees Regulations.

2 It is expected that EU students who are ROI nationals resident in ROI will be eligible for NI tuition fees. The tuition fee set out above is provisional and subject to the publication of the Northern Ireland Assembly student fees Regulations.

3 EU Other students (excludes Republic of Ireland nationals living in GB, NI or ROI) are charged tuition fees in line with international fees.

All tuition fees quoted are for the academic year 2021-22, and relate to a single year of study unless stated otherwise. Tuition fees will be subject to an annual inflationary increase, unless explicitly stated otherwise.

More information on postgraduate tuition fees.

Physics costs

Depending on the area of research chosen there may be extra costs which are not covered by tuition fees.

Additional course costs

All Students

Depending on the programme of study, there may also be other extra costs which are not covered by tuition fees, which students will need to consider when planning their studies . Students can borrow books and access online learning resources from any Queen's library. If students wish to purchase recommended texts, rather than borrow them from the University Library, prices per text can range from £30 to £100. Students should also budget between £30 to £100 per year for photocopying, memory sticks and printing charges. Students may wish to consider purchasing an electronic device; costs will vary depending on the specification of the model chosen. There are also additional charges for graduation ceremonies, and library fines. In undertaking a research project students may incur costs associated with transport and/or materials, and there will also be additional costs for printing and binding the thesis. There may also be individually tailored research project expenses and students should consult directly with the School for further information.

Bench fees

Some research programmes incur an additional annual charge on top of the tuition fees, often referred to as a bench fee. Bench fees are charged when a programme (or a specific project) incurs extra costs such as those involved with specialist laboratory or field work. If you are required to pay bench fees they will be detailed on your offer letter. If you have any questions about Bench Fees these should be raised with your School at the application stage. Please note that, if you are being funded you will need to ensure your sponsor is aware of and has agreed to fund these additional costs before accepting your place.

How do I fund my study?

1.PhD Opportunities

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