Deuterium fractionation in ultracold collisions using trapped molecular ions

One of the key elements of the quantum theory developed in the early 20th century is the concept of wave-particle duality. Thus particles, such as electrons, can be diffracted in a manner similar to X-rays, and conversely light can be considered to be composed of particle-like energy packets known as photons .In this project we aim to explore chemical processes at temperatures very close to absolute zero where these concepts of wave-particle duality become very important. When matter is cooled, the constituent atoms and molecules have less kinetic energy and move more slowly, hence their momentum is decreased. In the quantum theory this implies that their wavelength increases and their wave-like characteristics are enhanced. The conventional 'particle-view' of chemical processes describes these as occurring through collisions of molecules with one another, breaking and making new chemical bonds. At very low temperatures, however, the fundamental physical description of a chemical transformation changes; the picture of classical collisions needs to be replaced with a description in terms of wave motion, with the implied effects of superposition and interference. The specific goal of this interdisciplinary project is the precise measurement of ultracold reactions of ammonia cations and methyl cations in the gas phase with neutral species such as H2O and NH3. We intend to explore how the intrinsic wave properties of the reaction like tunneling, barrier reflection and interference become important in determining the rate of the reaction as a function of temperature. Ion-neutral reactions tend to have very low energy barriers and hence can be very fast even at the lowest temperatures. The low kinetic energy makes the behaviour of the reaction highly sensitive to the long-range forces between the molecules. Ultracold collisions offer the possibility of new forms of control using electromagnetic fields. This is therefore a very unusual regime for investigating chemical dynamics, but also one well suited for testing quantum theories of chemical reactivity. At temperatures near 10 K the studies become relevant to understand the rich and diverse chemistry of interstellar gas. We will study reactions in which one or two of the hydrogen atoms in the neutral molecule have been replaced with a deuterium atom. Reactions of this type are very important in the context of the interstellar medium because they determine the concentrations of deuterated molecules in space and these can be related back to fundamental theories of the formation of the universe.Experimentally we employ novel devices for controlling the temperature and motion of the reacting species. The ions are trapped in a highly localized region of a vacuum chamber using radiofrequency fields and are maintained at very low temperatures using the technique of laser cooling. The ions interact with a beam of neutral molecules provided by a Stark decelerator , a device which can decelerate polar molecules. Combining this with an ion trap provides us with a unique and internationally leading set-up to measure ultracold reactive collisions with unparalleled control. In order to measure the rates of reactions, novel techniques will be developed to measure the change in the numbers and species of the ions over time. This can be done by applying additional electric fields which shake the trapped ions. The resultant motion of the ions is characteristic of the number of reactant and product ions present and their masses. New laser-based techniques will also be used to control and monitor the internal motions of the molecule. The techniques which will be developed in the scope of this project are novel and timely and will undoubtedly have high impact in one of the most rapidly developing fields of chemical physics. The reaction rate measurements will also have potential impact in Astrophysics as well as computational chemistry.

1.) Academic Impact: The proposed research is expected to impact in the academic sphere. Cold and ultracold collisions are subjects of intense experimental and theoretical activity. The UK is strong in this area and the unique experimental programme proposed here will further strengthen this role. The beneficiaries in the fields of cold matter physics, reactions dynamics and quantum chemistry, spectroscopy and metrology and in astrophysics/astrochemistry are described under Academic Beneficiaries. 2.) Economic and industrial Impact: Immediate Impact: Some equipment which will be designed for this project may become viable products especially for specialised companies. Matthias Keller is currently in discussions with a UK distributor to market specialised electronic equipment and opto-mechanical components which were designed and built for other research projects. The funding for this project is likely to catalyse the development of more devices which may become commodities and may lead to spin-offs. Long-term Vision: The dream of many chemists is to be able to understand and fully control chemical processes at a molecular level. With a little imagination, we can foresee a world where certain types of chemical transformation are no longer performed in the traditional reaction vessel, but in an environment where each reacting particle and the interactions between those particles are perfectly controlled. While it is unlikely that this will ever replace bulk chemistry, it may well enable delicate chemical transformations to be performed which are beyond the scope of operating in a thermal entropy driven environment. Studying chemical processes under ultracold conditions provides a promising approach towards this goal of complete control. The work proposed in this programme will advance our ability to control chemical processes under ultracold conditions. This project may ultimately facilitate a revolution in the production of specialised target molecules and thus have long-term impact on the chemical industry. The techniques developed in this project may also be used to identify molecules on a single particle level and thus may be developed into a device to analyse substances with ultra-high sensitivity. Such a product may be used to analyse tiniest amounts of trace gases. 3.) Skills, training and knowledge economy: The current proposal employs cutting edge techniques in atomic, molecular and laser physics and the students and postdocs engaged with this project will benefit from the high quality training in these methods. The postdoc, in spending two years in each institution will have the opportunity to benefit from the interdisciplinary collaboration between chemists and physicists and thus gain multiple perspectives on the work programme and the field more generally. Also undergraduate and postgraduate students will benefit from the training they will receive during the project. All personnel will be able to take part in the transferable skills training programmes of their host university. They will also be exposed to and interact with the international community of scientists working in this field which comprises many of the leading physics and physical chemistry groups in the world. 4) Impact in society The proposed research has great potential to engage the public in the fascination of fundamental science and we will use the results as a springboard to public outreach work. The work can be presented in a highly visual format - for example single ions can be imaged and reactions observed in real time through the disappearance of these ions. The interpretation of this work is strongly linked to the 'mysterious world' of quantum physics. We have the opportunity to convey the excitement of the connection between this microscopic world and the macroscopic world of chemical transformations.