Chemical kinetics of interstellar and planetary atmospheres

Summary

The Atmospheric, Planetary and Theoretical Chemistry (APTC) research group within the School of Chemistry are pleased to advertise one PhD studentship, fully-funded by the Science and Technology Facilities Council (STFC), in the general area of astrochemistry. One focus of the project is laboratory measurements of the chemical kinetics of elementary reactions under conditions encountered in the interstellar medium and the atmospheres of exoplanets, over a range of temperatures but particularly at very cold temperatures. Another focus of the project is the calculation of kinetic parameters using theoretical methods, and simulations of interstellar chemistry and planetary atmospheres using a numerical model. New kinetic data determined by experiment or using theory will be incorporated into astrochemical models to calculate the abundances of key molecules in space, for comparison with telescope observations, for example made by ALMA or JWST. The project will be of interest to post-graduate researchers wishing to work at the cutting edge of interstellar and exoplanet chemistry, and at the interface of physical chemistry and astrophysics.

The ATPC group has researchers working at the forefront of cold interstellar chemistry, dust formation in the circumstellar envelopes of evolved stars; the chemistry of metals which ablate from cosmic dust, and the formation of ice clouds in the atmospheres of Mars and Venus; and the atmospheric composition, chemistry and habitability of rocky exoplanets. There may also be opportunities to work in these areas. Full description Almost 300 molecules have been discovered in interstellar space, the atmospheres of other planets (including exoplanets) and comets using modern astronomical telescopes, such as ALMA (Atacama Large Millimeter/submillimetre Array) and the more recently launched JWST (James Webb Space Telescope). The telescopes can detect molecules containing a wide range of chemical functional groups at ever increasing spatial resolution (Molecules in Space). A major open question in astrochemistry (the branch of chemistry associated with the extreme conditions in space) concerns the mechanisms for the formation and subsequent fate of these chemical species, for example complex organic molecules (COMs) which are considered the building blocks of larger pre-biotic molecules essential for life. Are these molecules created on the surface of cold-grains, or is gas-phase chemistry playing an important role?

At Leeds we have discovered that reactions between two neutral species in the gas-phase play a key role in the astrochemistry of low temperature environments such as the interstellar medium (ISM), dense molecular clouds, protoplanetary disks, and planetary atmospheres, where temperature can be as low as 10 K. An example is the reaction of the hydroxyl radical with methanol, which despite the presence of a significant activation barrier to reaction proceeds rapidly to products at extremely low temperatures [1]. The Arrhenius equation tells us that the rate coefficient for this reaction should be vanishingly small at these temperatures. However, the formation of a weakly-bound pre-reactive van der Waals complex, which is sufficiently long-lived to undergo quantum mechanical tunnelling to form products, facilitates a dramatic increase in the rate coefficient at very low temperatures. Following further work by ourselves and other groups, these exciting findings have been found to also be applicable to reactions of OH and other small radical species with a variety of molecules [2-5].

In this project, you will explore the chemical mechanisms of key astrochemical reactions of small O-, N- and C- containing species with a variety of relevant astrochemical molecules (e.g. COMs) using both experimental and computational methods. The experimental part of the project involves using a pulsed Laval nozzle apparatus [1-5], which uses laser flash-photolysis combined with laser-induced fluorescence (LIF) spectroscopy to study the kinetics of reactions down to temperatures as low as ~ 20 K. A variety of nozzles enables a range of temperatures to be studied up to ~ 150 K and you may also use other apparatus in our laboratory to study reactions at higher temperatures, representative, for example, of the atmospheres of exoplanets. Collaboration is also envisaged with other groups using complementary experimental methods to probe the products of these reactions.

In the computational part of the project, you will calculate the potential energy surfaces (PESs) for the reactions using ab initio quantum methods utilising the Gaussian suite of computer programmes. You will use these PESs to compute rate coefficients as a function of temperature and pressure, together with product branching ratios using the MESMER rate theory software package [6], which was developed in Leeds. You will compare experiment and theory [3-5] in order to elucidate the mechanisms of these reactions, and you will develop parameterisations of rate parameters. In the project, you will use these parameters as input into astrochemical models to calculate the abundance of key molecules for comparison with telescope observations, and to investigate the astrochemical importance of these reactions.

There is flexibility within the project for you to work on other areas of astrochemical research in the Atmospheric, Planetary and Theoretical Chemistry research group, for example dust formation in the circumstellar envelopes of evolved stars, the chemistry of metals which ablate from cosmic dust, and the formation of ice clouds, in the atmospheres of Mars and Venus; and the atmospheric composition, chemistry and habitability of exoplanets.

References

[1] Shannon, R. J., M. A. Blitz, A. Goddard and D. E. Heard, Accelerated chemistry in the reaction between the hydroxyl radical and methanol at interstellar temperatures facilitated by tunnelling, Nature Chemistry, 2013, 5, 745-749.

[2] Heard, D. E., Rapid Acceleration of Hydrogen Atom Abstraction Reactions of OH at Very Low Temperatures through Weakly Bound Complexes and Tunneling, Accounts of Chemical Research, 2018, 51, 2620.

[3] Douglas, K. M.; Li, L. H. D.; Walsh, C.; Lehman, J. H.; Blitz, M. A.; Heard, D. E., Experimental, theoretical, and astrochemical modelling investigation of the gas-phase reaction between the amidogen radical (NH2) and acetaldehyde (CH3CHO) at low temperatures. Faraday Discussions, 2023, 245, 261-283.

[4] West, N. A., Li, L. H. D., Millar, T. J., Van de Sande, M., Rutter, E., Blitz, M. A., Lehman, J. H., Decin, L., Heard, D.E., Experimental and theoretical study of the low-temperature kinetics of the reaction of CN with CH2O and implications for interstellar environments, Phys. Chem. Chem. Phys., 2023, 25, 7719–7733.

[5] Douglas, K. M., D. Lucas, C. Walsh, M. A. Blitz, and D. E. Heard, Experimental and Theoretical Investigation of the Reaction of C2H with Formaldehyde (CH2O at Very Low Temperatures and Application to Astrochemical Models, ACS Earth Space Chem., 2024, 8, 2428−2441.

[6] Glowacki, D. R.; Liang, C. H.; Morley, C.; Pilling, M. J.; Robertson, S. H., MESMER: An Open-Source Master Equation Solver for Multi-Energy Well Reactions. Journal of Physical Chemistry. A, 2012, 116 (38), 9545-9560.

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