Welcome by the Royal Society & lead organiser
Hydrogen molecular ions: setting the scene
Professor Jonathan Tennyson FRS, University College London, UK
The H3+ molecular ion is the simplest stable molecular ion of hydrogen. It is rapidly formed by collisions between H2 and H2+. Its role in the interstellar medium and the ionospheres of gas giant planets is now well established but careful studies of its spectra are providing valuable information on issues as diverse as the cosmic ray ionisation rate in different environments and wind speeds in planetary upper atmospheres.
H3+ is the electronically simplest stable polyatomic molecule and therefore provides a benchmark system for testing high accuracy ab initio methods. While impressive accuracy has been achieved, calculations on the isoelectronic H2 molecule remain many orders of magnitude more accurate; this problem is not directly due to issues with the multi-dimensional nuclear motion problem, which is capable of high accuracy solution, but more due to treating various subtle effects in many dimensions.
H5+ may seem superficially similar to H3+ but it is a fluxional molecule: one for which there is facile conversion between multiple equilibrium geometries leading to complicated and delocalised wave functions. H5+, and indeed the higher hydrogen molecular ions, thus raise their particular issues especially in terms of predicting and interpreting their spectral signatures. Reactions between ionised and neutral hydrogenic species, such as H+ + H2 or H2+ + H2, are of importance for studying hydrogen plasmas both on Earth and in the interstellar medium. These reactions also raise their own issues with fundamental physics.
Modern experimental techniques, such as the ability to study atoms and molecules at extremely low temperatures, allow these processes to be studied with increasing detail, which raises new challenges for theory to address. The original discovery of H3+ is now well over a century ago but there clearly remains a whole host of issues to be studied both using and involving the molecular ions of hydrogens.
Quantum computations on H5+ and H7+ ions
Dr Rita Prosmiti, IFF-CSIC, Madrid, Spain
The protonated hydrogen dimer, H5+, is the smallest system including proton transfer, and of longstanding interest since its first laboratory observation in 1962. H5+ and its isotopologues are the intermediate complexes in the deuterium fractionation reactions, and of central importance in molecular astrophysics. Recently, the IR spectra of H5+/D5+ and H7+/D7+ have been recorded revealing a rich vibrational dynamics of the cations, that present a challenge for the standard theoretical approaches. Although both of them are few-electron ions, which makes highly accurate electronic structure calculations tractable, the construction of ab initio-based potential energy and dipole moment surfaces has proved a hard task. In the same vein, the difficulties to treat the nuclear motion could also turn cumbersome as the dimensionality, floppiness and/or symmetry of the system increase. These systems are prototypical examples of studying large amplitude motions; as they are highly delocalised, interconverting between equivalent minima though rotation and proton transfer motions requiring state-of-the-art treatments. Recent advances in the vibrational spectroscopy of the H5+ cations and beyond will be reported from full quantum spectral simulations, providing important information in a rigorous manner, and open perspectives for further future investigations.
Dissociative recombination of H3+ and D5+
Professor Mats Larsson, Stockholm University, Sweden
Hydrogen has played a very important role in the development of the physical sciences during the 20th century, and continues to do so into the next millennium. Although well-known from the citation of the 1932 Nobel Prize in Physics, it is probably not widely understood why the discovery of the allotropic forms of hydrogen was singled out as the most important consequence of Heisenberg’s quantum theory. In this talk, the dissociative recombination of H3+ and D5+ will be discussed. The former has been the subject of many experimental and theoretical investigations, whereas the latter has received far less attention. Molecular hydrogen in its ionised forms is continuously reinventing itself, in recent years not least by its presence in “hot Jupiter” exoplanets.
Lightning and charge processes in brown dwarf and exoplanet atmospheres
Dr Christiane Helling, Centre for Exoplanet Science, University of St Andrews, UK
Exoplanet science is moving from object discoveries into characterisation and analysis. Transit spectroscopy has shown that exoplanet atmospheres form clouds which can be composed of minerals instead of water, and their phase curves indicate the presence of winds driven by the external, host-star irradiation. The large diversity of the more than 3000 exoplanets (of which a subclass resembles brown dwarfs) suggests that the atmospheres of these planets will differ considerably and so will their weather patterns and their chemical composition. Dr Helling will present results from 3D atmosphere simulations, providing insight into the complexity of exoplanet clouds, which are the precondition for lightning to occur. Lightning chemistry calculations show the strong feedback on the local chemistry, enabling the discussion of chemical lightning tracers. The occurrence of lightning is also linked to the ionisation of the atmospheres by external radiation. Dr Helling will present results that show that external radiation can cause the formation of a global but shallow ionosphere on brown dwarfs or of a deep but locally confined ionosphere on highly irradiated exoplanets. She will discuss the emergence of H3+ resulting from Aurora on brown dwarfs.
Probing the connecting seam between ground and first excited H3+ potentials by photodissociation and charge exchange
Professor Xavier Urbain, Université catholique de Louvain, Belgium
Earlier work on charge transfer in proton dihydrogen collisions demonstrated the decisive role of the seam connecting the ground and first excited potential energy surfaces of H3+. This phenomenon stems from the difference in dissociation energy between H2 and H2+ exceeding that of ionisation potential between H2 and H. While protons probe this connecting seam in a full collision, the photodissociation of H3+ is actually probing it from within, as fragments depart from the classical turning point accessed via a vertical transition from the ground state potential well. Detailed experiments have been conducted for both reactions, making extensive use of three-dimensional imaging of dissociation products, that allows for the precise determination of vibrational distributions of reactants and products. While proton dihydrogen charge transfer leaves excited H2+ products in specific vibrational distributions, photodissociation of hot H3+ in the near ultraviolet produces comparatively colder H2+ products. Modelling the wavepacket dynamics along the repulsive potential surface is expected to account for the repopulation of the ground potential energy surface on its way to H2 + H+ products. The role of the connecting seam will be emphasised and its importance for the astrophysically relevant H2+ hydrogen charge transfer reaction underlined.