Advances in hydrogen molecular ions: H3+, H5+ and beyond

09:00-09:05 Welcome by the Royal Society & lead organiser

09:05-09:25 Hydrogen molecular ions: setting the scene

Professor Jonathan Tennyson FRS, University College London, UK

Abstract

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.

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09:25-09:50 Quantum computations on H5+ and H7+ ions

Dr Rita Prosmiti, IFF-CSIC, Madrid, Spain

Abstract

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.

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09:50-10:15 Dissociative recombination of H3+ and D5+

Professor Mats Larsson, Stockholm University, Sweden

Abstract

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.

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10:15-10:45 Discussion

10:45-11:15 Coffee

11:15-11:40 Lightning and charge processes in brown dwarf and exoplanet atmospheres

Dr Christiane Helling, Centre for Exoplanet Science, University of St Andrews, UK

Abstract

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.

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11:40-12:05 Probing the connecting seam between ground and first excited H3+ potentials by photodissociation and charge exchange

Professor Xavier Urbain, Université catholique de Louvain, Belgium

Abstract

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.

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12:05-12:30 Discussion

13:30-13:55 The Motion and Distribution of the Gas in the Central 300 pc of the Galaxy as Revealed by Spectra of H3+

Dr Thomas R Geballe, Gemini Observatory, USA

Abstract

Analysis of infrared absorption line profiles of H₃⁺ toward stars in the ~300 pc diameter central molecular zone (CMZ) of the galaxy show that most of the front half of the CMZ is filled with a few million solar masses of warm diffuse gas. The variation of the velocity profiles of the H₃⁺ lines across the CMZ demonstrate that this gas is moving radially outward from the centre at speeds of up to ~140 km/s. This is consistent with past interpretations of high velocity molecular gas observed at cm wavelengths in the galactic centre as arising in an expanding ring of gas at the outer edge of the CMZ. The characteristic time scale (r/vmax) of the expanding gas is roughly one million years, significantly less than the ages of the three clusters of hot and massive stars located near the centre, whose stellar winds and supernovae must contribute to powering the radial expansion of the diffuse gas. The observed velocities are significantly less than those found in the winds of massive stars and in supernova ejecta, suggesting that the ejected gas from stars and supernovae is impeded by the CMZ’s gas and decelerated by the gravity of its stars.

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13:55-14:20 H3+, the ideal probe for in situ measurements of cosmic rays

Professor Emeritus Takeshi Oka FRS, Enrico Fermi Institute and University of Chicago, USA

Abstract

Cosmic rays are mysterious particles mostly atomic nuclei with extremely high energy from 106 eV (MeV) to 1021 eV (Zev). Their energy spectra for many nuclei are known in detail from the measurements on the earth. To measure cosmic rays in the galaxy, however, we need a chemical method using spectroscopy. H₃⁺, trihydrium, provides the ideal probe for this purpose because of (1) its ubiquity, (2) simple chemistry, and (3) concise spectrum. For about 30 years from the classic paper by Spitzer & Tomasko (1968) when H⁺ was used as the probe, the cosmic ray ionisation rate of H₂ was thought to be on the order of ζ  ~ 10-17 s-1 and uniform throughout the galaxy. When in 1997 H₃⁺ was discovered in diffuse clouds and in the galactic centre (GC), however, this picture has gone down the drain. It is now established that ζ in diffuse clouds is 10 times higher than in dense clouds and ζ in the central molecular zone of the GC is 1000 times higher. The uniformity of cosmic ray energy density throughout the galaxy which was once thought to be reasonable because of its high penetrability has been completely negated.

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14:20-14:45 Deuterated forms of H3+ and their importance in astrochemistry

Professor Paola Caselli, Max Planck Institute for Extraterrestrial Physics, Garching, Germany

Abstract

At the low temperatures (~10 K) and high densities (~100,000 H2 molecules per cc) of molecular cloud cores and protostellar envelopes, a large amount of molecular species (in particular molecules containing C and O) freeze-out onto dust grain surfaces. It is in these regions that the deuteration of H₃⁺ becomes very efficient, with a sharp abundance increase of H₂D+ and D₂H⁺. The multi-deuterated forms of H₃⁺ participate in an active chemistry: (i) their collision with neutral species produces deuterated molecules such as the commonly observed N₂D⁺, DCO⁺ and multi-deuterated NH₃; (ii) their dissociative electronic recombination increases the D/H atomic ratio by several orders of magnitude above the D cosmic abundance, thus allowing deuteration of molecules (eg CH₃OH and H₂O) on the surface of dust grains. Deuterated molecules are the main diagnostic tools of dense and cold interstellar clouds, where the first steps toward star and protoplanetary disk formation take place. Recent observations of deuterated molecules will be presented and discussed in view of astrochemical models inclusive of spin-state chemistry. Models assuming complete scrambling to calculate branching ratio tables for reactions between chemical species that include protons and/or deuterons will be compared to models that assume non-scrambling, and with observations.

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14:45-15:15 Discussion

15:15-15:45 Tea

15:45-16:10 Hydrogen molecular ions and the violent past of the Solar System

Professor Cecilia Ceccarelli, Université Grenoble Alpes / IPAG, France

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16:10-16:35 H3+ as an ionospheric sounder of Jupiter and giant planets

Dr Pierre Drossart, LESIA, Observatoire de Paris, Meudon, France

Abstract

As the main infrared emitter of the ionosphere of hydrogen atmospheres, H₃⁺ is an ideal probe for these evanescent atmospheric layers. Ionospheres are transition layers between the deeper atmosphere and outer space. As such, they are of particular importance to understand the energetic processes at work from above and below. H₃⁺ controls the temperature of the ionosphere, by ensuring an energy balance between the infrared radiative emission and the energy input from external or internal sources. The source of heating are twofold: UV and external particle precipitation, with all the aeronomy reactions induced, and gravity waves dissipation from internal sources. Understanding the balance between these different processes is an active research field in astrophysics, for giant planets and exoplanets. The spatial variability of these phenomena is inherent to their sources: auroral precipitations, wave activity related to the dynamics, or even meteoroids. The ability to obtain high spatial resolution images in H₃⁺ emission from ground-based telescopes is therefore of high interest for 3D models of thermal global circulation models, in association with other observational techniques (UV observations, radio occultation). Mapping H₃⁺ emissions on giant planets will ultimately address the question of the coupling between the magnetosphere and the ionosphere.

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16:35-17:00 Discussion

17:00-18:00 Poster reception sponsored by the UK Collaborative Computational Project on Quantum Dynamics (CCPQ)

09:00-09:25 A last look at Saturn during Cassini’s Grand Finale

Dr Tom Stallard, University of Leicester, UK

Abstract

This talk will present analysis of Saturn’s aurora taken during the final month of the Cassini space mission. Saturn was observed from two contrasted perspectives. As Cassini flew in its Grand Finale orbits, sweeping closer and closer over the polar regions, the infrared VIMS instrument observed the auroral regions at incredibly high spatial resolution. In contrast to this, simultaneous observations made by the Keck telescope on Mauna Kea scanned the auroral region using the NIRSPEC instrument, providing a wider view of the entire auroral region at very high spectral resolution. The unprecedented Keck Cassini Grand Finale support programme scanned Saturn’s auroral region from Earth over seven separate orbits, providing the first ever maps of temperature and ion wind structure. This talk will describe the thermal and ion wind structures observed by Keck, comparing these with Cassini-VIMS observations of the aurora, and placing them in the context of the solar wind conditions observed during the final stages of Cassini’s mission. It will also reveal the first evidence of how Saturn’s ionospheric and thermospheric structures are controlled by changing solar wind conditions.

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09:25-09:50 JUNO/JIRAM’s view of Jupiter’s H3+ emissions

Dr Bianca Maria Dinelli, ISAC-CNR, Bologna, Italy

Abstract

The instrument JIRAM (Jovian Infrared Auroral Mapper), on board the NASA spacecraft Juno, is both an imager and a spectrometer. Two distinct detectors are used for imaging and spectroscopy. The imager acquires Jupiter images in two bands, one of which (L band, 3.3-3.6 μm) is devoted to monitor the H₃⁺ emission. The spectrometer covers the spectral region from 2 to 5 μm (average spectral resolution 9 nm) with a 256 pixels slit, that can observe the same scene of the L band imager with some delay. JIRAM scientific goals are the exploration of the Jovian aurorae and the planet’s atmospheric structure, dynamics and composition. Starting early July 2016 Juno is orbiting around Jupiter. Since then, JIRAM has provided an unprecedented amount of measurements, monitoring both Jupiter’s atmosphere and aurorae. In particular the camera has monitored Jupiter’s poles with unprecedented spatial resolution, providing new insights in both its aurorae and the polar dynamic. The main findings obtained by the L imager are detailed pictures of Jupiter’s aurorae showing an extremely complex morphology of the H₃⁺ distribution in the main oval and in the moon’s footprints. The spectrometer has enabled to measure the distribution of both H₃⁺ concentration and temperature.

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09:50-10:15 H3+ Emissions related to Jupiter’s Great Red Spot

Dr Licia Ray, Lancaster University, UK

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10:15-10:45 Discussion

10:45-11:15 Coffee

11:15-11:40 Understanding Uranus

Dr Henrik Melin, University of Leicester, UK

Abstract

Emissions from the molecular ion H₃⁺ was first discovered at Uranus in May 1992, three months before it was discovered at Saturn. Near-infrared ground-based observations can be used to monitor the physical conditions in the upper atmosphere of the planet over time. The orbital period of Uranus about the Sun is 84 years and it is expected that the seasonal and thermal time-scales are very long. Between 1992 and 2010, a period of long-term cooling was observed, from ~750 K to ~550 K. This was initially attributed to be seasonal in nature, with the expectation that the trend would reverse to a sustained period of heating at some point after the 2007 equinox. However, analysis of recent observations, obtained between 2011 and 2018, using a range of ground-based facilities, reveal that the cooling has continued. This is a surprise, and the potential reasons for this are explored in this presentation. Looking forward, the launch of the James Webb Space Telescope in 2021 promises to revolutionise our understanding of H₃⁺ emissions from Uranus, with full disk mapping observations already in the pipeline as part of the Guaranteed Time Observing (GTO) programme.

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11:40-12:05 Modelling H3+ in planetary atmospheres

Dr Luke Moore, Boston University, US

Abstract

The molecular ion H₃⁺ has proved to be a remarkably useful probe of the upper atmospheres of the giant planets. This atmospheric region is otherwise difficult to monitor remotely, and encodes signatures of the complex coupling between the vacuum of space above and the dense atmosphere below. There are three main aspects of remote H₃⁺ observations that provide valuable insight to models of giant planet ionospheres. First, unconstrained proton chemical loss rates lead to uncertainty in modelled electron densities. Protons, electrons and H₃⁺ dominate giant planet ionospheres, and so constraints on H₃⁺ densities help to reduce this uncertainty. Second, modelled H₃⁺ densities are tightly coupled to the influxes of external material and to atmospheric mixing. Intense auroral structures near the magnetic poles are due to precipitation of energetic particles. At lower latitudes, influxes of less energetic particles, such as interplanetary dust grains or charged dust from Saturn’s rings, can modify ionospheric chemistry, leaving an imprint in the measured H₃⁺ densities. Thus, careful modelling of H₃⁺ observations can also provide insight into external influxes that would otherwise be difficult to constrain. Finally, H₃⁺ observations are also one of the few means by which giant planet upper-atmospheric temperatures can be measured.

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12:05-12:30 Discussion

13:30-13:55 Ultracold scattering for strongly interacting atoms and molecules: A new computational methodology

Dr Laura McKemmish, University of New South Wales, Sydney, Australia

Abstract

Experiments by Carrington in 1993 probed by near-threshold photodissociation of H₃⁺ revealed a dense and complex resonance structure that is still poorly understood and completely unassigned. Coupled with the desire to theoretically support upcoming experiments into ultracold collisions over deep potential wells, we have been developing a new theoretical framework and computational methodology to treat heavy particle collisions (e.g. H⁺ with H₂) based on the computable R-matrix method that has proven outstandingly successful for the study of electron collisions with atoms and molecules. R-matrix theory involves the division of space into an inner region encompassing the whole collision complex and an outer region where species involved in the scattering can be separately identified. In the computable R-matrix method, the Schrodinger equation for the restricted inner region is solved once and for all for each scattering symmetry, independent of the precise scattering energy.  For collisions between H⁺ and H₂, this is particularly efficient as the H₃⁺ potential energy surface is very deep. In the energy-dependent outer region, it is then only necessary to treat a few partial waves, facilitating propagation of solutions to extremely large inter-particle separations and high energy resolution.

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13:55-14:20 The spectroscopy of molecules related to H3+

Professor Stephan Schlemmer, Universität zu Köln, Germany

Abstract

The proton affinity of molecular hydrogen is small and therefore H₃⁺ readily passes on its loosely bound proton to most other molecules in the interstellar medium. This leads to the formation of abundant molecular ions like HCO⁺ or N₂H⁺ which are used as diagnostic tools in many astronomical studies. Through the same process also other molecular ions are formed. In particular protonated forms of very abundant neutral species, like O2H⁺, CH₅⁺, or protonated methanol should be expected. However, the observational search for many of those ions is hampered since their spectra are not known. Recording spectra of such transient species in the laboratory is an experimental challenge. Action spectroscopy in cold ion traps as developed continuously over the last 20 years circumvents these difficulties. Here, the interaction of the ions with light is recorded by a chemical alteration of the ion cloud composition of only a few thousand mass selected, cold molecular ions by mass spectrometric means. Recent examples of spectra recorded via light induced reactions in cold ion traps and related methods will be discussed.

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14:20-14:45 On the low-temperature behaviour of simple reactions involving H+ and H2+

Professor Frédéric Merkt, ETH Zurich

Abstract

The talk will present experimental studies of molecular-hydrogen ion chemistry that exploit techniques developed in the context of research on cold molecules. In these experiments, the researchers try to characterise the elementary reactions through which the molecules H₂⁺, H₂ and H₃⁺ are formed, in particular the reactions H₂ + H₂+ into H₃⁺ + H and H + H⁺ into H₂⁺ + hn. To access the temperature regime below 10 K, the researchers suppress heating effects by stray electric fields by studying the ion-neutral reaction systems within the orbit of a highly excited Rydberg electron and verify that the Rydberg electron does not significantly affect the outcome of the reactions. For this research, the researchers have, over the years, developed dedicated radiation sources and chip-based experimental platforms to control the external and internal degrees of freedom of Rydberg atoms and molecules, which will also be presented.

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14:45-15:15 Tea

15:15-15:40 Non-adiabatic effects in the H3+ spectrum

Professor Alexander Alijah, University of Rheims, France

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15:40-16:05 Astrochemical studies at the Cryogenic Storage Ring

Dr Holger Kreckel, MPI Kern Physik, Germany

Abstract

The new Cryogenic Storage Ring (CSR) at the Max Planck Institute for Nuclear Physics in Heidelberg is moving from the commissioning phase toward scientific operation. The CSR provides long storage times at extremely high vacuum and low temperatures for atomic and molecular ions of almost arbitrary mass. The experimental vacuum chambers of the CSR can be cooled down to 5K, and it has been shown that within a few minutes of storage infrared active molecular ions (eg, CH₊ and OH^-) will cool to their lowest rotational states by spontaneous emission of radiation. Equipped with a novel ion-neutral collision setup and a low-energy electron cooler, the CSR offers unique possibilities for astrochemical experiments under true interstellar conditions. In this talk, an overview of the capabilities of the CSR will be presented along with first experimental results. Furthermore, the potential for astrochemical studies involving H₃⁺ (and deuterated versions of the triatomic hydrogen cation) will be discussed.

16:05-16:30 Discussion

16:30-17:00 Panel discussion/Overview (future directions)