Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - CICERO (Cold Ion Chemistry - Experiments within a Rydberg Orbit)

Teaser

This research project is devoted to the development of a new experimental approach to study ion-molecule reactions, i.e., reactions between neutral atoms or molecules and positively charged atoms or molecules, at very low temperatures, down to100 mK, and to investigate how...

Summary

This research project is devoted to the development of a new experimental approach to study ion-molecule reactions, i.e., reactions between neutral atoms or molecules and positively charged atoms or molecules, at very low temperatures, down to100 mK, and to investigate how quantum-mechanical effects affect these reactions close to 0 K. The key idea behind our approach to reach extremely low temperatures is to study the ion-molecule reactions within the orbit of the highly excited electron (called Rydberg electron) of a Rydberg state. This Rydberg electron effectively shields the reaction from stray electric fields and thus prevents the heating of the ions by such fields that has so far prevented studies of ion-molecule reactions below 10 K. At the same time, the Rydberg electron, which orbits at very large distance around the reaction system, does not influence the outcome the ion-molecule reaction: It acts as a spectator. A further advantage offered by Rydberg states is their large electric dipole moments, which we shall exploit to precisely control the motion of the reactants using chip-based devices generating moving electric traps for Rydberg atoms and molecules. In this project, we shall explore this new approach in detail, test its limitations, and hopefully improve the current understanding of ion-molecule chemistry at low temperature.
Because quantum mechanical effects are more pronounced in systems containing light atoms, the focus of our studies is placed on studies of reactions involving atomic and molecular hydrogen (H, H+, H2, H2+) and helium (He and He+). Such reactions are also of relevance in astrophysics because H and He are by far the two most abundant elements in the universe. Reactions involving hydrogen and helium in neutral and positively charged forms are at the origin of the chemical processes that have led to the formation of more complex molecules in the interstellar medium. So far, laboratory studies of ion-molecule reactions have typically been limited to temperatures above 50 K and hardly any data are available in the temperature range 3 K – 50 K characteristic of the interstellar clouds in which molecules are formed. At very low temperature, one may expect the reaction rates to be influenced by the wave nature of the reactants. In the case of slow reactions, this wave nature is expected to manifest itself through resonances, i.e., sharp collision-energy-dependent or temperature-dependent variations of the rate constants. In the case of fast reactions, the wave nature may cause strong deviations from the behavior expected on the basis of classical models, particularly at temperatures below 1 K.

The scientific objectives of the project are (i) a fundamental understanding of ion-molecule reactions in a range of temperature extending below 1 K that has so far not been explored, (ii) the determination of rate constants for reactions of astrophysical relevance for future use in the modelling of chemical processes in the interstellar medium, and (iii) the development of an original experimental approach to cold ion-molecule chemistry based on the manipulation of Rydberg atoms and molecules at the surface of chips.

Work performed

Overview:

The overall project is articulated around a series of tasks related to the development of the required experimental infrastructure, the characterization of the new experimental methods, and the study of specific ion-molecule reactions involving hydrogen and helium at low temperatures. These tasks and tentative deadlines for their completion are listed in the document “Description of Action (DoA)” on page 19 of Annex 1 of the Grant Agreement. Our progress in the first 18 months of this project is described below by making reference to these tasks. Information on the material and personnel resources that have made this progress possible is provided in detail in Part B of the Periodic Financial Report, which the following paragraph only briefly summarizes before the scientific results are presented.

The research team involved in this project during the first 18 months included four PhD students (Maximilian Beyer (MB), Ugo Jacovella (UJ), Katharina Höveler (KH) and Dominik Wehrli (DW)), two postdocs (Dr. Paul Jansen (PJ) and Dr. Johannes Deiglmayr (JD)) and myself (FM). MB, UG and DW were financed by the project, whereas KH, PJ and JD were financed by ETH funds. At the beginning of the project, and while we were ordering the components needed for the realization of our main experimental setup and waiting for their delivery, we decided to use and adapt several components of an existing experimental setup, with which we had carried out preparatory proof-of-principles experiments, in order to start with the scientific research without delay. This decision has had a very positive impact on the progress of our research and enabled us to make advance simultaneously on two fronts: While we were developing and testing the new experimental setup (tasks 1 and 4), we used and modified the pre-existing proof-of-principle setup to study the radiative association reactions involving H, D, H+ and D+ and the reactions H2 + H2+ and D2 + H2+. We also used this experimental setup to study the high Rydberg states of H2 and He2 to verify the postulated spectator role of the Rydberg electron. Particular highlights of the research carried out in these first 18 months were
(i) the determination of the rate constant of the H+ + H -> H2+ + hnu and and D+ + D -> D2+ + hnu over the entire range of temperature from 10 mK to 10’000 K (Beyer and Merkt, Phys. Rev. X 8, 031085 (2018)),
(ii) the determination of the scattering length of the H+ + H collision and the discovery that it differs both in magnitude and sign from the value that was accepted so far (Beyer and Merkt, J. Chem. Phys. 149, 214301 (2018)), and
(iii) the study, at very high resolution, of high Rydberg states of H2, which have enabled us to make the most accurate determination of the dissociation energy of a molecule ever carried out so far (Cheng et al., Phys. Rev. Lett. 121, 013001 (2018)).
After 18 months, we have completed tasks 1-4 and reached the corresponding research goals. In addition, we have made significant advances towards reaching the goals specified in tasks 5, 8 and 9. We are thus ahead of the planned schedule. This situation has enabled us to consider additional scientific goals related to our project. Questions of particular interest are linked to the spectator role of the highly excited Rydberg electron: Under which conditions can one safely assume that this Rydberg electron does not influence the properties (structure, dynamics, reactivity) of the ion core around which it orbits? Can deviations from this purely spectator role be quantified and linked to the degree of electronic excitation given by the principal and angular momentum quantum numbers n and l of the Rydberg electron? To answer these questions, we have decided to add two new tasks (now tasks 13 and 14) in addition to the original 12 tasks proposed, which we hope to also complete in the realm of this project:
-Task 13: Study the high Rydberg states of H2 at high values of the principal quantum

Final results

During the first 18 months, the project has developed as we had hoped. The new strategy we are following to study ion-neutral reactions has started yielding new results on fundamental reactions in a range of temperature range that had not been accessible to experimental studies.
With our results on the radiative association reactions H + H+ forming H2+ and D + D+ forming D2+, we have demonstrated the possibility of studying ion-neutral reactions at temperatures down to 10 mK. The reaction rates we have determined for this very slow reaction at the lowest temperatures reveal the importance of shape resonances (Phys. Rev. X 8, 031085 (2018)). An astonishing and important result was the determination of the scattering length of the H + H+ collision, which we found to differ both in magnitude and sign from the accepted value. The reason for the discrepancy is the breakdown of the g/u symmetry caused by the hyperfine interactions (J. Chem. Phys. 149, 214301 (2018)).
Our results on the extremely fast H2+ + H2 and D2+ + D2 reactions forming H3+ + H and D3+ + D, respectively, obtained with a much improved experimental setup compared to our early proof-of-principle setup, indicates a clear deviation from classical Langevin-capture behavior at the lowest collision energies. We have not yet reached the single-partial-wave regime, but are making good progress towards this goal. Our new data cover the entire temperature range of interest for the modelling of chemical process in interstellar clouds.
The research goals listed in our original proposal will continue to be the focus of our studies for the next period. However, the good progress we have made in this initial phase, has generated some room for studying several aspects related to our project in detail. We therefore plan to address two additional questions in future work: (1) We have become fascinated by the properties of double Rydberg states and planetary atoms and think that our Rydberg-electron-as-spectator approach might provide a new experimental access to study these systems. (2) Establishing the limits of this Rydberg-electron-as-spectator approach also appears to be a fascinating scientific problem. We believe that extremely precise spectroscopic measurements of the Rydberg spectra of atoms and molecules represents the best route to study this problem. We are currently exploring ways to carry out such measurements, which will require excellent control over stray electric fields and frequency-metrology tools. Our initial steps in both new directions (Phys. Rev. Lett. 121, 013001 (2018) and Int. J. Mass. Spectrom. 435, 209 (2019)) are promising. It is possible that future steps along these lines will necessitate the development or acquisition of specific new equipment. The flexibility allowed for by the ERC advanced grant program in this respect represents ideal conditions, and we conclude this 18-months progress report by expressing our deep gratitude.