Interactions of cold and trapped negative hydrogen ions

The simplest of all atoms, the Hydrogen atom, gives rise to two well-studied molecular forms, as a diatomic neutral molecule and as a positively charged triatomic molecular ion. Recently it has become clear that also a triatomic anion, i.e. a negatively charged hydrogen triatomic molecule, is stable, but very little is known about its properties. This is particularly dissatisfying, because hydrogen is the most abundant atom in our universe; it makes up for 90 percent of all existing atoms. Whether hydrogen negative ions play any role in the evolution of the gas in our and other galaxies is therefore totally unclear. To provide answers about the properties of these elusive molecular species and their role in astronomy, we have formed a team of theoreticians and experimentalists in Orsay, Bordeaux and Innsbruck. We will carry out calculations on the structure and on collision processes of hydrogen anion molecules. Using these data we will prepare the anions experimentally in an ion trap that operates at low temperature typical of the gas composing the interstellar medium. The calculations will predict where the anions absorb infrared light and this will be tested by irradiating the ions in the ion trap. By feeding this information back to the calculations a detailed prediction of the properties of the anions will be given. Such information is needed for astronomical searches for hydrogen anion complexes. In addition, the team will work on the chemical reactions that form and destroy the hydrogen anion complexes. This will allow for predictions of the abundance of these species in typical interstellar environments. As a result this project will provide a new level of understanding of the structure and stability of different hydrogen molecular anion complexes and the role of the hydrogen anion in cold, dark areas of the interstellar medium.

Hydrogen atoms and molecules represent the simplest constituents of the interstellar medium. Their interactions have been widely studied in the case of neutral species or positively charged ions. It is speculated that negatively charged hydrogen atoms also exist in interstellar space, but a detection has not been possible so far. In this project we have studied the interactions of negatively charged hydrogen atoms and similar negative ions with molecular hydrogen. We have searched for means to produce and investigate negatively charged hydrogen complexes, which could aid in the search for hydrogen negative ions in space. This work was conducted in a collaboration between experimentalists at the University of Innsbruck and theoreticians at the Laboratoire Aime Cotton in Orsay and the University of Bordeaux. We have carried out several experiments on the interactions of negative ions with hydrogen molecules and compared them to high level quantum calculations by our collaborators. Notably, we have investigated the tunneling reaction of hydrogen negative ions with neutral hydrogen and determined an extremely small reaction probability that agrees very well with calculations. We also investigated pathways for the formation of negative ion complexes by collisions at low temperature and determined the formation rates for chlorine-hydrogen complexes. In several spectroscopic studies in the far infrared region, again in comparison with calculations, we unraveled interesting quantum properties of these complexes, such as the large influence of their nuclear magnetic moments on the vibrational motion in the complex. The experiments in Innsbruck have employed different radiofrequency ion traps that allow us to confine negative ions for many seconds and cool them to temperatures only a few Kelvin above absolute zero. We also achieved a substantial technological improvement with the development of a wire ion trap that allowed us to achieve lower ion temperatures than were possible in our earlier trap design. The close collaboration between the groups in Innsbruck, Orsay, and Bordeaux and their complementary expertise was crucial for the success of this project. As a result we have obtained a new level of understanding of the structure and stability of hydrogen molecular ion complexes as well as of the importance of quantum mechanics for their properties.