Interactions of cold and trapped negative hydrogen ions – COLD HMINUS

Hydrogen, the simplest of all atoms, forms the stable and well-known diatomic hydrogen
molecule, H2. Hydrogen also forms a very stable triatomic positive ion, H3+, which is a very
important ingredient for interstellar molecular clouds and has been studied for many years. It
is much less well known that also a negatively charged form of triatomic hydrogen exists.
Experimentally it has only been seen a few times and also theoretically it has only been the
topic of a small number of studies up to now. In this project, the formation, the properties,
and the fragmentation of the H3- system and the related hydrogen anion complexes HCO-,
HN2-, and HNO- will be studied using a closely interacting team of experimentalists and
theoreticians. We will target the formation processes, the structure and stability of these
negative ions, and their fragmentation pathways after photodetachment. This information will
be very useful to search for an interstellar appearance of these anions.

We will perform calculations in the Orsay and Bordeaux groups to obtain quantitative
rate coefficients for of the association of H- with H2, CO, NO and N2, both radiatively and by
three-body collisions down to a temperature of a few Kelvin. This information will be used to
plan the experiments in Innsbruck and demonstrate the formation of the anion complexes in
a cryogenic radiofrequency ion trap. Spectroscopic studies in different near- and far-infrared
spectral regions will then be carried in the laboratory to obtain rotational and vibrational
transition frequencies for comparison with ab initio calculations performed in France. The
comparison will be used to refine the structure calculations and obtain high quality
predictions of the entire eigenstate spectrum of these anions. In addition, these data will be
made available to astronomers to search for emission lines of the hydrogen anion complexes
in radioastronomical observations. To study the stability of the negative ion complexes
precise calculations of their binding energy with respect to dissociation as well as their
adiabatic and vertical electron affinities will be performed using up-to-date quantum
chemistry approaches. These calculations will be tested in experiments on thermally
activated collision induced dissociation of the anions and on photodetachment near
threshold, from which electron affinities can be deduced. By including the formation and
fragmentation rates into models of interstellar chemistry, the abundance of H- relative to H3-
will be derived and compared to neutral hydrogen using either detected H3- line emission or
upper limits of H3- column densities.

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.