ECAMP 15

The absorption spectrum of diffuse interstellar clouds displays a rich set of lines - the diffuse interstellar bands (DIBs). The origin of the DIBs is intensely investigated since their first observation in 1922 [1] and remains elusive even today. Already with the discovery of C$_{60}$, Kroto et al. [2] proposed C$_{60}$ as a possible carrier for the DIBs. This hypothesis was later confirmed by laboratory studies, which assigned the first DIBs to C$_{60}^+$ [3]. Further evidence from laboratory investigations and observational studies using the Hubble Space Telescope has strengthened confidence in the existence of C$_{60}^+$ in the interstellar medium (ISM) and its contribution to the DIBs. However, the origin and destruction pathways of C$_{60}^+$ in the ISM remain a puzzle. For this, reaction kinetic data is needed to model the C$_{60}^+$ chemistry. Obtaining laboratory data with astrophysical relevance requires experimental conditions comparable to those in the ISM, such as low temperatures and intermediate densities [4].
The electrostatic Cryogenic Storage Ring (CSR) [5] at the Max Planck Institute for Nuclear Physics in Heidelberg is a suitable experimental environment to mimic the cold ISM, since it reaches vacuum chamber temperatures of < 10 K. In the electron-ion merged-beams setup, the stored ion beam is overlapped with a low-temperature electron beam produced by a photocathode [6], and electron-ion collisions can be studied at well-defined and tunable collision energies. We stored C$_{60}^+$ ions in the CSR and studied electron collisions at collision energies between few meV and $\sim$ 85 eV. Long ion beam storage times of up to 500 s allowed to investigate the reaction dynamics for an evolving internal excitation of the stored C$_{60}^+$ ions. We observed various collisional processes such as recombination, ionization, fragmentation and fragmentation-ionization. In comparison to collisions of smaller ionized molecules with electrons, C$_{60}^+$ shows a different recombination behavior. In the same way, we studied interactions of C$_{70}^+$ with free electrons. Future electron-ion collision experiments that are planned at the CSR include measurements of other DIB carrier candidates, for example Polycyclic Aromatic Hydrocarbons. Additionally, recombination experiments with smaller fullerene species are considered.
[1] Heger, M. L., Lick Observatory Bulletin 10, no. 337, pp. 146-147 (1922)
[2] Kroto, H., Heath, J., O’Brien, S. et al., Nature 318, pp. 162–163 (1985)
[3] Campbell, E., Holz, M., Gerlich, D. et al., Nature 523, pp. 322–323 (2015)
[4] Herbst, E. and van Dishoeck, E.F., Annual Review of Astr. and Astroph. 47, pp. 427-480 (2009)
[5] von Hahn, R. et al., Rev. Sci. Instrum. 87, 063115 (2016)
[6] Shornikov, A. et al., Phys. Rev. ST Accel. Beams 17, 042802 (2014)