Spectroscopy of C60+ and Complex Organic Molecular Ions: from Mid-IR Emission to the Diffuse Interstellar Bands

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The laboratory spectroscopy of many molecular species that are relevant for the chemistry of the interstellar medium (ISM) still poses a great challenge. Consequences of the lack of reliable experimental data are numerous unidentified interstellar spectra and long-standing mysteries like the Diffuse Interstellar Bands (DIBs) and the Unidentified Infrared Emission (UIR) features. It has been recognized in the field that a tremendous effort in laboratory astrophysics will be necessary to provide the spectroscopic information to understand the vast amount of data produced by modern telescopes in the coming years. A particularly difficult field is the spectroscopy of large molecular ions like fullerenes and Polycyclic Aromatic Hydrocarbons (PAHs). While PAH ions have long been suspected to be responsible for the observed emission features in the mid-infrared, laboratory emission spectra on these important species are all but non-existent. Likewise, since more than 20 years there is an intriguing claim that C60+ is responsible for two DIB transitions, yet the confirmation or rejection of that claim requires reliable gas phase spectroscopic data, which so far have been elusive.We propose to develop a novel type of gas phase spectroscopy that is based on laser-induced vibrational emission. We intend to store fullerene and PAH ions in a cryogenic ion beam trap at ultra-high vacuum. Inside the trap the ions will be exposed to tunable laser radiation from the near-infrared to the UV range. The laser radiation will heat the molecules, which will redistribute the absorbed energy among their normal modes and re-emit it via vibrational transitions in the mid-IR. This is the same process that is responsible for the heating and cooling of complex organic molecules in the interstellar medium. We will observe this process in real time by detecting the emitted mid-IR radiation using ultra-sensitive blocked-impurity-band Si:As diodes. The detectors are based on the same technique that is used for mid-IR observations in space. By recording the emitted mid-IR intensity as a function of the wavelength of the laser, we will be able to obtain true gas phase spectra of large molecular ions. The existing cryogenic ion beam trap at MPIK provides an ultimate vacuum system (p < 10-12 torr) at very low temperatures (around 10 K), which is an ideal environment for background-free emission measurements. While our first target molecule will be C60+, we intend to extend the technique to large PAH ions and carbon chains. As electrostatic ion traps are gaining acceptance worldwide, the successful demonstration of the technique proposed here will provide a new pathway for gas phase spectroscopy of complex organic molecules, and further our understanding of the carbon balance of the ISM.