The ν lowest bending mode of linear C and of all its C-substituted isotopologues was recorded using a terahertz-supersonic jet spectrometer in combination with a laser ablation source. Sixty-five ro-vibrational transitions between 1.8 and 1.9 THz have been assigned to linear C12C12C12, C12C12C13, C12C13C12, C13C13C12, C13C12C13, and C13C13C13. For each isotopologue, molecular parameters were obtained and the C-C-bond length was derived experimentally. All results are in excellent agreement with recent ab initio calculations [B. Schröder and P. Sebald, J. Chem. Phys. 144, 044307 (2016)]. The new measurements explain why the interstellar search for singly substituted C12C12C13 has failed so far. A spectral line list with recommended transition frequencies based on global data fits is given to foster future interstellar detections.

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Astrophysical observations of radioactive isotopes, like 26 Al, 44 Ti, or 60 Fe, provide insight into the nucleosynthesis of stellar cores. The detection of characteristic γ-photons, which are released during radioactive decay, is used to map their spatial distribution on large scale. In general, the assignment to certain stellar objects fails due to limited sensitivity. An alternative approach is the observation of molecules containing radioactive isotopes. Radio-telescope facilities, like ALMA, can identify these species via their rotational transitions. In the outer atmosphere of late-type stars, the molecular condensation starts with simple diatomic particles containing oxides of refractory elements. The astrophysical detection of diatomic radioactive molecules requires highly accurate rotational transition frequencies, which can be obtained from laboratory measurements of stable isotopologues using mass-independent Dunham parameters. In this work, systematic studies are presented for 26 AlO, 44 TiO, and 60 FeO, as most promising tracers of nucleosynthesis in stellar environments, based on high-resolution measurements on the rotational transitions of their abundant stable isotopologues. Experiments were performed when a solid target (Al, Ti, Fe) is evaporated by a pulsed laser into an oxygenrich buffer gas to form simple metal oxides. An adiabatically planar expansion of the gas into a vacuum chamber cools the gas to a few tens of Kelvin and subsequently, Doppler-free rotational absorption spectra are recorded in the frequency range up to 400 GHz. A global data analysis, which also includes results from the literature, reveals the molecular structure beyond the Born-Oppenheimer (BO) limit, resulting in experimentally derived BO correction coefficients of these species for the first time. Based on this analysis, the rotational transitions of the radioactive molecules are determined with high accuracy at the sub-MHz level, which enables their unambiguous identification in stellar environments.

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Short carbon chains are fundamental for the chemistry of stellar and interstellar ambiences. The linear carbon chain molecule C 3 has been found in various interstellar and circumstellar environments, encompassing diffuse interstellar clouds, star forming regions, shells of late type stars, as well as cometary tails. Due to the lack of a permanent dipole moment C 3 can only be detected by electronic transitions in the visible spectral range or by vibrational bands in the mid and far-infrared region. We performed experiments where C 3 was produced via laser-ablation of a graphite rod with a 3 bar He purge and a subsequent adiabatic expansion into a vaccuum resulting in a supersonic jet. We report laboratory measurements of the lowest bending mode transitions of six 13 C-isotopologues of the linear C 3 molecule. Fifty-eight transitions have been measured between 1.8-1.9 THz with an accuracy of better than 1 MHz. Molecular parameters have been derived to give accurate line frequency positions of all 13 C isotopologues to ease their future interstellar detection. A dedicated observation for singly substituted 13 CCC is projected within the SOFIA airborne observatory mission.