**5. Conclusions**

Ablation-dominated switching arcs have been investigated in a model circuit breaker with CO2 atmosphere as well as in a long PTFE nozzle under ambient conditions. Optical emission spectroscopy and broadband optical absorption spectroscopy were carried out using either intensified or high-speed cameras. As a main result, we have shown that specific molecules are detectable in the wavelength range between 400 nm and 800 nm under strongly varying conditions. It was demonstrated that depending on nozzle geometry and discharge current the C2 Swan bands can be observed by their emission (i) near to the wall only, (ii) distributed over the full arc diameter with the highest intensity in the center, or (iii) by their absorption of continuum radiation from the arc plasma. Although an emission was found until a few hundreds of microseconds before CZ, no absorption of the Swan bands could be detected around and after CZ. Even the occurrence of C2 radiation can be used an indicator for intermediate temperatures of around 5000 K according to composition calculations. An accurate determination of quantities like rotational temperatures and densities can be performed by comparison with spectra calculation. This was out of the scope of the present paper and could be tackled in the future, demanding two-dimensional inverse Abel transformation of multiband spectra. Additionally, these findings could be applied e.g., for the study of fluxes and distribution of evaporated material and for the verification of erosion models of the PTFE wall. The molecule CuF could be expected when copper vapor from the electrode erosion is mixed with the dissociated PTFE vapor from the

nozzle ablation. To our knowledge, it was the first observation of CuF molecules in high-current arcs burning in nozzles under ablation-dominated regime. The agreemen<sup>t</sup> of spectral features between the literature on CuF and our experiments seem very plausible. However, further investigations are necessary to confirm these findings. The CuF molecular absorption could be applied as an alternative to investigating electrode erosion and distribution of electrode material within the discharge area, especially around CZ when the emission of atomic copper lines fades out.

**Author Contributions:** Conceptualization, R.M. and D.U.; methodology, validation, and formal analysis, R.M.; investigation with setup (a), R.M.; investigation with setup (b), R.M. and N.G.; writing—original draft preparation, R.M. and D.U.; project administration, D.U. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Deutsche Forschungsgemeinschaft gran<sup>t</sup> numbers UH 106/13-1 and SCHN 728/16-1.

**Acknowledgments:** The authors would like to thank Steffen Franke and Alireza Khakpour for experimental help and fruitful discussions. The calculation of plasma composition was realised by Sergey Gortschakow (all Leibniz Institute for Plasma Science and Technology).

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
