Recent advances in helium tagging assisted optical absorption spectroscopy of ions

Optical absorption of individual molecular ions and clusters can be measured by tagging with helium or other rare gas atoms. In case a resonance is hit by the laser frequency, the molecular ion is electronically excited, and the large amount of internal energy leads to boiling off helium. The signal that can be measured is the ratio in intensity between mass spectra with and without illumination, and extrapolation can be used to accurately approximate the gas phase spectrum. These experiments require the preparation of van-der-Waals bound complexes at very low temperatures. C₆₀⁺ could finally and unambiguously be confirmed as a carrier of diffuse interstellar bands by the Maier group using a trap with helium buffer gas cooling [1]. Alternatively, the cold complexes can also be produced in helium nano-droplets, where an atomically resolved phase transition could be measured in the Scheier group [2]. A record shall be given from the viewpoint of an interested observer of these experiments and of theoretical approaches and ongoing calculations for such systems. 

[1] Campbell, E. K., Holz, M., Gerlich, D. & Maier, J. P. Laboratory confirmation of C₆₀⁺ as the carrier of two diffuse interstellar bands. Nature 523, 322-323, doi:10.1038/nature14566 (2015).

[2] M. Kuhn, M. Renzler, J. Postler, S. Ralser, S. Spieler, M. Simpson, H. Linnartz, A. G. G. M. Tielens, J. Cami, A. Mauracher, Y. Wang, M. Alcamí, F. Martín, M. K. Beyer, R. Wester, A. Lindinger, P. Scheier, Nature Communication, 2016, DOI: 10.1038/ncomms13550