Observation and theory of electric-dipole-forbidden infrared transitions in cold molecular ions

Investigation of fundamental scientific questions such as possible variations of fundamental constants [1] or the search for new fundamental interactions [2] by spectroscopic methods, requires the measurement of very narrow atomic and molecular spectroscopic lines. One type of transitions exhibiting such narrow line widths are electric‐dipole‐forbidden transitions. Although such transitions are routinely used in experiments with atomic ions, in molecular ions their low intensity has until now precluded their detection. We have recently [3] measured the electric‐quadrupole infrared fundamental line in trapped, sympathetically cooled N 2+  ions. I will present the experimental setup and techniques that made this measurement possible, show our results, as well as shortly explain the theory of hyperfine resolved vibrational quadrupole lines needed to interpret such spectra. Finally, I will give an outlook of the next steps towards high precision spectroscopy with this system.

[1] M. Kajita et al., Phys. Rev. A, 89, 032509 (2014).
[2] E. J. Salumbides et al., Phys. Rev. D, 87, 112008 (2013).
[3] M. Germann, X. Tong and S. Willitsch, Nature Phys., doi: 10.1038/nphys3085 (2014).