Addressing and manipulation of individual hyperfine states in cold   trapped molecular ions and application to HD^{+} frequency metrology
  Advanced techniques for manipulation of internal states, standard in atomic physics, are demonstrated for a charged molecular species for the first time. We address individual hyperfine states of ro-vibrational levels of a diatomic ion by optical excitation of individual hyperfine transitions, and achieve controlled transfer of population into a selected hyperfine state. We use molecular hydrogen ions (HD^{+}) as a model system and employ a novel frequency-comb-based, continuous-wave 5 \mum laser spectrometer. The achieved spectral resolution is the highest obtained so far in the optical domain on a molecular ion species. As a consequence, we are also able to perform the most precise test yet of the ab-initio theory of a molecule. 
U. Bressel, A. Borodin, J. Shen, M. Hansen, I. Ernsting, S. Schiller
physics.atom-ph quant-ph