Two-photon rotational action spectroscopy of cold OH − at 1 ppb accuracy. Jusko, P., Asvany, O., Wallerstein, A.-C., Brünken, S. Microwave Spectroscopy (Dover, New York, NY, 1975). Laser-cooled mercury ion frequency standard. Manipulation of individual hyperfine states in cold trapped molecular ions and application to HD + frequency metrology. Sympathetic cooling of 9Be + and 24Mg + for quantum logic. Sympathetic crystallization of trapped ions. Observation of ordered structures of laser-cooled ions in a quadrupole storage ring. Raizen, M., Gilligan, J., Bergquist, J., Itano, W. This approach enables a wide range of high-accuracy measurements on molecules, both on rotational and, as we project, vibrational transitions. The results represent the long overdue extension of Doppler-free microwave spectroscopy of laser-cooled atomic ion clusters 6 to higher spectroscopy frequencies and to molecules. As an application, we demonstrate the most precise test of ab initio molecular theory and the most accurate (1.3 × 10 −9) determination of the proton mass using molecular spectroscopy. We achieve a linewidth of 1 × 10 −9 fractionally and 1.3 kHz absolute, an improvement of ≃50-fold over the previous highest resolution in rotational spectroscopy. It makes use of the strong radial spatial confinement of molecular ions when trapped and crystallized in a linear quadrupole trap, providing the Lamb–Dicke regime for rotational transitions. Here we introduce a general and accessible approach that enables Doppler-free rotational spectroscopy. However, this potential remains untapped so far, with the best resolution achieved being not better than 5 × 10 −8 fractionally, due to residual Doppler broadening being present in ion clusters even at the lowest achievable translational temperatures 5. For molecular ions, it promises vastly enhanced spectroscopic resolution and accuracy. Sympathetic cooling of trapped ions has been established as a powerful technique for the manipulation of non-laser-coolable ions 1, 2, 3, 4.
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