A novel experimental method is presented for detection of a trace of optical absorption of mass-selected ions stored in an ion trap. Photon-trap spectroscopy (a generalized scheme of cavity ring-down spectroscopy) is the key technique, where a storage lifetime of photons in a high-Q cavity provides a ppm-level high sensitivity. Experiment was performed on the manganese ion, Mn+, to record 7P2,3,4 7S 3 absorption spectra at around 260 nm. Zeeman splitting and Faraday rotation of the polarization plane were observed as well in the presence of a magnetic field. The high-resolution spectra show complex structures, which are fully interpreted by the quantum theory incorporating hyperfine interaction due to the Mn nuclear spin (I = 5/2). The present technique provides a novel approach to electronic and magnetic structures of mass-selected atomic, molecular, and cluster ions.
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