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An approach to determine the kinetic beam energy at the ${10}^{\ensuremath{-}5}$ level is presented, which corresponds to an improvement by more than one order of magnitude compared with conventional methods. Particularly, collinear fluorescence and resonance-ionization spectroscopy measurements on rare-isotope beams, where the beam energy is a major contribution to the uncertainty, can benefit from this method. The approach is based on collinear spectroscopy and requires no special equipment besides a wavelength meter, which is commonly available. Its advent is demonstrated in a proof-of-principle experiment on a Ni beam. In preparation for the energy measurement, the rest-frame transition frequencies of the $3{d}^{\phantom{\rule{0.16em}{0ex}}9}4s{\phantom{\rule{0.16em}{0ex}}}^{3}{D}_{3}\ensuremath{\rightarrow}3{d}^{\phantom{\rule{0.16em}{0ex}}9}4p{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{2}$ transitions in neutral nickel isotopes have been identified to be ${\ensuremath{\nu}}_{0}{(}^{58}\mathrm{Ni})=850\phantom{\rule{0.16em}{0ex}}343\phantom{\rule{0.16em}{0ex}}678\phantom{\rule{0.16em}{0ex}}(20)$ MHz and ${\ensuremath{\nu}}_{0}{(}^{60}\mathrm{Ni})=850\phantom{\rule{0.16em}{0ex}}344\phantom{\rule{0.16em}{0ex}}183\phantom{\rule{0.16em}{0ex}}(20)$ MHz.

Collinear laser spectroscopy was performed on an atomic beam of stable 55Mn. An ion beam of 55Mn+ was generated in an ion source, accelerated to 15 keV, and neutralized via charge-exchange reactions with a Na vapor. A long-lived metastable state of Mn I, near-resonantly populated in the charge-exchange process, was investigated via laser probing in addition to a laser excitation from the ground state in Mn I. The relative population of the Mn I metastable state compared to that of the ground state was found to be 0.7 ± 0.3. A theoretical calculation, which included feeding to the ground state and the metastable state from higher-energy excited electronic states populated in the charge-exchange process, agreed with the present result. The deduced A and B hyperfine coupling constants agreed with literature values, where available.