We use a multistage Zeeman decelerator to generate slow beams (v 100 m/s) of translationally cold, spin-polarized metastable a 3 Σ + u He2 molecules and perform a precision measurement of their spin-rotation fine structure. The spin polarization results from the elimination of the high-fieldseeking J = N spin-rotational component of each rotational level (rotational quantum number N ) by the Zeeman deceleration process. By repopulating the J = N component from the J = N ± 1 low-field-seeking

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... ponents using radio-frequency radiation, we measured the spin-rotation fine structure of 13 rovibrational levels with v = 0, 1 and N = 1 − 21. The low beam velocity and the resulting long interaction times with the radio-frequency radiation were exploited to determine the transition frequencies with a precision of 300 Hz.