Molecular-beam resonance method with Zeeman-decelerated samples: Application to metastable helium molecules

We use a multistage Zeeman decelerator to generate slow beams ($v\ensuremath{\simeq}100\phantom{\rule{0.16em}{0ex}}\mathrm{m}/\mathrm{s}$) of translationally cold, spin-polarized metastable $a\phantom{\rule{0.16em}{0ex}}^{3}\mathrm{\ensuremath{\Sigma}}_{u}^{+}\phantom{\rule{4pt}{0ex}}{\mathrm{He}}_{2}$ molecules and perform a precision measurement of their spin-rotation fine structure. The spin polarization results from the elimination of the high-field-seeking $J=N$ spin-rotational component of each rotational level (rotational quantum number $N$) by the Zeeman deceleration process. By repopulating the $J=N$ component from the $J=N\ifmmode\pm\else\textpm\fi{}1$ low-field-seeking components using radio-frequency radiation, we measured the spin-rotation fine structure of 13 rovibrational levels with $v=0$, 1 and $N=1\text{--}21$. 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.