Slow and velocity-tunable beams of metastable He2 by multistage Zeeman deceleration

We report on the use of multistage Zeeman deceleration to generate beams of He-2 molecules in the metastable a (3)Sigma(+)(u) state with velocities tunable down to 100 m/s. The metastable molecules are generated by striking a discharge in a supersonic expansion of pure helium gas from a pulsed valve held at cryogenic temperature. The velocity and internal-state distributions of the metastable He-2 molecules are measured for nozzle temperatures of 300, 77, and 10 K by high-resolution photoelectron and photoionization spectroscopy. The deceleration process does not exhibit any rotational state selectivity with rotational levels up to N-n=21 being populated, but eliminates molecules in spin-rotational sublevels with J(n)=N-n from the beam, where J(n) and N-n are the total and the rotational angular momentum quantum number, respectively. The lack of rotational state selectivity is attributed to the fact that the Paschen-Back regime of the Zeeman effect in the rotational levels of He2 is already reached at fields of only 0.1 T.