Accumulation of Stark-decelerated NH molecules in a magnetic trap

Here we report on the accumulation of ground-state NH molecules in a static magnetic trap. A pulsed supersonic beam of NH (a1Δ) radicals is produced and brought to a near standstill at the center of a quadrupole magnetic trap using a Stark decelerator. There, optical pumping of the metastable NH radicals to the X3Σ− ground state is performed by driving the spin-forbidden A3Π ← a1Δ transition, followed by spontaneous A → X emission. The resulting population in the various rotational levels of the ground state is monitored via laser induced fluorescence detection. A substantial fraction of the ground-state NH molecules stays confined in the several milliKelvin deep magnetic trap. The loading scheme allows one to increase the phase-space density of trapped molecules by accumulating packets from consecutive deceleration cycles in the trap. In the present experiment, accumulation of six packets is demonstrated to result in an overall increase of only slightly over a factor of two, limited by the trap-loss and reloading rates.