Accumulating NH radicals in a magnetic trap

Recently, we have succeeded in trapping neutral dipolar molecules in an electrostatic trap and in a storage ring exploiting the interaction of these molecules with time-varying inhomogeneous electric elds. To be able to study molecular interactions and collective e ects in these trapped cold molecular gases, with the experimental realization of molecular Bose-Einstein condensation and coherent molecular beams as the ultimate goal, the phasespace density of the trapped gas needs to be further increased, i.e., the number density needs to be made higher and/or the temperature needs to be reduced. For our technique, the most straightforward method to increase the phase-space density of trapped molecules would be the accumulation of several packages of molecules in the trap. Simply re-loading the trap, however, requires opening up the trapping potential thereby losing the molecules that are already stored. We propose a novel scheme to accumulate fermionic or bosonic NH radicals (NH or NH, respectively) in a magnetic trap in order to increase phase-space density. NH molecules prepared in the long-lived metastable a state that exhibits a large Stark e ect can be brought to a standstill using Stark deceleration. The molecules can then be forced to spontaneously decay to the X electronic ground state by exciting the spin-forbidden A a transition. In the ground state the NH molecules are no longer sensitive to electric elds but show a signi cant Zeeman e ect instead, enabling con nement and reloading of ground state molecules in a magnetic trap.