We present the design and performance of two supersonic radical beam sources: a conventional pinhole-discharge source and a dielectric barrier discharge (DBD) source, both based on the Nijmegen pulsed valve. Both designs have been characterized by discharging water molecules seeded in the rare gases Ar, Kr, or Xe. The resulting OH radicals have been detected by laser-induced fluorescence. The measured OH densities are (3.0 ± 0.6) × 10 11 cm -3 and (1.0 ± 0.5) × 10 11 cm -3 for the pinholedischarge and DBD sources, respectively. The beam profiles for both radical sources show a relative longitudinal velocity spread of about 10%. The absolute rotational ground state population of the OH beam generated from the pinhole-discharge source has been determined to be more than 98%. The DBD source even produces a rotationally colder OH beam with a population of the ground state exceeding 99%. For the DBD source, addition of O 2 molecules to the gas mixture increases the OH beam density by a factor of about 2.5, improves the DBD valve stability, and allows to tune the mean velocity of the radical beam. Published by AIP Publishing. [http://dx

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Trapped cold molecules represent attractive systems for precision-spectroscopic studies and for investigations of cold collisions and chemical reactions. However, achieving their confinement for sufficiently long timescales remains a challenge. Here, we report the long-term trapping of Stark-decelerated OH radicals in their X 2 Π 3/2 (ν = 0, J = 3/2, M J = 3/2, f) state in a permanent magnetic trap. The trap environment is cryogenically cooled to a temperature of 17 K to suppress black-body-radiation-induced pumping of the molecules out of trappable quantum states and collisions with residual background gas molecules which usually limit the trap lifetime. The cold molecules are thus confined on timescales approaching minutes, an improvement of up to two orders of magnitude compared with room temperature experiments, at translational temperatures of ∼25 mK. The present results pave the way for new experiments using trapped cold molecules in precision spectroscopy, in studies of slow chemical processes at low energies and in the quantum technologies.

The Stark deceleration technique can produce molecular beams with very low velocities. In order to maximize the density of decelerated molecules, experimental parameters such as the velocity, the velocity spread and the spatial spread of the initial molecular beam as well as the operation characteristics of the decelerator have to be chosen appropriately. In this tutorial review, we describe procedures for the optimization of the density of Stark decelerated radicals for low-velocity applications which are of interest in, e.g., molecule trapping and cold-collision studies. Keywords: Cold molecules, Molecular beams, Stark deceleration, Radicals ReviewTranslationally cold molecules have become an attractive subject of research in recent years. A number of techniques for the generation of cold molecules has been developed [1-5] among which Stark deceleration is one of the most important [3,[6][7][8]. This method finds a broad range of applications in spectroscopy [9][10][11][12], collision-dynamics studies [13][14][15][16][17][18][19][20][21][22][23][24][25][26] and trap loading experiments [27][28][29][30][31][32][33]. The principle of Stark deceleration has been well documented [2,3], and a number of operation schemes have been developed for the optimization of Stark-decelerated molecular beams in different types of experiments [34][35][36][37].A Stark decelerator employs time-varying inhomogeneous electric fields produced by an array of dipolar electrodes to slow down pulsed beams of polar molecules [3,6]. When a packet of molecules approaches a set of electrodes, they are switched to high electric potential. Molecules in low-field-seeking Stark states experience a force which reduces their kinetic energy. The voltages on the electrodes are switched off before the molecules reach the maximum of the dipole potential in order to prevent their re-acceleration after they have passed the electrodes. This procedure is repeated at every pair of electrodes along the decelerator until the molecules have reached their target velocity at the exit of the assembly.The final velocity of the packet of molecules is controlled by a parameter referred to as phase angle 0 which corresponds to a scaled position of a "synchronous molecule" at which the high voltages on the electrodes of the decelerator are switched. Successful

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