The resolution of any spectroscopic or interferometric experiment is ultimately limited by the total time a particle is interrogated. Here we demonstrate the first molecular fountain, a development which permits hitherto unattainably long interrogation times with molecules. In our experiments, ammonia molecules are decelerated and cooled using electric fields, launched upwards with a velocity between 1.4 and 1.9 m/s and observed as they fall back under gravity. A combination of quadrupole lenses and bunching elements is used to shape the beam such that it has a large position spread and a small velocity spread (corresponding to a transverse temperature of < 10 μK and a longitudinal temperature of < 1 μK) when the molecules are in free fall, while being strongly focused at the detection region. The molecules are in free fall for up to 266 ms, making it possible, in principle, to perform sub-Hz measurements in molecular systems and paving the way for stringent tests of fundamental physics theories.

We study collisions between neutral, deuterated ammonia molecules (ND 3 ) stored in a 50 cm diameter synchrotron and argon atoms in copropagating supersonic beams. The advantages of using a synchrotron in collision studies are twofold: (i) By storing ammonia molecules many round-trips, the sensitivity to collisions is greatly enhanced; (ii) the collision partners move in the same direction as the stored molecules, resulting in low collision energies. We tune the collision energy in three different ways: by varying the velocity of the stored ammonia packets, by varying the temperature of the pulsed valve that releases the argon atoms, and by varying the timing between the supersonic argon beam and the stored ammonia packets. These give consistent results. We determine the relative, total, integrated cross section for ND 3 þ Ar collisions in the energy range of 40-140 cm −1 , with a resolution of 5-10 cm −1 and an uncertainty of 7%-15%. Our measurements are in good agreement with theoretical scattering calculations. DOI: 10.1103/PhysRevLett.120.033402 The crossed molecular beam technique, pioneered by Herschbach and Lee, has yielded a detailed understanding of how molecules interact and react [1,2]. Until recently, these crossed molecular beam studies were limited by the velocities of the molecular beams to collision energies above a few hundred cm −1 (1 cm −1 ≃ 1.4 K). Over the past years, however, a number of ingenious methods [3][4][5] have been developed to study collisions in the cold regime. These advances are important for several reasons. First, the temperatures of interstellar clouds are typically between 10 and 100 K; collision data of simple molecules at low temperatures are thus highly relevant for understanding the chemistry in these clouds [6]. Furthermore, quantum effects become important at low temperatures, where few partial waves contribute and the de Broglie wavelength associated with the relative velocity becomes comparable to or larger than the intermolecular distances. Of particular interest are resonances of the collision cross section as a function of the collision energy [7][8][9][10]. The position and shape of these resonances are very sensitive to the exact shape of the PES and thus serve as precise tests of our understanding of intermolecular forces.The ability to control the velocity of molecules using time-varying electric fields has allowed studies of inelastic collisions of OH and NO molecules with rare gas atoms at low collision energies [11][12][13][14]. Using cryogenically cooled beams under a small (and variable) crossing angle, inelastic collisions of O 2 and CO with H 2 and helium at energies between 5 and 30 K have been studied [15]. Even lower temperatures can be obtained by using magnetic or electric guides to merge two molecular beams into a single beam. This technique has been used to study Penning ionization reactions of various atoms and molecules with metastable helium [16][17][18][19] and collisions between ground state hydrogen molecules and hydrogen molecules in ...

Here we report on the accumulation of ground-state NH molecules in a static magnetic trap. A pulsed supersonic beam of NH (a 1 ∆) 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 X 3 Σ − ground state is performed by driving the spin-forbidden A 3 Π←a 1 ∆ 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.

The resolution of any spectroscopic or interferometric experiment is ultimately limited by the total time a particle is interrogated. We here demonstrate the first molecular fountain, a development which permits hitherto unattainably long interrogation times with molecules. In our experiments, ammonia molecules are decelerated and cooled using electric fields, launched upwards with a velocity between 1.4 and 1.9 m/s and observed as they fall back under gravity. A combination of quadrupole lenses and bunching elements is used to shape the beam such that it has a large position spread and a small velocity spread (corresponding to a transverse temperature of <10 µK and a longitudinal temperature of <1 µK) when the molecules are in free fall, while being strongly focused at the detection region. The molecules are in free fall for up to 266 milliseconds, making it possible to perform sub-Hz measurements in molecular systems and paving the way for stringent tests of fundamental physics theories.

We have recently demonstrated a general and sensitive method to study low energy collisions that exploits the unique properties of a molecular synchrotron (Van der Poel et al., Phys Rev Lett 120:033402, 2018). In that work, the total cross section for ND3 + Ar collisions was determined from the rate at which ammonia molecules were lost from the synchrotron due to collisions with argon atoms in supersonic beams. This paper provides further details on the experiment. In particular, we derive the model that was used to extract the relative cross section from the loss rate, and present measurements to characterize the spatial and velocity distributions of the stored ammonia molecules and the supersonic argon beams.