Cold Collisions in a Molecular Synchrotron

Abstract

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 ...

Crossover from the Ultracold to the Quasiclassical Regime in State-Selected Photodissociation

“…Approaches to exploring the formation and breaking of molecular bonds across the low energy regime include a variety of techniques such as photoassociation [1], ultracold atomic collisions [2,3], ultracold molecular collisions [4], state-selected atomic and molecular beams with relative velocity control [5][6][7][8][9][10], and molecule collisions with cold trapped ions [11]. Each method has its benefits such as high tunability of collision energies, the ability to access ultralow energies, or the possibility to handle diverse molecular species.…”

A detailed account of the measurements of cold collisions in a molecular synchrotron

“…This correction factor is calculated by combining our knowledge of the trapping potential with the differential collision cross section (the cross section as a function of scattering angle) from quantum close-coupling calculations of ND 3 + Ar collisions, performed by Loreau and van der Avoird [43]. A detailed explanation is given in the supplementary material to Van der Poel et al [32].…”

“…Our approach combines the low collision energies obtained in experiments that use merged molecular beams with the high sensitivity of experiments that monitor trap loss. In Van der Poel et al [32], the total cross section for ND 3 + 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 this experiment.…”

“…2. The signals of the probe (red, S probe ) and reference packets (blue, S ref ), using numbers from the experiment that was discussed in Van der Poel et al [32]. , are shown as a function of storage time in the synchrotron.…”

“…After a given number of round-trips rt , the loss rate due to collisions can be found using

Fig. 2Model of the probe (red) and reference (blue) signals (using numbers from the experiment discussed in Van der Poel et al [32]), and the signal-to-noise ratio (orange, right y-axis) that results when these signals are used to calculate the loss rate due to collisions k col of the probe packets

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