Unraveling Cold Molecular Collisions: Stark Decelerators in Crossed‐Beam Experiments

Abstract

In the last two decades, enormous progress has been made in the manipulation of molecular beams. In particular, molecular decelerators have been developed with which advanced control over neutral molecules in a beam can be achieved. By using arrays of inhomogeneous and time-varying electric (or magnetic) fields, bunches of molecules can be produced with a tunable velocity, narrow velocity spreads, and almost perfect quantum-state purity. These monochromatic or "tamed" molecular beams are ideally suited to be used in crossed-molecular-beam scattering experiments. Here, we review the first generation of these "cold and controlled" scattering experiments that have been conducted in the last decade and discuss the prospects for this emerging field of research in the years to come.

High-Resolution Imaging of C + He Collisions using Zeeman Deceleration and Vacuum-Ultraviolet Detection

“… 11 − 15 In this regard, the high resolution afforded by the combination of Stark 16 , 17 or Zeeman 16 , 18 , 19 deceleration to control collision partners and velocity map imaging (VMI) to probe collision products has enabled the observation of delicate features in angular scattering distributions. 20 − 23 However, the sparse availability of efficient near-threshold resonance-enhanced multiphoton ionization (REMPI) schemes, a prerequisite for obtaining high-resolution scattering images, still limits the number of systems for which the full potential of this approach can be exploited.…”

Cold and controlled chemical reaction dynamics

“…Since the pioneering work of Herschbach and Lee, 45,46 crossed molecular beams have become a cornerstone of gas-phase chemical reaction dynamics studies. 12,16,21,[47][48][49][50][51] Such experiments have traditionally seen the collision of two collimated beams of particles, usually at a 90 • crossing angle, with the properties of the resulting products probed using laser-based detection methods. At high collision energies, a large number of quantum states can be populated as a result of the collision and, therefore, many partial waves contribute to the scattering process.…”

“…The position and shape of these resonances is exceptionally sensitive to the long-range, attractive part of the underlying potential energy surface (PES) and, as a result, low-crossing-angle beam scattering experiments provide stringent tests for quantum scattering theory. 12,16,21,51 Scattering resonances have been observed in the state-to-state inelastic cross sections of several systems, including CO + H 2 and O 2 + H 2 , using a crossed molecular beam apparatus with a variable angle of intersection. 52,53 The lowest crossing angle of 12.5 • enabled collision energies as low as 3 cm −1 (≈ 4 K) to be studied.…”

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

“…In recent years, several experiments have managed to measure low energy collisions by leveraging the unique properties of the systems they study. For instance, by exploiting the extreme state-purity of Stark-decelerated beams combined with sensitive ion-detection techniques, van de Meerakker and co-workers have measured quantum-state changing collisions of OH and NO molecules with rare gas atoms to temperatures as low as 5 K [11–14]. Costes and co-workers have studied inelastic collisions of O 2 and CO with H 2 molecules and helium at energies between 5 and 30 K using cryogenically cooled beams under a small (and variable) crossing angle [15–17].…”