Near-Threshold Inelastic Collisions Using Molecular Beams with a Tunable Velocity

Abstract

Molecular scattering behavior has generally proven difficult to study at low collision energies. We formed a molecular beam of OH radicals with a narrow velocity distribution and a tunable absolute velocity by passing the beam through a Stark decelerator. The transition probabilities for inelastic scattering of the OH radicals with Xe atoms were measured as a function of the collision energy in range of 50 to 400 wavenumber, with an overall energy resolution of about 13 wavenumbers. The behavior of the cross sections for inelastic scattering near the energetic thresholds was accurately measured, and excellent agreement was obtained with cross-sections derived from coupled-channel calculations on ab initio computed potential energy surfaces.The study of collisions between gas-phase atoms and molecules is a well-established method of gathering detailed information about their individual structures and mutual interaction (1 ). The level of detail obtained by these studies depends on the quality of preparation of the collision partners before the collision (2-4 ) and on how accurately the products are analyzed afterward (5-7 ). In recent years, it has become increasingly possible to control the internal and external degrees of freedom of the scattering partners, allowing the potential energy surfaces that govern the molecular collisions to be probed in ever greater detail. The most detailed information is obtained when crossed molecular beams are used to produce intense jets of molecules with a well-defined velocity, confined to only a few internal quantum states. Further state selection can be achieved by optical preparation of a single quantum state or by purification of the beam with the use of electrostatic or magnetic multipole fields (2, 3 ). These methods allow the orientation of the molecules to be controlled before the collision (8, 9 ) and the orientation of the scattered products can be measured (10 ).One of the most important parameters describing a scattering event is the collision energy of the scatterers. However, control over the collision energy has been a difficult experimental task. Since the 1980's, ingenious crossed beam machines have been engineered to vary the crossing angle of the intersecting beams, allowing variation of the collision energy while maintaining particle densities high enough for scattering (11 ). It was thereby possible to measure threshold behavior of rotational energy transfer (12, 13 ), or to tune the collision energy over the reaction barrier for reactive scattering (14, 15 ). These methods led to considerable improvement in the control over collision energy at high energiesfor example to probe short-range interactions. However, a similar level of control over collisions at low energies, which are sensitive probes for long-range interactions, is generally lacking. The angle of the 1

Multistage Zeeman decelerator for molecular-scattering studies

“…By operating the decelerator in the so-called s = 3 mode [14], in which only one third of the electrode pairs are used for deceleration while the remaining pairs are used for transverse focusing, instabilities are effectively eliminated [12,15]. The high-particle densities afforded by this method have recently enabled a number of high-resolution crossed beam scattering experiments, for instance [16][17][18][19]. For multistage Zeeman decelerators, several advanced switching protocols have been proposed and tested to mitigate losses.…”

A cold and slow molecular beam

“…Cold and ultracold collisions reveal resonances in the few partial wave regime, 1,2 and quantum threshold laws at cold and ultracold temperatures, [3][4][5] as well as new atom-molecule interactions. 1,3 Cold molecules also provide the opportunity for tests of fundamental laws of physics 6,7 and access to cold controlled chemistry with external fields. 8 In addition, long range and anisotropic electric dipolar interactions can lead to new quantum phases 9 and may be applied to quantum computing.…”

Cold SO2 molecules by Stark deceleration

“…To achieve this, one could accumulate the fragments (which have triplet ground states) in a magnetic trap by photodissociating the magnetic field insensitive, SO 2 within a magnetic trap. Decelerated SO 2 molecules can also serve as interesting collision partners, since the rotational structure is dense and thresholds for inelastic collisions will therefore appear at lower energy than in the previous study OH [5].…”

“…For these molecules Stark deceleration has proved to be a very efficient method to generate cold samples [1,2]. The decelerated molecules can be confined for long times [3,4] and can be used for collision studies with precise control over the translational degrees of freedom [5].…”