Jolijn Onvlee

State-resolved diffraction oscillations imaged for inelastic collisions of no radicals with he, ne and ar

Just as light scattering from an object results in diffraction patterns, the quantum mechanical nature of molecules can lead to the diffraction of matter waves during molecular collisions. This behaviour manifests itself as rapid oscillatory structures in measured differential cross-sections, and such observable features are sensitive probes of molecular interaction potentials. However, these structures have proved challenging to resolve experimentally. Here, we use a Stark decelerator to form a beam of state-selected and velocity-controlled NO radicals and measure state-to-state differential cross-sections for inelastic collisions of NO with He, Ne and Ar atoms using velocity map imaging. The monochromatic velocity distribution of the NO beam produced scattering images with unprecedented sharpness and angular resolution, thereby fully resolving quantum diffraction oscillations. We found excellent agreement with quantum close-coupling scattering calculations for these benchmark systems.

Resolving rainbows with superimposed diffraction oscillations in NO + rare gas scattering: experiment and theory

A Stark decelerator is used in combination with velocity map imaging to study collisions of NO radicals with rare gas atoms in a counterpropagating crossed beam geometry. This powerful combination of techniques results in scattering images with extremely high resolution, in which rotational and L-type rainbows with superimposed quantum mechanical diffraction oscillations are visible. The experimental data are in excellent agreement with quantum mechanical scattering calculations. Furthermore, hard-shell models and a partial wave analysis are used to clarify the origin of the various structures that are visible. A specific feature is found for NO molecules colliding with Ar atoms that is extremely sensitive to the precise shape of the potential energy surface. Its origin is explained in terms of interfering partial waves with very high angular momentum, corresponding to trajectories with large impact parameters.

Collision dynamics of symmetric top molecules: A comparison of the rotationally inelastic scattering of CD3 and ND3 with He

We compare rotationally inelastic scattering of deuterated methyl radicals (CD3) and ammonia (ND3) in collisions with helium using close-coupling quantum-mechanical scattering calculations performed with ab initio potential energy surfaces (PESs). The theoretical methods have been rigorously tested against angle-resolved experimental measurements obtained using crossed molecular beam apparatuses in combination with velocity map imaging [O. Tkáč, A. G. Sage, S. J. Greaves, A. J. Orr-Ewing, P. J. Dagdigian, Q. Ma, and M. H. Alexander, Chem. Sci. 4, 4199 (2013); O. Tkáč, A. K. Saha, J. Onvlee, C.-H. Yang, G. Sarma, C. K. Bishwakarma, S. Y. T. van de Meerakker, A. van der Avoird, D. H. Parker, and A. J. Orr-Ewing, Phys. Chem. Chem. Phys. 16, 477 (2014)]. Common features of the scattering dynamics of these two symmetric top molecules, one closed-shell and the other an open-shell radical, are identified and discussed. Two types of anisotropies in the PES influence the interaction of an atom with a nonlinear polyatomic molecule. The effects of these anisotropies can be clearly seen in the state-to-state integral cross sections out of the lowest CD3 rotational levels of each nuclear spin symmetry at a collision energy of 440 cm(-1). Similarities and differences in the differential cross sections for the ND3-He and CD3-He systems can be linked to the coupling terms derived from the PESs which govern particular initial to final rotational level transitions.

Imaging diffraction oscillations for inelastic collisions of NO radicals with He and D-2

We present state-to-state differential cross sections for collisions of NO molecules (X2Π1/2,j=1/2,f) with He atoms and ortho-D2 (j = 0) molecules as a function of collision energy. A high angular resolution obtained using the combination of Stark deceleration and velocity map imaging allows for the observation of diffraction oscillations in the angular scattering distributions. Differences in the differential cross sections and, in particular, differences in the angular spacing between individual diffraction peaks are observed. Since the masses of D2 and He are almost equal and since D2(j = 0) may be considered as a pseudo-atom, these differences directly reflect the larger size of D2 as compared to He. The observations are in excellent agreement with the cross sections obtained from quantum close-coupling scattering calculations based on accurate ab initio NO-He and NO-D2 potential energy surfaces. For the latter, we calculated a new NO-D2 potential energy surface.

Probing Scattering Resonances in (Ultra)Cold Inelastic NO-He Collisions

We theoretically study inelastic collisions between NO radicals and He atoms at low collision energies, focusing on the occurrence of scattering resonances. We specifically investigate de-excitation of rotationally excited NO radicals (X (2)Π1/2, v = 0, j = 3/2, f) at collision energies ranging from 10(-3) to 20 cm(-1) and compute integral and differential cross sections using quantum mechanical close-coupling calculations. Although unconventional, we show that the measurement of rotational de-excitation cross sections brings several advantages to experiments that aim to study rotational energy transfer at temperatures approaching zero kelvin. We analyze the nature and partial wave composition of the quasi-bound states associated with each individual resonance and compute the scattering wave functions. The differential cross sections contain the partial wave fingerprints of the scattering process and are found to change drastically as the collision energy is varied over the resonances. The prospects for measuring these differential cross sections in inelastic de-excitation collisions at low energies are discussed.

Analysis of velocity-mapped ion images from high-resolution crossed-beam scattering experiments: a tutorial review

A Stark decelerator produces beams of molecules with high quantum state purity, and small spatial, temporal and velocity spreads. These tamed molecular beams are ideally suited for high-resolution crossed beam scattering experiments. When velocity map imaging is used, the Stark decelerator allows the measurement of scattering images with unprecedented radial sharpness and angular resolution. Differential cross sections must be extracted from these high-resolution images with extreme care, however. Common image analysis techniques that are used throughout in crossed beam experiments can result in systematic errors, in particular in the determination of collision energy, and the allocation of scattering angles to observed peaks in the angular scattering distribution. Using a high-resolution data set on inelastic collisions of velocity-controlled NO radicals with Ne atoms, we describe the challenges met by the high resolution, and present methods to mitigate or overcome them.

Scattering resonances in bimolecular collisions between NO radicals and H-2 challenge the theoretical gold standard

Over the last 25 years, the formalism known as coupled-cluster (CC) theory has emerged as the method of choice for the ab initio calculation of intermolecular interaction potentials. The implementation known as CCSD(T) is often referred to as the gold standard in quantum chemistry. It gives excellent agreement with experimental observations for a variety of energy-transfer processes in molecular collisions, and it is used to calibrate density functional theory. Here, we present measurements of low-energy collisions between NO radicals and H2 molecules with a resolution that challenges the most sophisticated quantum chemistry calculations at the CCSD(T) level. Using hitherto-unexplored anti-seeding techniques to reduce the collision energy in a crossed-beam inelastic-scattering experiment, a resonance structure near 14 cm−1 is clearly resolved in the state-to-state integral cross-section, and a unique resonance fingerprint is observed in the corresponding differential cross-section. This resonance structure discriminates between two NO–H2 potentials calculated at the CCSD(T) level and pushes the required accuracy beyond the gold standard.Calculations at the theoretical gold standard generally yield accurate results for a variety of energy-transfer processes in molecular collisions. Using anti-seeding methods in a crossed-beam inelastic scattering experiment, a resonance structure is clearly resolved for NO–H2 collisions, pushing the required accuracy for theoretical potentials beyond the gold standard.