No abstract

The rotationally inelastic collisions of NO(X) with Ar, in which the NO bond-axis is oriented side-on (i.e. perpendicular) to the incoming collision partner, are investigated experimentally and theoretically. The NO(X) molecules are selected in the |j = 0.5, Ω = 0.5, = −1, f state prior to bond-axis orientation in a static electric eld.The scattered NO products are then state selectively detected using velocity-map ion imaging. The experimental bond-axis orientation resolved dierential cross sections and integral steric asymmetries are compared with quantum mechanical calculations, and are shown to be in good agreement. The strength of the orientation eld is shown to aect the structure observed in the dierential cross sections, and to some extent 1 also the steric preference, depending on the ratio of the initial e and f Λ-doublets in the superposition determined by the orientation eld. Classical and quantum calculations are compared and used to rationalise the structures observed in the dierential cross sections. It is found that these structures are due to quantum mechanical interference eects, which dier for the two possible orientations of the NO molecule due to the anisotropy of the potential energy surface probed in the side-on orientation. Side-on collisions are shown to maximise and aord a high degree of control over the scattering intensity at small scattering angles (θ < 90 • ), whilst end-on collisions are predicted to dominate in the backward scattered region (θ > 90 • ).

The concept of the steric effect in molecular collisions is central to chemistry. In this Perspective article we review some of the progress made in studying the steric effect in inelastic and reactive collisions involving relatively small isolated atomic and molecular species. We overview the theoretical framework used to quantify the steric effect, and outline some of the key experimental approaches that can be employed to study the dynamics and mechanism of collisions involving oriented and aligned molecules. We illustrate the discussion by highlighting a few recent studies of inelastic and reactive scattering. Finally, we conclude with some reflections on possible future directions of interest.

The integral steric asymmetry for the inelastic scattering of NO(X) by a variety of collision partners was recorded using a crossed molecular beam apparatus. The initial state of the NO(X, v = 0, j = 1/2, Ω=1/2, ϵ=-1,f) molecule was selected using a hexapole electric field, before the NO bond axis was oriented in a static electric field, allowing probing of the scattering of the collision partner at either the N- or O-end of the molecule. Scattered NO molecules were state selectively probed using (1 + 1') resonantly enhanced multiphoton ionisation, coupled with velocity-map ion imaging. Experimental integral steric asymmetries are presented for NO(X) + Ar, for both spin-orbit manifolds, and Kr, for the spin-orbit conserving manifold. The integral steric asymmetry for spin-orbit conserving and changing transitions of the NO(X) + O system is also presented. Close-coupled quantum mechanical scattering calculations employing well-tested ab initio potential energy surfaces were able to reproduce the steric asymmetry observed for the NO-rare gas systems. Quantum mechanical scattering and quasi-classical trajectory calculations were further used to help interpret the integral steric asymmetry for NO + O. Whilst the main features of the integral steric asymmetry of NO with the rare gases are also observed for the O collision partner, some subtle differences provide insight into the form of the underlying potentials for the more complex system.