Rotational alignment effects in NO(X) + Ar inelastic collisions: An experimental study

Rotational angular momentum alignment effects in the rotationally inelastic collisions of NO(X) with Ar have been investigated at a collision energy of 66 meV by means of hexapole electric field initial state selection coupled with velocity-map ion imaging final state detection. The fully quantum state resolved second rank renormalized polarization dependent differential cross sections determined experimentally are reported for a selection of spin-orbit conserving and changing transitions for the first time. The results are compared with the findings of previous theoretical investigations, and in particular with the results of exact quantum mechanical scattering calculations. The agreement between experiment and theory is generally found to be good throughout the entire scattering angle range. The results reveal that the hard shell nature of the interaction potential is predominantly responsible for the rotational alignment of the NO(X) upon collision with Ar

Rotational alignment effects in NO(X) + Ar inelastic collisions: A theoretical study

Rotational angular momentum alignment effects in the rotational inelastic scattering of NO(X) with Ar have been investigated by means of close-coupled quantum mechanical, quasi-classical trajectory, and Monte Carlo hard shell scattering calculations. It has been shown that the hard shell nature of the interaction potential at a collision energy of Ecoll = 66 meV is primarily responsible for the rotational alignment of the NO(X) molecule after collision. By contrast, the alternating trend in the quantum mechanical parity resolved alignment parameters with change in rotational state Δj reflects differences in the differential cross sections for NO(X) parity conserving and changing collisions, rather than an underlying difference in the collision induced rotational alignment. This suggests that the rotational alignment and the differential cross sections are sensitive to rather different aspects of the scattering dynamics. The applicability of the kinematic apse model has also been tested and found to be in excellent agreement with exact quantum mechanical scattering theory provided the collision energy is in reasonable excess of the well depth of the NO(X)-Ar potential energy surface

Side-impact collisions of Ar with NO

In the realm of molecular collision dynamics, stereochemistry refers to the dependence of the collision outcome on the mutual orientation of the colliding partners. This may involve directed end-on collisions along a molecular bond axis or encounters in which the partner approaches the bond of an oriented molecule from the side. Using both experiment and theory, we show here that in the simplest case of an inelastic collision between an atom and a nearly homonuclear diatom, in which the two atoms have almost the same mass, the scattering dynamics are very distinct for impacts on either side of the molecule. By recording the direction of the scattered particles after the collision, we demonstrate that the intensity of products scattered in the forward direction, near parallel to the initial motion, can be substantially controlled and even maximized by altering the side-on orientation of the quantum state selected NO molecules that collide with Ar atoms. In addition, our findings provide valuable information about the preferred collision mechanism and reveal interesting quantum interference effects

Steric Effects in the Inelastic Scattering of NO(X) + Ar: Side-on Orientation.

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, Omega=0.5, epsilon=-1,f> state prior to bond-axis orientation in a static electric field. The scattered NO products are then state selectively detected using velocity-map ion imaging. The experimental bond-axis orientation resolved differential 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 field is shown to affect the structure observed in the differential cross sections, and to some extent also the steric preference, depending on the ratio of the initial e and f Lambda-doublets in the superposition determined by the orientation field. Classical and quantum calculations are compared and used to rationalise the structures observed in the differential cross sections. It is found that these structures are due to quantum mechanical interference effects, which differ 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 afford a high degree of control over the scattering intensity at small scattering angles (theta<90 degrees), whilst end-on collisions are predicted to dominate in the backward scattered region (theta>90 degrees)

Side-impact collisions of Ar with NO

In the realm of molecular collision dynamics, stereochemistry refers to the dependence of the collision outcome on the mutual orientation of the colliding partners. This may involve directed end-on collisions along a molecular bond axis or encounters in which the partner approaches the bond of an oriented molecule from the side. Using both experiment and theory, we show here that in the simplest case of an inelastic collision between an atom and a nearly homonuclear diatom, in which the two atoms have almost the same mass, the scattering dynamics are very distinct for impacts on either side of the molecule. By recording the direction of the scattered particles after the collision, we demonstrate that the intensity of products scattered in the forward direction, near parallel to the initial motion, can be substantially controlled and even maximized by altering the side-on orientation of the quantum state selected NO molecules that collide with Ar atoms. In addition, our findings provide valuable information about the preferred collision mechanism and reveal interesting quantum interference effects

Product lambda-doublet ratios as an imprint of chemical reaction mechanism

In the last decade, the development of theoretical methods has allowed chemists to reproduce and explain almost all of the experimental data associated with elementary atom plus diatom collisions. However, there are still a few examples where theory cannot account yet for experimental results. This is the case for the preferential population of one of the Λ-doublet states produced by chemical reactions. In particular, recent measurements of the OD((2)Π) product of the O((3)P)+D2 reaction have shown a clear preference for the Π(A') Λ-doublet states, in apparent contradiction with ab initio calculations, which predict a larger reactivity on the A'' potential energy surface. Here we present a method to calculate the Λ-doublet ratio when concurrent potential energy surfaces participate in the reaction. It accounts for the experimental Λ-doublet populations via explicit consideration of the stereodynamics of the process. Furthermore, our results demonstrate that the propensity of the Π(A') state is a consequence of the different mechanisms of the reaction on the two concurrent potential energy surfaces

Stereodynamical control of a quantum scattering resonance in cold molecular collisions

Cold collisions of light molecules are often dominated by a single partial wave resonance. For the rotational quenching of HDandnbsp;(vandnbsp;=andnbsp;1,andnbsp;jandnbsp;=andnbsp;2) by collisions with ground stateandnbsp;para-H2, the process is dominated by a singleandnbsp;Landnbsp;=andnbsp;2 partial wave resonance centered around 0.1andnbsp;K. Here, we show that this resonance can be switched on or off simply by appropriate alignment of the HD rotational angular momentum relative to the initial velocity vector, thereby enabling complete control of the collision outcome.</p

Controlling the spin–orbit branching fraction in molecular collisions

The collision geometry, that is, the relative orientation of reactants before interaction, can have a large effect on how a collision or reaction proceeds. Certain geometries may prevent access to a given product channel, while others might enhance it. In this Letter, we demonstrate how the initial orientation of NO molecules relative to approaching Ar atoms determines the branching between the spin–orbit changing and the spin–orbit conserving rotational product channels. We use a recently developed quantum treatment to calculate differential and integral branching fractions, at any arbitrary orientation, from theoretical and experimental data points. Our results show that a substantial degree of control over the final spin–orbit state of the scattering products can be achieved by tuning the initial collision geometry

Inelastic collision dynamics of oriented NO molecules with Kr atoms

Building on our previous work on NO + Ar, this paper presents a complete set of orientation measurements and quantum mechanical calculations for the NO + Kr collision system, including both spin-orbit conserving and changing collisions, and both side-on (x-axis) and end-on (z-axis) orientations. While many of the trends observed in the oriented differential and integral scattering distributions, as well as in the spin-orbit branching fractions, are similar to the ones seen previously for NO + Ar, a direct comparison with the Ar data reveals subtle differences in the scattering dynamics, which we rationalise with the more extended attractive regions on the NO + Kr potential energy surfaces. High-impact parameter collisions that lead to low scattering angles in the spin-orbit conserving manifold are particularly sensitive to the topology in the attractive parts of the potential, whereas more impulsive, low-impact parameter trajectories, which sample the repulsive parts of the potential, produce very similar features in the oriented differential cross sections for the Ar and Kr systems, especially for spin-orbit changing collisions

Stereodynamical control of a quantum scattering resonance in cold molecular collisions

Cold collisions of light molecules are often dominated by a single partial wave resonance. For the rotational quenching of HD (v = 1, j = 2) by collisions with ground state para-H2, the process is dominated by a single L = 2 partial wave resonance centered around 0.1 K. Here, we show that this resonance can be switched on or off simply by appropriate alignment of the HD rotational angular momentum relative to the initial velocity vector, thereby enabling complete control of the collision outcome.</p