Inelastic Collisions Of NO Research Articles

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, Ω = 0.5, ε = -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 Λ-doublets in the superposition determined by the orientation field. Classical and quantum calculations are compared and used to rationalize 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 maximize and afford a high degree of control over the scattering intensity at small scattering angles (θ < 90°), while end-on collisions are predicted to dominate in the backward scattered region (θ > 90°).

Rotational Orientation Effects in NO(X) + Ar Inelastic Collisions.

Rotational angular momentum orientation effects in the rotationally inelastic collisions of NO(X) with Ar have been investigated both experimentally and theoretically at a collision energy of 530 cm(-1). The collision-induced orientation has been determined experimentally using a hexapole electric field to select the ϵ = -1 Λ-doublet level of the NO(X) j = 1/2 initial state. Fully quantum state resolved polarization-dependent differential cross sections were recorded experimentally using a crossed molecular beam apparatus coupled with a (1 + 1') resonance-enhanced multiphoton ionization detection scheme and subsequent velocity-map imaging. To determine the NO sense of rotation, the probe radiation was circularly polarized. Experimental orientation polarization-dependent differential cross sections are compared with those obtained from quantum mechanical scattering calculations and are found to be in good agreement. The origin of the collision-induced orientation has been investigated by means of close-coupled quantum mechanical, quantum mechanical hard shell, quasi-classical trajectory (QCT), and classical hard shell calculations at the same collision energy. Although there is evidence for the operation of limiting classical mechanisms, the rotational orientation cannot be accounted for by QCT calculations and is found to be strongly influenced by quantum mechanical effects.

Steric effects and quantum interference in the inelastic scattering of NO(X) + Ar.

Rotationally inelastic collisions of NO(X) with Ar are investigated in unprecedented detail using state-to-state, crossed molecular beam experiments. The NO(X) molecules are selected in the Ω = 0.5, j = 0.5, f state and then oriented such that either the 'N' or 'O' end of the molecule is directed towards the incoming Ar atom. Velocity map ion imaging is then used to probe the scattered NO molecules in well-defined quantum states. We show that the fully quantum state-resolved differential steric asymmetry, which quantifies how the relative efficiency for scattering off the 'O' and the 'N' ends of the molecule varies with scattering angle, is strongly affected by quantum interference. Significant changes in both integral and differential cross sections are found depending on whether collisions occur with the N or O ends of the molecule. The results are well accounted for by rigorous quantum mechanical calculations, in contrast to both classical trajectory calculations and more simplistic models that provide, at best, an incomplete picture of the dynamics.

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.

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.

Fully quantum state-resolved inelastic scattering between He and NO(XΠ2)

Quantum mechanical close-coupling calculations have been used to obtain fully quantum state-resolved differential cross sections and opacity functions for the rotationally inelastic collisions of NO(X (2)Pi) with He at collision energies of 63 and 147 meV using the most recent ab initio potential energy surfaces of Kłos et al. [J. Chem. Phys. 112, 2195 (2000)]. Double peaks observed in the Lambda-doublet resolved differential cross sections are shown to be related to the presence of analogous peaks in the corresponding opacity functions. These structures can be linked directly to a specific expansion term in the potential, and reflect the fact that NO is not quite homonuclear.

Investigation of steric effects in inelastic collisions of NO(X 2Π) with Ar

Cross sections were determined for collisions of Ar with oriented NO(X 2Π), based on full close-coupled calculations and new ab initio potential energy surfaces (PESs). Collisions in which the NO molecules are initially oriented so that the O end preferentially points toward the Ar atom are more effective in promoting spin–orbit changing transitions. The magnitude of the steric asymmetry is consistent with earlier calculations based on a previous PES, and agrees well with experiment. Various modifications of the full PESs were used to explore the origin of the observed features in the steric asymmetries, in particular the striking oscillatory pattern seen in the variation of the steric asymmetry with final state.

Direct observation of Λ-doublet and hyperfine branching ratios for rotationally inelastic collisions of NO–He at 4.2 K

A 4.2 K collisionally cooling cell and time-resolved millimeter-wave/millimeter-wave double resonance have been used to directly measure the Λ-doublet and hyperfine branching ratios and absolute cross-sections of rotationally inelastic collisions of NO–He. The data were analyzed in the context of angular momentum recoupling, which is based on the `spectator' role of the nuclear spin in the collision, and tensor opacities, which characterize the electrostatic collision. The propensities expected for the approximately homonuclear NO (the largest of which is the e/f symmetry conserving, ΔJ=0 rate) were observed and the four K=1 tensor opacities measured.

Polarization and steric effects in inelastic collisions of NO(X 2Π) with Ar and He

Differential mj resolved cross-sections were determined for collisions of NO(X2Π) with Ar and He based on full close-coupled calculations and abinitio potential energy surfaces. As a function of the scattering angle, the predicted alignment becomes increasingly negative as the scattering angle increases. Integrated over all scattering angles, the sign of the product alignment becomes increasingly negative as Δj increases. For collisions with Ar, the product alignment is more negative for spin–orbit changing, as compared to spin–orbit conserving, transitions. For collisions with He, the calculated product alignment, both differential and integral, is in excellent agreement with experiment.

State-to-state differential cross sections for rotationally inelastic collisions of NO(2Π1/2,j⩽2.5) with CO(1Σ+) and O2(3Σg−) at a kinetic energy of 442 cm−1

Crossed molecular beam measurements of state-resolved differential cross sections for NO+O2 and NO+CO inelastic collisions at a relative kinetic energy of 442 cm−1 are reported. The initial states (NO 2Π1/2, ν=0, j⩽2.5, CO 1Σ+, ν=0, O2 3Σg−, ν=0) were prepared by pulsed supersonic expansions of pure NO, O2, and CO gas. Scattered NO products were detected by resonance enhanced two-photon ionization. Product distributions were measured in both center-of-mass scattering angle and final rotational state (j′). Intensity maxima were found in both types of scans and comparable populations were found in both of the spin–orbit manifolds (2Π1/2 and 2Π3/2). The results are compared to previous inelastic scattering experiments of NO collisions with NO, Ar, CO, and O2.

Differential cross sections for rotationally inelastic collisions of NO(2Π1/2, j′⩽2.5) with NO(2Π1/2, j′⩽2.5) at a kinetic energy of 442 cm−1

We report crossed molecular beam measurements of state-resolved differential cross sections for NO+NO collisions at relative kinetic energy of 442 cm−1. The initial state (NO 2Π1/2v=0,j⩽2.5) was prepared in both beams by pulsed supersonic expansion of pure NO gas. Scattered products were detected by resonance enhanced two-photon ionization. NO product distributions were measured in both scattering angle and final rotational state. Intensity maxima were found in both types of scans, and comparable populations were found in both of the spin-orbit manifolds (2Π1/2 and 2Π3/2). The results obtained here are compared to previous NO+Ar scattering experiments and theory, NO dimer studies, and NO+NO bulb kinetics experiments.

State-to-state differential cross sections for rotationally inelastic collisions of NO(2Π1/2,j=0.5) with Ar at kinetic energies between 117 cm−1 and 1694 cm−1

State-to-state differential cross sections have been measured for scattering of NO(2Π1/2,j=0.5) by Ar at kinetic energies of 117, 149, 442, and 1694 cm−1. The differential cross sections at each collision energy are presented as a function of final state (Ω′,j′) at constant center-of-mass scattering angle. Center-of-mass angular distributions are also given for final rotational states (2Π1/2, j′=1.5, 2.5, 8.5, 12.5, and 14.5) at a kinetic energy of 442 cm−1, and for j′=18.5 at a kinetic energy of 1694 cm−1. Rotational rainbow structure is seen in both types of data. The results are generally in good agreement with quantum scattering calculations carried out by Alexander [J. Chem. Phys. 99, 7725 (1993)] using newly calculated ab initio potential energy surfaces, and thus may serve as a new benchmark for the microscopic dynamics of molecular energy transfer in open-shell molecules.

Inelastic scattering of [formula omitted] with atomic and molecular colliders. Rotational and fine-structure excitations

The inelastic collisions of NO ( T rot ⩽ 5 K) with Ar, Xe, CO, N 2, O 2, N 2O and CO 2 were studied in molecular beams at center-of-mass collision energies 750–2500 cm −1. Rotational, Λ-doublet and spin-orbit distributions of scattered NO( 2 Π 1 2 3 2 ) were determined. In all the scattering experiments (with the possible exception of Ar) no preferences wer observed in the Λ-doublet populations. The rotational distributions all appear Boltzmann-like, with somewhat different rotational temperatures. The populations of the 2 Π 1 2 and 2 Π 3 2 spin-orbit states depend on the nature of the collider, but do not show any clear relationship with the extent of rotational excitation. No evidence of long-range attractive interactions is revealed in any of the systems studied.