Molecular beam scattering of NO+Ne: A joint theoretical and experimental study

The near-IR spectrum of NO(X̃(2)Π)-Ne detected through excitation into the Ã-state continuum: A joint experimental and theoretical study

We present new measurements of the near IR spectrum of NO-Ne in the region of the first NO overtone transition. The IR absorption is detected by exciting the vibrationally excited complex to the Ã-state dissociation continuum. The resulting NO(A) fragment is subsequently ionized in the same laser pulse. Spectra of the two lowest bands, A and B, are recorded. The spectra are compared with calculated spectra based on bound states derived from a new set of high level ab initio potential energy surfaces (PESs). For the calculation, the PESs are used with either fixed NO intermolecular distance or averaged for the vibrational states of NO (X̃, v = 0 or 2). Spectra based on the new PESs reproduce the experimental spectra better than theoretical spectra based on the older PESs of M. H. Alexander et al. [J. Chem. Phys. 114, 5588 (2001)]. Especially, spectra based on the two different vibrationally averaged PESs show a marked improvement in comparison to the one based on the fixed internuclear NO-distance. A fitted set of spectroscopic constants allows to reproduce most of the finer details of the measured spectra. Monitoring simultaneously the NO fragment ion and the parent ion channels while scanning the UV wavelength through the NO A-X hot-band region enabled us to confirm the NO-Ne Ã-state dissociation limit of 44233 ± 5 cm(-1). These measurements also confirm the absence of a structured NO-Ne spectrum involving the Ã-state.

Rotational alignment of NO (A2Σ+) from collisions with Ne

We report the direct angle-resolved measurement of collision-induced alignment of short-lived electronically excited molecules using crossed atomic and molecular beams. Utilizing velocity-mapped ion imaging, we measure the alignment of NO in its first electronically excited state (A(2)Σ(+)) following single collisions with Ne atoms. We prepare A(2)Σ(+) (v = 0, N = 0, j = 0.5) and by comparing images obtained using orthogonal linear probe laser polarizations, we experimentally determine the degree of alignment induced by collisional rotational excitation for the final rotational states N' = 4, 5, 7, and 9. The experimental results are compared to theoretical predictions using both a simple classical hard-shell model and quantum scattering calculations on an ab initio potential energy surface (PES). The experimental results show overall trends in the scattering-angle dependent polarization sensitivity that are accounted for by the simple classical model, but structure in the scattering-angle dependence that is not. The quantum scattering calculations qualitatively reproduce this structure, and we demonstrate that the experimental measurements have the sensitivity to critique the best available potential surfaces. This sensitivity to the PES is in contrast to that predicted for ground-state NO(X) alignment.

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.

New findings regarding the NO angular momentum orientation in Ar-NO(2Π(1/2)) collisions

This article reports a theoretical study of the stereodynamics of Ar + NO(X(2)Π, v = 0, j = 1/2, Ω = 1/2, ε = ±1) rotationally inelastic collisions. First, quantum scattering data are used to calculate all differential polarisation moments of the reagent and product molecules; this leads to the observation that the orientations of the reagent and product angular momenta are very strongly correlated. Next, canonical collision mechanisms theory [Aldegunde et al., Phys. Chem. Chem. Phys., 2008, 10, 1139] is used to separate and characterise the stereodynamics of the two independent collision mechanisms that contribute to the collision dynamics; this leads to the observation that the average product orientation is determined by the relative contributions of the two canonical mechanisms, which have comparable importance but are associated with starkly contrasting angular momentum orientations. These observations lead to a new and rigorous explanation of the experimental results reported a decade ago by Lorenz et al. [Science, 2001, 293, 2063]. The central fact of the new explanation is the incoherent, interference-free superposition of two independent collision mechanisms. This makes the new explanation radically different from the only one previously suggested, namely that the experimental observations might be due to quantum interference in a single collision mechanism.

Joint experimental-theoretical investigation of the lower bound states of the NO(X2Pi)-Kr complex

We describe the first measurement of the near IR spectrum of the NO-Kr van der Waals complex. A variant of IR-REMPI double-resonance spectroscopy is employed in which the IR and UV lasers are scanned simultaneously in such a way that throughout the scan the sum of the two photon energies is kept constant, matching a UV resonance of the system. In the region of the first overtone vibration of the NO monomer, we observe several rotationally resolved bands for the NO-Kr complex. In addition to the origin band located at 3723.046 cm(-1), we observe excited as well as hot bands involving the excitation of one or two quanta of z-axis rotation. Another band is assigned to the excitation of one quantum of bending vibration. The experimental spectra are compared with results of bound-state calculations for a new set of potential energy surfaces calculated at the spin-restricted coupled cluster level. For the average vibration-rotation energies, there is excellent agreement between the theoretical results based on the coupled states (CS) approximation and the full close-coupling (CC) treatment. Finer details like the electrostatic splitting and the P-type doubling of the rotational levels are accounted for only within the CC formalism. The comparison of the CC results with the measured spectra confirms the high quality of the PESs. However, the high resolution of the experiments is sufficient to identify some inaccuracies in the difference between the potential energy surfaces of A' and A'' reflection symmetry.

Quantum Interference as the Source of Steric Asymmetry and Parity Propensity Rules in NO−Rare Gas Inelastic Scattering

Rotationally inelastic scattering of rare gas atoms and oriented NO molecules exhibits a remarkable alternation in the sign of steric asymmetry between even and odd changes in rotational quantum number. This effect has also been found in full quantum-mechanical scattering calculations. However, until now no physical picture has been given for the alternation. In this work, a newly developed quasi-quantum treatment (QQT) provides the first demonstration that quantum interferences between different orientations of the repulsive potential (that are present in the oriented wave function) are the source of this alternation. Further, from application of the treatment to collisions of nonoriented molecules, a previously unrecognized propensity rule is derived. The angular dependence of the cross sections for excitation to neighboring rotational states with the same parity is shown to be similar, except for a prefactor. Experimental results are presented to support this rule. Unlike conventional quantum-mechanical (or semiclassical) treatments, QQT requires no summation over the orbital angular momentum quantum number l or integration over the impact parameter b. This eliminates the need to solve large sets of coupled differential equations that couple l and rotational state channels among which interference can occur. The QQT provides a physical interpretation of the scattering amplitude that can be represented by a Legendre moment. Application of the QQT on a simple hard-shell potential leads to near-quantitative agreement with experimental observations.