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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.

The rotationally resolved magnetic dipole absorption spectrum of the oxygen A-band b1Σ + g (v=0) ←X3Σ- g (v=0) perturbed by collisions with helium was studied theoretically using the impact approximation. To calculate the relaxation matrix, scattering calculations were performed on a newly computed helium-oxygen (b1Σ + g ) interaction potential as well as on a helium-oxygen (X3Σ- g ) interaction potential from the literature. The calculated integrated line cross sections and broadening coefficients are in good agreement with experimental results from the literature. Additionally, cavity ring-down experiments were performed in the wings of the spectral lines for a quantitative study of line-mixing, i.e., the redistribution of rotational line intensities by helium-oxygen collisions. It is shown that inclusion of line-mixing in the theory is required to reproduce the experimentally determined absolute absorption strengths as a function of the density of the helium gas. [ABSTRACT FROM AUTHOR]

We present detailed calculations on resonances in rotationally and spin-orbit inelastic scattering of OH (X 2Π, j = 3/2, F1, f) radicals with He and Ne atoms. We calculate new ab initio potential energy surfaces for OH-He, and the cross sections derived from these surfaces compare well with the recent crossed beam scattering experiment of Kirste et al. [Phys. Rev. A 82, 042717 (2010)]. We identify both shape and Feshbach resonances in the integral and differential state-to-state scattering cross sections, and we discuss the prospects for experimentally observing scattering resonances using Stark decelerated beams of OH radicals. [ABSTRACT FROM AUTHOR]

We theoretically study slow collisions of NH3 molecules with He atoms, where we focus in particular on the observation of scattering resonances. We calculate state-to-state integral and differential cross sections for collision energies ranging from 10-4 cm-1 to 130 cm-1, using fully converged quantum close-coupling calculations. To describe the interaction between the NH3 molecules and the He atoms, we present a four-dimensional potential energy surface, based on an accurate fit of 4180 ab initio points. Prior to collision, we consider the ammonia molecules to be in their antisymmetric umbrella state with angular momentum j = 1 and projection k = 1, which is a suitable state for Stark deceleration. We find pronounced shape and Feshbach resonances, especially for inelastic collisions into the symmetric umbrella state with j = k = 1. We analyze the observed resonant structures in detail by looking at scattering wavefunctions, phase shifts, and lifetimes. Finally, we discuss the prospects for observing the predicted scattering resonances in future crossed molecular beam experiments with a Stark-decelerated NH3 beam. [ABSTRACT FROM AUTHOR]

Relative integrated cross sections are measured for rotationally inelastic scattering of NO(2Π1/2), hexapole selected in the upper Λ-doublet level of the ground rotational state (j=0.5), in collisions with He at a nominal energy of 514 cm-1. Application of a static electric field E in the scattering region, directed parallel or antiparallel to the relative velocity vector v, allows the state-selected NO molecule to be oriented with either the N end or the O end towards the incoming He atom. Laser-induced fluorescence detection of the final state of the NO molecule is used to determine the experimental steric asymmetry, SA≡(σv↑↓E-σv↑↑E)/(σv↑↓E+σv↑↑E), which is equal to within a factor of (-1) to the molecular steric effect, Si→f≡(σHe→NO-σHe→ON)/(σHe→NO+σHe→ON). The dependence of the integral inelastic cross section on the incoming Λ-doublet component is also observed as a function of the final rotational (j′), spin-orbit (Ω′), and Λ-doublet (ε′) state. The measured steric asymmetries are significantly larger than previously observed for NO-Ar scattering, supporting earlier proposals that the repulsive part of the interaction potential is responsible for the steric asymmetry. In contrast to the case of scattering with Ar, the steric asymmetry of NO-He collisions is not very sensitive to the value of Ω′. However, the Λ-doublet propensities are very different for [Ω=0.5(F1)→Ω′=1.5(F2)] and [Ω=0.5(F1)→Ω′=0.5(F1)] transitions. Spin-orbit manifold conserving collisions exhibit a propensity for parity conservation at low Δj, but spin-orbit manifold changing collisions do not show this propensity. In conjunction with the experiments, state-to-state cross sections for scattering of oriented NO(2Π) molecules with He atoms are predicted from close-coupling calculations on restricted coupled-cluster methods including single, double, and noniterated triple excitations [J. Klos, G. Chalasinski, M. T. Berry, R. Bukowski, and S. M. Cybulski, J. Chem. Phys. 112, 2195 (2000)] and correlated electron-pair approximation [M. Yang and M. H. Alexander, J. Chem. Phys. 103, 6973 (1995)] potential energy surfaces. The calculated steric asymmetry Si→f of the inelastic cross sections at Etr=514 cm-1 is in reasonable agreement with that derived from the present experimental measurements for both spin-manifold conserving (F1→F1) and spin-manifold changing (F1→F2) collisions, except that the overall sign of the effect is opposite. Additionally, calculated field-free integral cross sections for collisions at Etr=508 cm-1 are compared to the experimental data of Joswig et al. [J. Chem. Phys. 85, 1904 (1986)]. Finally, the calculated differential cross section for collision energy Etr=491 cm-1 is compared to experimental data of Westley et al. [J. Chem. Phys. 114, 2669 (2001)] for the spin-orbit conserving transition F1 (j=0.5)→F1f (j′=3.5). © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A joint theoretical and experimental study of state-to-state rotationally inelastic polarization dependent differential cross sections (PDDCSs) for CO (v = 0, j = 0, 1, 2) molecules colliding with helium is reported for collision energies of 513 and 840 cm-1. In a crossed molecular beam experiment, velocity map imaging (VMI) with state-selective detection by (2 + 1) and (1 + 1′) resonance enhanced multiphoton ionization (REMPI) is used to probe rotational excitation of CO due to scattering. By taking account of the known fractions of the j = 0, 1, and 2 states of CO in the rotationally cold molecular beam (Trot ≈ 3 K), close-coupling theory based on high-quality ab initio potential energy surfaces for the CO-He interaction is used to simulate the differential cross sections for the mixed initial states. With polarization-sensitive 1 + 1′ REMPI detection and a direct analysis procedure described by Suits et al. (J. Phys, Chem. A 2015, 119, 5925), alignment moments are extracted from the images and the latter are compared with images simulated by theory using the calculated DCS and alignment moments. In general, good agreement of theory with the experimental results is found, indicating the reliability of the experiment in reproducing state-to-state differential and polarization-dependent differential cross sections. [ABSTRACT FROM AUTHOR]

An interaction potential with vibrational coordinate dependence is presented for the He-O[sub 2](X [sup 3]Σ[sub g][sup -]) van der Waals complex. The interaction is calculated with the partially spin-restricted open-shell single and double excitation coupled cluster method with perturbative triples [RCCSD(T)] in the supermolecule approach, with correction for the basis set superposition error. The augmented correlation consistent polarized triple zeta basis set is used with an additional set of bond orbitals. The single reference configuration method breaks down at an O-O separation of rapprox. 3.5 a[sub 0]. No points with r>3.1 a[sub 0] are used in the construction of the potential surface. A high quality 120 parameter fit to 754 ab initio points is presented. Special attention is given to the extraction of the long range coefficients. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In molecular collisions, resonances occur at specific energies at which the colliding particles temporarily form quasibound complexes, resulting in rapid variations in the energy dependence of scattering cross sections. Experimentally, it has proven challenging to observe such scattering resonances, especially in differential cross sections. We report the observation of resonance fingerprints in the state-to-state differential cross sections for inelastic NO-He collisions in the 13 to 19 centimeter-1 energy range with 0.3 centimeter-1 resolution. The observed structures were in excellent agreement with quantum scattering calculations. They were analyzed by separating the resonance contributions to the differential cross sections from the background through a partitioning of the multichannel scattering matrix. This revealed the partial-wave composition of the resonances and their evolution during the collision. [ABSTRACT FROM AUTHOR]