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Physical Review A

Oscillatory alignment phenomena in Rydberg-atom–rare-gas collisions

William A. Isaacs, Michael A. Morrison

Published Jan 1, 1998• 13 citations
Publisher

Abstract

Retention of orbital alignment during near-resonant energy transfer collisions of rare-gas projectiles with excited alkaline-earth-metal atoms is explored theoretically for scattering of Xe from Rydberg states of Ca. For such collisions, conventional interpretations of alignment phenomena, which are based on potential curves for a transient molecule thought to form during the collision, are not relevant. Theoretical partial magnetic sublevel cross sections for ${\mathrm{Ca}}^{**}(4s17d{}^{1}{D}_{2}\ensuremath{\rightarrow}4s18p{}^{1}{P}_{1})$ transitions confirm the existence of alignment effects, as demonstrated experimentally by Spain et al. [J. Chem. Phys. 102, 9532 (1995)], at a mean relative velocity of 914 m/s. Theory further predicts heretofore unobserved oscillatory structures in these cross sections as a function of relative velocity and that these oscillations depend strongly on the initial magnetic quantum number of the Rydberg electron.

DOI
10.1103/physreva.57.r9

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Physical Review Letters•May 30, 1994•12 citations
Retention of orbital alignment during near-resonant energy transfer collisions of rare-gas projectiles with excited alkaline-earth-metal atoms is explored theoretically for scattering of Xe from Rydberg states of Ca. For such collisions, conventional interpretations of alignment phenomena, which are based on potential curves for a transient molecule thought to form during the collision, are not relevant. Theoretical partial magnetic sublevel cross sections for Ca∗∗(4s17d1D2\ensuremath→4s18p1P1){\mathrm{Ca}}^{**}(4s17d{}^{1}{D}_{2}\ensuremath{\rightarrow}4s18p{}^{1}{P}_{1})Ca∗∗(4s17d1D2​\ensuremath→4s18p1P1​) transitions confirm the existence of alignment effects, as demonstrated experimentally by Spain et al. [J. Chem. Phys. 102, 9532 (1995)], at a mean relative velocity of 914 m/s. Theory further predicts heretofore unobserved oscillatory structures in these cross sections as a function of relative velocity and that these oscillations depend strongly on the initial magnetic quantum number of the Rydberg electron.
Recent experimental and theoretical studies of near-resonant energy transfer collisions involving rare-gas atoms and alkali or alkaline earth atoms which have been initially excited to an aligned state via one or more linearly polarized rasters have yielded a wealth of insight into orbital alignment and related effects. We have extended this inquiry to initially aligned Rydberg states, examining state-to-state and alignment-selected cross sections using quantum collision theory augmented by approximations appropriate to the special characteristics of the Rydberg state (e.g., the quasi-free-electron model and the impulse approximation).
A recent quantum mechanical study [W. Isaacs and M. A. Morrison, Phys. Rev. A 57, R9 (1998)] discovered pronounced oscillations in cross sections for near-resonant energy transfer collisions of rare-gas atoms with initially aligned Rydberg atoms. We analyze such collisions for 17d(m)-->18p(m(')) transitions in the Ca-He system semiclassically and show that the oscillations arise from a phase interference process unique to Rydberg target states. In addition to explaining the origin of these structures, this analysis explains their disappearance when the relative Ca-He velocity goes to infinity and/or the energy defect vanishes and their dependence on the initial and final magnetic quantum numbers of the transition.
A classical ensemble model of three-body energy transfer in the point contact approximation is presented. This model yields cross sections for transitions between initial and final states defined by energy, magnitude of angular momentum, and projection of angular momentum along an axis of spatial quantization corresponding to the quantum numbers of the initial and final stationary states, n,l,m and n′,l′,m′. Using a cross section that is differential in the final-state quantum numbers, the spatial constraints imposed by conservation of energy and angular momentum can be investigated even for comparatively small quantum numbers. When applied to the Ca(17d)+He→Ca(18p)+He energy transfer processes, the model sheds light on recently discovered alignment phenomena in collisions of rare-gas atoms with initially aligned Rydberg atoms. Materials for the implementation of this model are available from the authors via the Internet.
The resonant mechanism of interaction of alkaline-earth atoms having a low electron affinity to Rydberg atoms in circular (l = |m| = n–1) and near-circular states has been studied. To describe the dynamics of resonant processes accompanied by nonadiabatic transitions between ionic and Rydberg covalent terms of a quasimolecule, an approach based on the integration of coupled equations for the probability amplitudes has been developed taking into account the possibility of the decay of an anion in the Coulomb field of the positive ionic core of a highly excited atom. The approach involves the specific features of the problem associated with the structure of the wavefunction of a Rydberg electron in states with high orbital angular momenta l ~ n–1. This approach provides a much more accurate description of the dynamics of electronic transitions at collisions between atoms than that within the modified semiclassical Landau–Zener model. In addition, this approach makes it possible to effectively take into account many channels of the problem. The cross sections for resonant quenching of Rydberg states of the Li(nlm) atom with given principal n, orbital l = n–1, and magnetic m quantum numbers at thermal collisions with the Ca(4s 2) and Sr(5s 2) atoms have been calculated. The dependences of the results on n, m, and angle α between the relative velocity of the atoms and the normal to the plane of the orbit of the Rydberg electron have been obtained. The influence of orientational effects on the efficiency of the collisional destruction of circular and near-circular states has been studied. The results indicate a higher stability of such states to their perturbations by neutral particles as compared to usually studied nl states with low values of l (l ≪ n).
We present a detailed study of the mechanism of spin–changing transitions between 1P and 3P electronic states corresponding to the nsn′p Rydberg state of an alkaline earth atom in collision with a structureless, spherical partner. This type of process takes place on four potential curves and involves collisional coupling between the electronic angular momentum L of the electronically excited atom, the spin S of the atom in the final 3P state, and the orbital angular momentum l associated with the relative motion of the collision partners. We review the use of Hund’s case coupling schemes in describing the collision and present a careful analysis of the initial state preparation under conditions of laser excitation in a crossed beam experiment. This underlies the development of a new statistical model for the dependence of the spin–changing cross section on the alignment of the pump laser. Finally, we describe a new technique to follow the redistribution of flux onto locally adiabatic states as the initially excited atom is approached by the closed-shell partner. This is used to show that dynamical corrections to our simple statistical model are small.
A theoretical study of Stueckelberg interference structure in inelastic, state-changing integrated cross sections for collisions of low-Rydberg Na (n10) atoms with ground state He atoms is reported. The dependence of the Stueckelberg interference structure on orbital geometry and alignment of the initial state is predicted. Interference oscillations are found in the cross sections for both initial ns and np states; in the latter case, the features show considerable sensitivity with respect to variations in the spatial alignment of the principal axis of the electron charge distribution at some collision velocities and near isotropy at others.