Orientational Dichroism in the Electron-Impact Ionization of Laser-Oriented Atomic Sodium

Abstract

We report the first experimental observation of "orientational dichroism" in electron impact-induced ionization. ͑e, 2e͒ experiments have been performed on pure angular momentum states 3P 3͞2 , m F 13 and m F 23 of the sodium atom, excited by right-and left-handed circularly polarized laser light. Our results show that the angular correlation of the final-state electron pair is strongly dependent on the m L sublevel populated. Comparison with calculation demonstrates the dependence of the dichroism on details of the scattering dynamics. [S0031-9007(97)04999-5] PACS numbers: 34.80.Dp A detailed knowledge of the ionization process is essential to our understanding of, and progress in, such diverse fields as discharge and plasma physics, fusion physics, laser physics, and the physics and chemistry of the upper atmosphere. The most accurate information on this process is derived from kinematically complete ͑e, 2e͒ experiments, in which the energies and momenta of all reaction participants and products are determined [1]. We report here on an ͑e, 2e͒ experiment in which the projection quantum number for an excited atomic target in a pure nonzero angular momentum state is also resolved. Using laser pumping techniques, an atomic sodium target is laser excited to the 3 2 P 3͞2 F 3, m F 13 or m F 23 hyperfine states. These states consist of maximal projections of the nuclear spin (I 63͞2), the orbital angular momentum (m L 61), and the electron spin (S 61͞2) along the laser beam direction.If neither the projection quantum numbers of the incident electron nor the target are resolved, the (e, 2e) differential cross section is simply a scalar invariant with respect to a joint rotation of the incoming and outgoing electron momenta p 0 , p a , and p b , respectively. This basic symmetry is destroyed by an initial orientation and/or alignment of the target. The (e, 2e) cross section is then irreducibly described by a set of tensorial parameters whose number is determined by the symmetry of the initially prepared target state [2]. Generally, the cross section for the ionization of a target with total angular momentum J can be written as [2]where L ͑i͒ 0 are tensorial components along the quantization axis of the target describing the dynamics of the ionization process and r i0 are the state multipoles describing the M-state population of the target. The transformation properties of L ͑i͒ 0 are determined by the rank i of the associated tensor. The solid angles of the momenta p a , p b with respect to p 0 are labeled by dV a , dV b , respectively.In the present case of the Na 3P 3͞2 state, neglecting spin-orbit interactions and for unpolarized primary electrons it suffices to consider Eq. (1) with respect to the orbital angular momentum L 1 for singlet and triplet scattering. Then the cross section is described only by three tensorial components L ͑i͒ 0 , which can be written in terms of the ionization cross sections for the individual magnetic substates s L,m L :L ͑2͒ 0 1 p 6 ͑s 1,1 2 2s 1,0 1 s 1,21 ͒ .The scalar component L ͑0...

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