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Physical Review Letters

Preparation of Molecular States in an Ion-Atom Crossed-Beam Scattering Experiment by Laser Excitation

Hartmut Schmidt, A. Bähring, Edmund Meyer, B. Miller

Published Apr 12, 1982• 27 citations
Publisher

Abstract

In ${\mathrm{Na}}^{+}$-${\mathrm{Na}}^{*}$ scattering in the energy range ${E}_{\mathrm{c}.\mathrm{m}.}=25\ensuremath{-}50$ eV the molecular states $\ensuremath{\sigma}$, ${\ensuremath{\pi}}^{+}$, and ${\ensuremath{\pi}}^{\ensuremath{-}}$ of the resulting ionic quasimolecule ${\mathrm{Na}}_{2}^{+}$ are prepared by laser-optical pumping of the sodium atom to the $3^{2}P_{\frac{3}{2}}$, $F=3$ state. Selection is made with linearly or circularly polarized light. With this novel approach, one can discuss the transition from the space-fixed to the body-fixed reference frame in detail. There is clear evidence for conservation of reflection symmetry with respect to the scattering plane and the implications are considered.

DOI
10.1103/physrevlett.48.1008

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In Na+{\mathrm{Na}}^{+}Na+-Na∗{\mathrm{Na}}^{*}Na∗ scattering in the energy range Ec.m.=25\ensuremath−50{E}_{\mathrm{c}.\mathrm{m}.}=25\ensuremath{-}50Ec.m.​=25\ensuremath−50 eV the molecular states \ensuremathσ\ensuremath{\sigma}\ensuremathσ, \ensuremathπ+{\ensuremath{\pi}}^{+}\ensuremathπ+, and \ensuremathπ\ensuremath−{\ensuremath{\pi}}^{\ensuremath{-}}\ensuremathπ\ensuremath− of the resulting ionic quasimolecule Na2+{\mathrm{Na}}_{2}^{+}Na2+​ are prepared by laser-optical pumping of the sodium atom to the 32P323^{2}P_{\frac{3}{2}}32P23​​, F=3F=3F=3 state. Selection is made with linearly or circularly polarized light. With this novel approach, one can discuss the transition from the space-fixed to the body-fixed reference frame in detail. There is clear evidence for conservation of reflection symmetry with respect to the scattering plane and the implications are considered.
Collisions studies using optically state selected beams is discussed. Measurements of spin resolved differential cross sections for superelastic scattering of electrons from excited sodium atoms (3P) are reported. The excited states are prepared by laser optical pumping with circularly polarized light thus giving a pure atomic state before the collision. A crossed beam arrangement of sodium atoms and electrons is used with a scattered electron detector. Scattered electron intensity as a function of scattering angle, incident electron energy, and relative orientation of the incident electronic and atomic spins is measured. 8 refs., 10 figs.
Electron scattering by excited sodium atoms has been carried out in a crossed-beam experiment. The atoms have been optically pumped into a well defined hyperfine level (3 2 P 3 2/F=3) by a tunable CW dye laser. For two of the most prominent scattering processes, the 3 2 P to 3 2 S and 3 2 P to 4 2 S transitions, the effect of the anisotropy of the initial state introduced by the laser light is studied in detail and is interpreted in terms of multipole moments describing the optical excitation and the collision process. Possibilities and limitations of the experiment are discussed. Electron collisions may be seen as a test case for the understanding of more involved interaction processes.
We report cross-section measurements of the associative-ionization process, Na(3p2{\mathit{p}}^{2}p2P3/2{\mathit{P}}_{3/2}P3/2​)+Na(3p2{\mathit{p}}^{2}p2P3/2{\mathit{P}}_{3/2}P3/2​)\ensuremath{\rightarrow}Na2+{\mathrm{Na}}_{2}^{+}Na2+​+e as a function of velocity, alignment, and orientation in the collision energy range from 300 to 5 K. Two independently tunable laser beams select narrow velocity classes of excited sodium in a well-collimated atomic beam. Circular and linear polarization of the excitation lasers permits alignment and orientation of the excited atoms in the two velocity classes. The excitation functions show pronounced differences reflecting different molecular pathways to the final product, and they bridge the energy range between conventional and ``cold'' collision dynamics.
The optical pumping parameters that modify the pure-state pseudo-Stokes parameters in an electron-superelastic-scattering experiment are calculated for the 3 2^{2}2S1/2{\mathit{S}}_{1/2}S1/2​(F=2)--32{3}^{2}32P3/2{\mathit{P}}_{3/2}P3/2​(F) transitions of Na. A QED model is used to describe the laser excitation. It is found, that while the magnitude of the pumping parameter K' associated with circularly polarized excitation is close to unity for other than weak laser intensities, the value of the parameter K, associated with linearly polarized light, differs significantly from unity and is dependent on such experimental conditions as laser intensity, laser detuning, laser beam profile, and the Doppler width of the atomic transition. Methods for measuring K are suggested.
An experimental investigation of electron - impact induced transitions from the laser - excited 3P state of sodium atom to the 4S and 3D excited states is presented, the main objective being measurement of the atomic collision parameters ( ACPs ) that are related to individual scattering amplitudes. These parameters are complementary to the differential excitation cross - sections and provide finer details on the collisional excitation process. The experiment involved the preparation of the 3P - state with coherent laser radiation of a particular polarisation and measuring the rate of electrons scattered from the laser - excited state as function of laser polarisations. The ACPs were then deduced from these measurements and optical pumping parameters, which account for the optical preparation of the 3P - state. To formulate different measurement schemes, a measurement theory based on the general description of transitions between states of different angular momentum using density matrix formalism is derived. The investigation of the 3P - 4S transition is very similar to the studies of the 3S - 3P excitation performed by superelastic method involving the 3P - 3S scattering in the 'time - inverse' geometry. 'Standard' ACPs are used and, similarly to the superelastic method, the measurements for the 3P - 4S excitation performed in the 'time - inverse' geometry also reveal ACPs for the 4S - 3P de -excitation process. For the more complex 3P - 3D transition, however, the 'standard' ACPs may be less informative. It is shown that certain sums of individual magnetic sublevel excitation probabilities can be measured which may be considered as atomic collision parameters for transitions between P and D - states.
The 3s-3p excitation of spin-polarized Na atoms by Ne+{\mathrm{Ne}}^{+}Ne+ ions has been studied for impact energies Elab=200{E}_{\mathrm{lab}=200}Elab=200​ eV to 6 keV, i.e., in the adiabatic regime. The total excitation cross section and the three Stokes polarization parameters of the fluorescence light have been measured. The linear polarization of the light shows a preferential excitation of the \ensuremath{\Vert}ml{m}_{l}ml​\ensuremath{\Vert}=1 magnetic substates. The circular polarization probes the spin orientation of the excited 3p state. At the highest impact energies investigated the experimental data are compatible with conservation of spin orientation during the collision. With decreasing impact energy, the spin polarization of the final 3p state becomes smaller than the spin polarization of the initial 3s state. This apparent spin depolarization is attributed to the exchange interaction between the Na valence electron and the unfilled Ne+{\mathrm{Ne}}^{+}Ne+ 2p5{p}^{5}p5 core in the quasimolecule formed during the collision.