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Journal of the Chemical Society Faraday Transactions

Four-vector study of the spin-changing collision Ba[(6s) 1 (6p) 1 , 1 P 1 →(6s) 1 (6p) 1 , 3 P 2 ]/H 2

J.‐M. Mestdagh, J. P. Visticot, P. Meynadier, O. Sublemontier, Arthur G. Suits

Published Jan 1, 1993• 12 citations
Publisher

Abstract

The spin-changing (6s)1(6p)1, 1P1→(6s)1(6p)1, 3P2 transition of barium induced by single collisions with H2 has been studied using a crossed beam apparatus. An mj-selective preparation and detection of barium was achieved using the pump and probe technique in the presence of a magnetic field that breaks the degeneracy of the barium magnetic sublevels. The probe laser was scanned across the (6s)1(6p)1, 3P2,mj′→(6s)1(7s)1, 3S1, mj″ multiplicity. Analysis of the Doppler profiles of each line reveals the angular distribution of scattered barium atoms, the polarization of which is selected both before and after the collision. This is therefore a four-vector correlation experiment, and it is the first to document spin-changing transitions of alkaline-earth-metal atoms induced by a molecular target. It must be emphasized that all four vectors in the present experiment do not refer to the same quantization axis, whereas in the standard notion of n-vector correlation experiments, the n-vectors refer to the same axis. The results of the present work are discussed in the light shed by other results obtained on the same experiments with only two and three vectors controlled. A collision mechanism is proposed that accounts for all the existing experimental results on this process.

DOI
10.1039/ft9938901413

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Experimental studies concerning elastic and inelastic electron scattering by coherently ensembles of Ba (...6s6p (sub 1)P(sub 1)) atoms with various degrees of alignment will be described. An in-plane, linearly-polarized laser beam was utilized to prepare these target ensembles and the electron scattering signal as a function of polarization angle was measured for several laser geometries at fixed impact energies and scattering angles. From these measurements, we derived cross sections and electron-impact coherence parameters associated with the electron scattering process which is time reverse of the actual experimentally studied process. This interpretation of the experiment is based on the theory of Macek and Herte. The experimental results were also interpreted in terms of cross sections and collision parameters associated with the actual experimental processes. Results obtained so far will be presented and plans for further studies will be discussed.

•Jan 1, 2000•0 citations
The spin-changing (6s)1(6p)1, 1P1→(6s)1(6p)1, 3P2 transition of barium induced by single collisions with H2 has been studied using a crossed beam apparatus. An mj-selective preparation and detection of barium was achieved using the pump and probe technique in the presence of a magnetic field that breaks the degeneracy of the barium magnetic sublevels. The probe laser was scanned across the (6s)1(6p)1, 3P2,mj′→(6s)1(7s)1, 3S1, mj″ multiplicity. Analysis of the Doppler profiles of each line reveals the angular distribution of scattered barium atoms, the polarization of which is selected both before and after the collision. This is therefore a four-vector correlation experiment, and it is the first to document spin-changing transitions of alkaline-earth-metal atoms induced by a molecular target. It must be emphasized that all four vectors in the present experiment do not refer to the same quantization axis, whereas in the standard notion of n-vector correlation experiments, the n-vectors refer to the same axis. The results of the present work are discussed in the light shed by other results obtained on the same experiments with only two and three vectors controlled. A collision mechanism is proposed that accounts for all the existing experimental results on this process.
In a crossed beam atomic energy transfer experiment, relative cross sections are measured between initial and final magnetic substates of atomic orbitals in a three vector correlation experiment. A pulsed laser beam prepares Ca(4s5p 1P1) in a single magnetic sublevel ‖j1mi〉 with respect to the laser polarization vector. Subsequent collision with He at a well-defined relative velocity yields Ca(4s5p 3P2). The near-resonant Ca(4s5p 3P2) is probed by a second polarized pulsed laser, revealing its magnetic sublevel ‖j2mf〉 distribution with respect to the probe laser polarization vector. The experiment is analyzed in the collision frame where the direction of the initial relative velocity vector serves as the quantization axis. In this frame, the initial and final Ca states are characterized by substates ‖j1m1〉 and ‖j2m2〉, respectively. Fourteen collision frame cross sections are needed to describe the energy transfer completely. Eight of these cross sections are real and positive (conventional type—describing population transfer) and six are complex (coherence type—containing phase information). By symmetry, only 15 unique parameters for the real and imaginary parts of the cross sections are required, nine of which are obtained here using linear polarizations and collinear laser beams. Possible cases for circularly polarized light are also tabulated; measurement of these cases would provide several more parameters. For initial parallel preparation of the 1P1 p orbital with respect to the relative velocity (m1=0), the final 3P2 state is highly aligned in the m2=±1 and m2=±2 sublevels. Initial preparation of the p orbital perpendicular to the relative velocity vector (m1=±1) produces an aligned 3P2 state with greater population in the m2=0 and m2=±1 sublevels compared to the m2=±2 sublevels. Using the coherence information and symmetry, orientation cross sections are obtained into the m2=±1 sublevels (i.e., m preserving 1→1 and m-changing 1→−1) and are nearly identical. Preparation of the Ca p orbital perpendicular to the initial relative velocity enhances the state-specific collisional transfer of 1P1→3P2 by a factor of 2.2±0.2 over initial parallel preparation.
Results of the quantum close-coupling calculations for the spin-changing transition in Ba(6s6p, 1 P 1 → 3 P 2 ) induced by collisions with argon are presented. The dependence on angular and collision energy of the calculated multi-structure differential cross-sections are discussed. The backward scattering is most prominent in a small energy range, particularly in the case of Π preparation of the initial Ba( 1 P 1 )-Ar molecular state. The similar characteristic features of differential cross-sections are observed in the experiment by Visticot et al (Visticot J P et al 1992 Phys. Rev. A 45 6371).
A laser pump–probe experiment is used to study the orbital alignment effects, orientation effects and vector correlations of collisional transfer of the Ca (4s)1(4p)1, 3P1 state to the Ca (4s)1(4p)1, 3P2, 0 levels. The experiment is configured in a single-collision crossed-beam arrangement between Ca and He, and multi-structure cross-sections are determined using appropriate combinations of linear and circular laser light for the pump/probe steps. Real and imaginary parts of coherence cross-sections are obtained along with the conventional population cross-sections for the m1→m2 magnetic sublevel transitions into the 3P2 level. The total relative cross-section ratio for the perpendicular (m1=±1) to parallel (m1= 0) polarization preparation of 3P1 transferring to 3P2 is 1.46 ± 0.15. For initial 3P1 preparation with laser light linearly polarized perpendicular to the initial relative velocity vector, the transfer into the m2-sublevels of the 3P2 state show a distinct preference for the sign-changing m1=+ 1 →m2=–1 transition. Preparation of Ca 3P1 with laser light linearly polarized parallel to the initial relative velocity vector produces population transfer into the 3P2 level that is completely aligned in the ±1 and ±2 sublevels, consistent with symmetry considerations. The magnitudes of the coherence cross-sections range from a few percent to greater than 100% of some of the population transfer conventional cross-sections. Study of the alignment effect into the final 3P0 state found a very large observed effect (σ|m|= 1/σ|m|= 0) of 23 ± 0.9. Interpretation of the energy transfer results indicates that the energy transfer obeys symmetry rules and follows predictions of curve crossings between the Σ and Π potentials, where for the transfer into 3P2 only indirect coupling can occur.
Intramultiplet mixing collisions have been investigated using polarized Ne(3p)\mathrm{Ne}(3p)Ne(3p) atoms with a well-defined initial relative velocity, i.e., two initial-state vectors. By analyzing the final-state center-of-mass velocity with a (Doppler-detuned) probe laser with a well-defined polarization and detecting the laser-induced fluorescence we measure the differential cross section and the polarization of the final state, i.e., two final-state vectors. Measurements have been done at a collision energy E=70meV,E=70\mathrm{meV},E=70meV, for both the ${\ensuremath{\alpha}}_{7}$ and ${\ensuremath{\alpha}}_{9}$ (Paschen numbering) initial states. For the elastic ${\ensuremath{\alpha}}_{7}\ensuremath{\rightarrow}{\ensuremath{\alpha}}_{7}$ collisions the experimental results are strongly hindered by the undesired response on elastic scattering of the metastable Ne(3s,3P0)\mathrm{Ne}{(3s,}^{3}{P}_{0})Ne(3s,3P0​) state. Inelastic collisions have been investigated for the ${\ensuremath{\alpha}}_{9}$ state, that forms a closed level system with the metastable Ne(3s,3P2)\mathrm{Ne}{(3s,}^{3}{P}_{2})Ne(3s,3P2​) state. For the ${\ensuremath{\alpha}}_{9}\ensuremath{\rightarrow}{\ensuremath{\alpha}}_{8}$ transition experimental results are presented for both parallel and perpendicular orientations of the polarizations of the pump and probe laser, respectively, as well as for circular polarized $({\ensuremath{\sigma}}^{+},{\ensuremath{\sigma}}^{+})$ and $({\ensuremath{\sigma}}^{+},{\ensuremath{\sigma}}^{\ensuremath{-}})$ configurations. The experiments are compared to a full quantum-mechanical coupled channels calculation of the scattering process, using the model potentials of Hennecart and Masnou-Seeuws as input. The output of this calculation is combined with a density matrix description of the initial- and final-state polarization. Both shape of the differential cross sections and their relative scaling for different laser polarizarion configurations agree well with experiment. This gives strong support for the quality of the model potentials used.
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.
Experimental studies concerning elastic and inelastic electron scattering by coherently ensembles of Ba (...6s6p (sub 1)P(sub 1)) atoms with various degrees of alignment will be described. An in-plane, linearly-polarized laser beam was utilized to prepare these target ensembles and the electron scattering signal as a function of polarization angle was measured for several laser geometries at fixed impact energies and scattering angles. From these measurements, we derived cross sections and electron-impact coherence parameters associated with the electron scattering process which is time reverse of the actual experimentally studied process. This interpretation of the experiment is based on the theory of Macek and Herte. The experimental results were also interpreted in terms of cross sections and collision parameters associated with the actual experimental processes. Results obtained so far will be presented and plans for further studies will be discussed.