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Variation of spin state due to collisions of Ca(5/sup 1//ital P//sub 1/) polarized atoms with He and Ne atoms

Alexander Z. Devdariani, A. L. Zagrebin, M. G. Lednev

Published Sep 1, 1988• 0 citations
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Abstract

An analysis of the effect of the polarization of Ca(5/sup 1//ital P//sub 1/) atoms on the reaction cross section of the variation of spin states during collisions with He and Ne atoms under the conditions of a pulsed experiment with laser radiation was made on the basis of computed quasimolecular terms. The effect of repeated crossing of terms and potential barriers is discussed.

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An analysis of the effect of the polarization of Ca(5/sup 1//ital P//sub 1/) atoms on the reaction cross section of the variation of spin states during collisions with He and Ne atoms under the conditions of a pulsed experiment with laser radiation was made on the basis of computed quasimolecular terms. The effect of repeated crossing of terms and potential barriers is discussed.
A formula has been obtained which describes the dependence of the overall cross section of the inelastic process in experiments with intersecting atomic and light beams on five parameters, two of which depend on the properties of the quasimolecule and the collisional energy, and three others on the degree of linear polarization of the exciting light and two angles which determine the geometry of the experiment. It was shown that measuring the overall cross section by varying the two latter parameters allows us to obtain additional information on the mechanism of the reactions. As an example, the experimentally studied process of spin transfer in Ca(4s5p/sup 1/P/sub 1/)+He and the process of Penning ionization of He(2/sup 1/P/sub 1/)+Cd(Zn) has been considered.
We consider collisions between (quasi-) one- or two-electron atoms, each of which has been prepared in a fixed distribution of hyperfine magnetic substates. The resulting ``polarization-dependent'' reaction rates are functions of (1) the (experimentally adjustable) orientation of the quantization axis associated with the hyperfine substates, (2) the hyperfine terms of the colliding atoms, and (3) the detailed reaction dynamics as characterized by preparation- and fine-structure-independent cross sections. These effects are deconvoluted and analyzed, and their use in elucidating reaction mechanisms is discussed. Particular emphasis is placed on the associative ionization of two Na(3p) atoms, each prepared by adsorption of resonant laser photons. The quasimolecular states of Na...Na that might react in this case are limited by selection rules. More information on the reaction mechanism is then obtained by using experimental data, in combination with the formal analysis, to draw further conclusions about the active quasimolecular states.
Time-resolved nsec kinetic methods are used to measure state specific electronic excitation transfer for the near-resonant process: Ca(4s5p /sup 1/P/sub 1/) +Ar /sup k//sub 1/arrow-right-left/sub k//sub -/1 Ca(4s5p /sup 3/P/sub 2,1,0/)+Ar. In stimulated emission experiments it is found that the Ca(4s5p /sup 3/P/sub 2/) level is preferentially populated. The rate constant k/sub 1/ for the specific /sup 3/P/sub 2/ channel is 5.2 +- 0.5 x 10/sup -11/ cm/sup 3/ molecule/sup -1/ sec/sup -1/. These results are discussed in terms of near-resonant spin changing collision mechanisms. Radiative lifetimes for Ca(4s5p /sup 1/P/sub 1/) and Ca(4s5p /sup 3/P/sub 1/) are also determined to be 77 +- 2 nsec and 67 +- 2 nsec, respectively.
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.