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The Journal of Chemical Physics

Coherent control of collision processes: Penning versus associative ionization

Carlos A. Arango, Moshe Shapiro, Paul W. Brumer

Published Sep 6, 2006• 18 citations
Publisher

Abstract

Coherent control theory is applied to the control of Ne*(3s,(3)P2)+Ar((1)S0) collisions and computations are shown that display extensive control over these processes. Indeed we demonstrate that it is possible to essentially turn on and off the cross sections for both the Penning and associative ionization processes. This facility arises from the interference between matter waves induced by creating a linear superposition of the degenerate M={-2,-1,0,1,2} Zeeman sublevels of the Ne*(3s,(3)P2) target atom. The computations, conducted at collision energies in the 1-8 kcal/mole range, are based on combining, within the "rotating atom approximation," empirically derived and ab initio ionization widths.

DOI
10.1063/1.2336430

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Coherent control theory is applied to the control of Ne*(3s,(3)P2)+Ar((1)S0) collisions and computations are shown that display extensive control over these processes. Indeed we demonstrate that it is possible to essentially turn on and off the cross sections for both the Penning and associative ionization processes. This facility arises from the interference between matter waves induced by creating a linear superposition of the degenerate M={-2,-1,0,1,2} Zeeman sublevels of the Ne*(3s,(3)P2) target atom. The computations, conducted at collision energies in the 1-8 kcal/mole range, are based on combining, within the "rotating atom approximation," empirically derived and ab initio ionization widths.
Coherent control techniques are computationally applied to cold (1  mK<T<1  K1\text{ }\text{ }\mathrm{mK}<T<1\text{ }\text{ }\mathrm{K}1  mK<T<1  K) and ultracold ($T<1\text{ }\text{ }\ensuremath{\mu}\mathrm{K}$) Ne∗(3s,3P2)+Ar(1S0){\mathrm{Ne}}^{*}(3s,^{3}P_{2})+\mathrm{Ar}(^{1}S_{0})Ne∗(3s,3P2​)+Ar(1S0​) collisions. We show that by using various initial superpositions of the Ne∗(3s,3P2){\mathrm{Ne}}^{*}(3s,^{3}P_{2})Ne∗(3s,3P2​) $M={\ensuremath{-}2,\ensuremath{-}1,0,1,2}$ Zeeman sublevels it is possible to reduce the Penning ionization and associative ionization cross sections by as much as 4 orders of magnitude. It is also possible to drastically change the ratio of these two processes. The results are based on combining, within the ``rotating atom approximation'', empirical and ab initio ionization widths.
We explore coherent control of Penning and associative ionization in cold collisions of metastable He∗(23S){\mathrm{He}}^{*}(2{\phantom{\rule{4pt}{0ex}}}^{3}S)He∗(23S) atoms via the quantum interference between different states of the He2∗{\mathrm{He}}_{2}^{*}He2∗​ collision complex. By tuning the preparation coefficients of the initial atomic spin states, we can benefit from the quantum interference between molecular channels to maximize or minimize the cross sections for Penning and associative ionization. In particular, we find that we can enhance the ionization ratio by 30% in the cold regime. This work is significant for the coherent control of chemical reactions in the cold and ultracold regimes.
We show that quantum interference-based coherent control is a highly efficient tool for tuning ultracold molecular collision dynamics that is free from the limitations of commonly used methods that rely on external electromagnetic fields. By varying the relative populations and phases of initial coherent superpositions of degenerate molecular states, we demonstrate complete coherent control over integral scattering cross sections in the ultracold s-wave regime of both the initial and final collision channels. The proposed control methodology is applied to ultracold O_{2}+O_{2} collisions, showing extensive control over s-wave spin-exchange cross sections and product branching ratios over many orders of magnitude.
Control of the combined Penning and associative ionization cross section is demonstrated with cold (~1mK) metastable Ne (3s 3 P 2 ) atoms in a magneto-optical trap (MOT). By illuminating the trapped atoms with a near resonant probe laser beam, increased ionization rates are observed at several detunings. The probe beam is swept through a region from +500MHz to −500MHz. The increase in the Penning and associative ionization cross section is observed in both the red and blue regions of the spectrum.
A thermal-energy beam of helium atoms in the metastable state is used to coherently excite ions of Sr, Ca, and Ba in a Penning collision. The anisotropic excited state distributions appears as Polarized line emission from the excited ions. The degree of ion alignment may be used to determine the populations of the quasi-molecular states of the ion-atom pair. The coherent emission is also utilized to obtain Hanle signals from which the radiative decay rate of the excited ion is determined. This is the first case in which coherence is produced in a collision process involving Penning ionization. The alignment of the Penning ions is an important new parameter in the description of these reactions. In an effort to extend such observations to molecular systems a nitrogen molecular metastable beam source was demonstrated. The use of metal vapor targets and the excitation rate of the excited states of Cd and Zn in energy transfer collisions with the N2/sub (m)/ enable us to determine the vibrational distribution of the A /sup 3/SIGMA state of N/sub 2/ in the beam. Energy transfer cross sections and some Penning cross sections were measured in the beam system.