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The Journal of Chemical Physics

Anisotropy quantum beat in two-photon ionization of coherently excited hyperfine states of Na

Young Jong Lee, Sung Haeng Cho, Nam Joon Kim, Seong Keun Kim

Published Jul 8, 2001• 3 citations
Publisher

Abstract

A new method is proposed that utilizes the quantum beat of polarization anisotropy in two-photon ionization of atoms and molecules. It was shown that the frequency and the amplitude of the beating anisotropy contain information on, respectively, the hyperfine constants of the electronically excited state and the branching ratio of the photoionization channels leading to different photoelectron angular momentum states in the ionization continuum. The method was demonstrated for the case of the Na 3p P3/22 state by the time-resolved two-photon ionization with two pulses of parallel and perpendicular polarizations.

DOI
10.1063/1.1379582

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A new method is proposed that utilizes the quantum beat of polarization anisotropy in two-photon ionization of atoms and molecules. It was shown that the frequency and the amplitude of the beating anisotropy contain information on, respectively, the hyperfine constants of the electronically excited state and the branching ratio of the photoionization channels leading to different photoelectron angular momentum states in the ionization continuum. The method was demonstrated for the case of the Na 3p P3/22 state by the time-resolved two-photon ionization with two pulses of parallel and perpendicular polarizations.
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We report in detail our complete characterization of atomic photoionization via a single measurement, namely, that of the two-photon photoelectron angular distribution using elliptically polarized light. Through these measurements we are able to determine the atomic parameters necessary to unambiguously describe two-photon ionization of atomic rubidium. These parameters include relative cross sections for photoionization into various outgoing channels as well as the phase difference between the continuum S and D partial waves. We present a derivation of the angular distribution with arbitrarily polarized light in terms of these cross sections and phases, demonstrate the advantages of using elliptically polarized light, discuss details of the experiment, and report the results of our measurements in this paper. While measured phase differences are in excellent agreement with expected values over the entire range of photoelectron kinetic energy of our measurements from 0 to 0.5 eV, ratios of cross sections for photoionization into the two D channels are anomalous, suggesting unexpected fine structure effects in these continua.
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.
We have studied the effect of reactant Na(3p) polarization on the rotational angular momentum alignment of product Na2{\mathrm{Na}}_{2}Na2​+{\mathrm{}}^{+}+ ions arising from associative-ionization (AI) collisions. Our results show that sensitivity of the AI rate constant to initial atomic polarization persists even when all hyperfine states are populated with broadband (3 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) pulsed laser excitation of Na(2{(\mathrm{}}^{2}(2P3/2{P}_{3/2}P3/2​) and that the spatial distribution of product rotational angular momentum vectors is anisotropic. We discuss a qualitative description of the collision process consistent with our measurements which indicates that \ensuremath{\sigma}-orbital symmetry is preferred to \ensuremath{\pi}-orbital symmetry as the colliding partners approach.