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Soviet Journal of Quantum Electronics

Analysis of the efficiency of isotopically selective laser excitation of potassium atoms

A. G. Bakhtadze, Leonid A Bol'shov, V. M. Vetsko, Andrei N Starostin

Published Nov 30, 1981• 1 citation
Publisher

Abstract

A numerical study is made of the interaction of laser radiation with different isotopic forms of potassium, 39K and 40K, for time intervals shorter than the relaxation times. The quantum efficiency and selectivity of the process are determined. The behavior of the atoms is considered using the Schrodinger equations for the probability amplitudes. Account is taken of the effects associated with hyperfine splitting of the levels, their degeneracy, and the Doppler effect. Excitation by a single traveling wave and by two oppositely traveling waves is considered and the excitation characteristics of the two regimes are compared.

DOI
10.1070/qe1981v011n11abeh008643

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Two-photon amplification and lasing in laser-driven potassium atoms is investigated theoretically using a rigorous model that takes into account the hyperfine level structure of the atoms for arbitrary intensity drive and lasing fields. The model correctly predicts most features of the recent experiments, such as the characteristics of the multiphoton gain resonances and the two-photon laser emission intensity and line shape. In addition, it constitutes a new tool for exploring behaviors that are difficult to address experimentally, such as interference effects between different quantum pathways. In contrast to single-photon lasers, the laser emission line shape is frequency pushed and has the form of a closed curve composed of a stable and an unstable branch.
Atomic selective ionization processes provide critical ionization probability and isotopic selectivity in laser isotope separation. Aimed at the general principles of transition cross sections and ionization probabilities under various atomic hyperfine structures and laser parameters, and the optimization of atomic multi-channel photo-excitation and ionization processes of non-zero nuclear spin isotopes, the semi-classical density matrix theory of an imaginary two-step photoionization process is established with the consideration of magnetic sublevels of near-resonantly excited hyperfine levels. For determining the dimensionless transition cross sections of hyperfine transition channels, two methods are proposed in this article, i.e. the saturated excitation line-shape comparison method and weak excitation signal strength method, and a good agreement is achieved between numerical dimensionless transition cross sections and experimental results for several non-zero nuclear spin isotopes. The dependence of saturated ionization probability on laser polarization is studied systematically, and the hyperfine transition of Δ F = +1 is found to be superior to Δ F = −1, 0 in terms of saturated ionization probability. A general formula of oversaturated ionization probability is summarized for different hyperfine transitions. An interesting phenomenon called “magnetic sublevel coherence trapping” is employed to explain the inadequate ionization probability of the Δ F = −1 hyperfine transition even under the oversaturated excitation condition, and its mechanism is discussed through the temporal evolution of the density matrix elements. These numerical studies can provide beneficial references for laser isotope separation of isotopes with non-zero nuclear spin, as well as atomic laser spectroscopy.