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Journal of Nuclear Science and Technology

Ionization Characteristics of Three-Level Atoms by Nonmonochromatic Lasers

Kazuyuki KATOH, Shuichi Hasegawa, Atsuyuki Suzuki

Published Jan 1, 1990• 4 citations
Publisher

Abstract

Ionization characteristics of two-step excitation by two intense nonmonochromatic (finite bandwidth) lasers are investigated. By approximating the laser fields by a stochastic model of a chaotic field, the stochastic atomic-density-matrix equations for three-level atoms are first obtained. Comparing with the analytical solution, the accuracy of our model is confirmed. By using this model, the ionization characteristics are studied in detail as a function of laser bandwidth. The dependence of laser bandwidth on the Doppler averaged ionization probability is also investigated.

DOI
10.1080/18811248.1990.9731205

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We derive a set of density matrix equations describing laser photo-excitation and ionization of atomic hydrogen in the presence of an external electric field. The equations are relevant as a description of multiphoton ionization of hydrogen (or deuterium) atoms in laser-produced transient plasmas. The laser wavelength is 243 nm, so that the photoionization is resonant with the 2 s 1/2 level. The electric field causes Stark mixing of the 2 s 1/2 level with the 2 p 1/2 and 2 p 3/2 levels. Because the electric field is taken in the same direction as the linear polarization of the laser, only states with equal m j are coupled, and the atom can be described with a four-level density matrix. The laser bandwidth is taken into account by using the stochastic model of a chaotic laser field introduced by Zoller [1]. We present a few calculations of the probabilities for ionization and excitation of a single hydrogen atom as a function of laser intensity and wavelength.
The 4-photon 3-step ionization scheme was analyzed by using Bloch equations. The effect of laser bandwidth was considered in accordance with the decorrelation method. For monochromatic lasers, the coherent population trapping was observed and the effects of detunings and Rabi frequencies on the ionization characteristics were studied. For nonmonochromatic lasers, the coherent population trapping was not observed and the enhancement of ionization at large detunings was observed. In order to evaluate the selectivity, the ionization probability of atoms which have detuning of 5 GHz for each excitation step was examined. Two small ionization peaks were observed for nonmonochromatic lasers while only peak was observed for monochromatic lasers. However sufficiently high selectivity can be expected over detunings.
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The atomic selective multi-step photoionization process is a critical step in laser isotope separation. In this work, we study three-step photoionization processes with non-monochromatic laser fields theoretically based on the semi-classical theory. Firstly, three bandwidth models, including the chaotic field model, de-correlation model, and phase diffusion model, are introduced into the density matrix equations. The numerical results are compared with each other comprehensively. The phase diffusion model is selected for further simulations in terms of the correspondence degree to physical practice. Subsequently, numerical calculations are carried out to identify the influences of systematic parameters, including laser parameters (Rabi frequency, bandwidth, relative time delay, frequency detuning) and atomic Doppler broadening, on photoionization processes. In order to determine the optimal match among different systematic parameters, the ionization yield of resonant isotope, and selectivity factor are adopted as evaluation indexes to guide the design and optimization process. The results in this work can provide a rewarding reference for laser isotope separation.