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Journal of Nuclear Science and Technology

Coherent Population Trapping in Four-Photon Three-Step Ionization Scheme

Kazuyuki KATOH, Atsuyuki Suzuki

Published Jan 1, 1990• 5 citations
Publisher

Abstract

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.

DOI
10.1080/18811248.1990.9731220

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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.
The four-color three-step selective photoionization process of atom is very important in laser isotope separation technology. The population trapping phenomena and their influences are studied theoretically in monochromatic and non-monochromatic laser fields based on the density matrix theory in this work. Time evolutions of the photoionization properties of the four-color, three-step process are given. The population trapping effects occur intensely in monochromatic excitation, while it gradually turns weak as the laser bandwidth increases. The effects of bandwidth, Rabi frequency, time delay, and frequency detuning on the population trapping effect are investigated in monochromatic and non-monochromatic laser fields. The effects of laser process parameters and atomic parameters on the effective selective photoionization are also discussed. The ionization probability and selectivity factors, as evaluation indexes, are difficult to improve synchronously by adjusting systematic parameters. Besides, the existence of metastable state may play a negative role when its population is low enough.
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.
The population inversion in a three-level atomic system, interacting with two coherent laser pulses, occurs from the first level to the third level at the optimal detunings of the laser frequencies near the two-photon resonance. An examination is performed to see whether the energy density and time-delay of the photoionizing laser can significantly influence the ion yield for the case where the atom interacts with two coherent pulses and one photoionizing pulse incoherently coupled. The well-known three-colour, three-step, three-photon ionization scheme, which is available for the selective photoionization of 168Yb, is chosen for use in this research with the Bloch equation. It is revealed that the proper time-delay and energy density enable us to achieve higher efficiency for the selective photoionization of 168Yb, regardless of the Doppler-broadening, though the efficiency and selectivity of coherent excitation are prominently enhanced by applying the optimal detunings.
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.
Optical selectivity calculations have been carried out for the 91Zr isotope for two-color resonant three-photon photoionization schemes. The density-matrix equations of motion were solved numerically for collinear, counterpropagating, linearly polarized laser beams having Gaussian temporal profile. Calculations were carried out incorporating the effects of Doppler broadening, magnetic sublevel degeneracy, and laser bandwidths. The selectivities and ion yields were determined for various Rabi frequencies and ionization rates. Thereby, the optimal Rabi frequencies for the first and second excitation steps and ionizaton rate for the ionizaton step were determined. The ionization efficiency as a function of detuning of the excitation lasers was also investigated. Two of the photoionization schemes investigated yielded a selectivity of ∼10, with an ion yield of ∼23%.