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Journal of Physics B Atomic Molecular and Optical Physics

Selective photoionization of168Yb in a three-level atomic medium by the collinear propagation of laser pulses

A. S. Choe, Yongjoo Rhee, Jongmin Lee, Marina A. Kuzmina, В. А. Мишин

Published Sep 14, 1995• 15 citations
Publisher

Abstract

The medium of the three-level Yb atom corresponding to the three-colour, three-step, three-photon scheme of 6 1 S 0 (0 cm -1 ) to 6 3 P 1 (17992008 cm -1 ) to (7/2, 3/2) 2 4f 13 6s 2 6p (35196.98 cm -1 ) to autoionization state (52353 cm -1 ) is chosen to examine the selective photoionization of 168 Yb. The Maxwell-Bloch equation is numerically solved, where the interacting laser pulses are supposed to be collinear and resonant with the transition lines of 168 Yb, and the effects of Doppler broadening and laser bandwidth are incorporated. Firstly, the thin medium dynamics are examined to determine the optimal energy of laser pulses for the selective photoionization of 168 Yb, and to determine Yb isotopes which have a large influence on the excitation of 168 Yb. Secondly, the thick medium dynamics are investigated, resulting in the fact that the laser pulse for the first transition is prominently deformed due to the near-resonant interaction with 171 Yb and 173 Yb, and the ion yield along the propagation of the laser beam is strongly dependent on the initial energy of pulse for the first transition and the time-delay between input pulses.

DOI
10.1088/0953-4075/28/17/018

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The medium of the three-level Yb atom corresponding to the three-colour, three-step, three-photon scheme of 6 1 S 0 (0 cm -1 ) to 6 3 P 1 (17992008 cm -1 ) to (7/2, 3/2) 2 4f 13 6s 2 6p (35196.98 cm -1 ) to autoionization state (52353 cm -1 ) is chosen to examine the selective photoionization of 168 Yb. The Maxwell-Bloch equation is numerically solved, where the interacting laser pulses are supposed to be collinear and resonant with the transition lines of 168 Yb, and the effects of Doppler broadening and laser bandwidth are incorporated. Firstly, the thin medium dynamics are examined to determine the optimal energy of laser pulses for the selective photoionization of 168 Yb, and to determine Yb isotopes which have a large influence on the excitation of 168 Yb. Secondly, the thick medium dynamics are investigated, resulting in the fact that the laser pulse for the first transition is prominently deformed due to the near-resonant interaction with 171 Yb and 173 Yb, and the ion yield along the propagation of the laser beam is strongly dependent on the initial energy of pulse for the first transition and the time-delay between input pulses.
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 pulse deformation and time delay, which appear when the laser propagates in a thick atom vapor, influence the ionization yield and selectivity of atomic multi-step photoionization process directly. In this paper, we study the propagation of laser pulse and atomic photoionization in a thick atom vapor medium according to the atom vapor laser isotope separation. The process of atomic multi-step photoionization and the propagation of laser in a thick medium are described by density matrix equation and Maxwell equations, respectively. The medium consists of target isotope and non-target isotope, which is non-resonantly excited. Through numerical solution of the coupled equations we illustrate the propagation characteristics of laser and the influences of atom vapor parameters and laser parameters on average ionization yield and average selectivity in a thick medium. The important results of calculation are as follows:when the atom vapor medium is rather thick, the average ionization yield increases while average selectivity decreases with the increase of laser power. When the atom vapor is relatively thin, the average ionization yield and average selectivity increase with the decrease of laser power simultaneously. Besides, there is a positive time delay between two laser pulses in which case the average ionization yield of target isotope reaches its maximum value. Moreover, when the parameters of atom vapor are constant, extending the width of laser pulses as great as possible can not only increase average ionization yield and average selectivity simultaneously, but also loosen the control accuracy of time delay between laser pulses.
A principle for the efficient excitation of a three-level medium in a selective photoionization method is examined. It is shown that with special detunings for a given set of the laser-atom interaction the third-level population can be maximized. The expression of the optimal detunings is derived near the two-photon resonance region. It is shown that if the optimal detuning method is applied to three-level systems of Yb and Ca, the first-level populations are inverted to their corresponding third levels by about 100% and 86%, respectively. Application of this method to a Doppler-broadened medium is discussed and it is found that the optimal detuning method with counterpropagating waves will be very effective for selective ionization of wanted atoms.
Selective photoionization of ytterbium isotope is studied numerically based on a three-step photoionization scheme, 4f146s2 1S0 (0 cm−1) → 4f146s6p 3P1 (17992.008 cm−1) → (4f136s26p) (7/2, 3/2)2 (35196.98 cm−1) → auto-ionization state (52353 cm−1) → Yb+, by the density matrix theory with the consideration of atomic hyperfine structures and magnetic sublevels. To examine the physical model, the numerical isotopic abundance of ytterbium is compared with that from mass spectroscopy experiment, showing that they are in good agreement with each other. The excitation process and ionization process of ytterbium, especially for odd isotopes, are discussed and analyzed in detail on this basis. The effects of frequency detuning, power densities, spectral bandwidths, polarization of two excitation lasers, and atomic Doppler broadening on the total ionization yield and isotopic abundance are investigated numerically and the optimal excitation conditions for 176Yb enrichment are identified semi-quantitatively.