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Journal of Physics B Atomic Molecular and Optical Physics

Isotopic selectivity calculations for multi-step photoionization of calcium atoms using narrow-band lasers

A. K. Pulhani, Gaurav Gupta, B. M. Suri

Published Aug 21, 2002• 3 citations
Publisher

Abstract

Isotopic selectivity calculations are carried out for minor calcium isotopes against the major isotope 40 Ca for the single-resonance two-step and double-resonance three-step photoionization schemes with narrow-band lasers by using spectral simulation (SS) and modified spectrum (MS) approaches. The results of these calculations are compared with the density matrix (DM) results reported in the literature. It is noted that the values of isotopic selectivity from the SS approach do not agree with those from the DM approach whereas the MS approach, considering hole burning in the Doppler-broadened atomic spectrum, predicts selectivity values which are in good agreement with the DM results. It is argued that one can adequately use the simple MS approach rather than the complex DM approach for the calculation of isotopic selectivity of multi-step photoionization with single-frequency lasers.

DOI
10.1088/0953-4075/35/17/305

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Isotopic selectivity calculations are carried out for minor calcium isotopes against the major isotope 40 Ca for the single-resonance two-step and double-resonance three-step photoionization schemes with narrow-band lasers by using spectral simulation (SS) and modified spectrum (MS) approaches. The results of these calculations are compared with the density matrix (DM) results reported in the literature. It is noted that the values of isotopic selectivity from the SS approach do not agree with those from the DM approach whereas the MS approach, considering hole burning in the Doppler-broadened atomic spectrum, predicts selectivity values which are in good agreement with the DM results. It is argued that one can adequately use the simple MS approach rather than the complex DM approach for the calculation of isotopic selectivity of multi-step photoionization with single-frequency lasers.
Optical selectivities of calcium isotopes have been calculated for the 4s 2 1 S 0 4s4p 1 P 1 4s4d 1 D 2 →Ca + double-resonance ionization scheme using the spectral simulation (SS) approach. Optical selectivities obtained by the SS approach are in good agreement with the results obtained by the density matrix formalism. Studies on the effect of laser detuning and linewidth on optical selectivities indicate that lasers with 50 MHz linewidth for both the excitation steps can yield an optical selectivity of 2.2 × 10 5 , which is adequate for selective excitation of the 41 Ca isotope for certain applications.
Optical selectivities have been calculated by use of the density matrix approach for ns2 1S0−nsnp 1P10− nsms (or np2) 1S0 double-resonance photoionization pathways to establish the possibility of selective ionization of rare calcium and strontium isotopes for continuous-wave laser excitation. Numerical integration of the density matrix equations for double-resonance ionization has been carried out by incorporation of the effects of Doppler broadening, velocity-dependent interaction times, time-varying Rabi frequencies, and laser bandwidths. The conditions for obtaining optimum selectivities have been evaluated. This study results in five new photoionization pathways (two for calcium and three for strontium) whose optical selectivities were found to be a few orders higher than the previously studied photoionization schemes. The effect of laser linewidth of the excitation lasers and Doppler width have also been investigated.
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.
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.
Numerical computations of ionization efficiency and isotope selectivity of 176 L u have been carried out for the following three-step ladder type photoionization scheme, 5 d 6 s 2 2 D 3 / 2 ( 0.0 c m − 1 ) − − − ⟶ 540.4068 n m ⁡ 5 d 6 s 6 p 4 F 5 / 2 o ( 18504.58 c m − 1 ) − − − ⟶ 535.0626 n m 5 d 6 s 7 s 4 D 3 / 2 ( 37193.98 c m − 1 ) − − − ⟶ 618.0061 n m 53375 c m − 1 A u t o i o n i z a t i o n S t a t e → L u + , by invoking the density matrix formalism for the laser–atom interactions. Equations of motion have been derived for the odd isotopes by inclusion of hyperfine levels. The line shapes and their effect on the ionization efficiency and isotope selectivity have been discussed in detail. The effects of power of excitation and ionization lasers, spectral bandwidths, pulse delays, Doppler broadening of the atomic ensemble on the ionization efficiency, and degree of enrichment have been presented. Optimum conditions for the power of excitation, detunings, and other system parameters for the selective yet efficient photoionization of 176 L u have been identified.
Seven new schemes have been theoretically studied using the density-matrix approach for the ns2(1S0)→nsnp(3P01 or 1P01)→nsms(1S0) and the ns2(1S0)→nsnp(3P01 or 1P01)→np2(1S0 or 1D2) double optical resonance photoionization pathways to establish the possibility of selective ionization of the rare strontium 90Sr isotope for continuous wave laser excitation. The double-resonance excitation is followed by photoionization using a 488 nm photon. Density-matrix equations for the double-resonance ionization have been numerically integrated by incorporating the effects of Doppler broadening, velocity-dependent interaction times, time varying Rabi frequencies, and laser bandwidths. All the optical selectivity calculations were carried out on a high-speed parallel processing machine. Theoretical computations on five previously studied photoionization pathways have also been performed and tabulated for the sake of completeness. The conditions for obtaining optimum optical selectivities have been evaluated. This study results in seven new photoionization pathways. The optical selectivities are a few orders higher compared to the previously studied photoionization schemes for five out of the seven schemes studied. The remaining two schemes are slightly better than the previously studied schemes.