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Journal of Physics Communications

Effect of magnetic field on isotope-selective photoionization of atomic samarium using polarized lasers

A C Sahoo, P.K. Mandal, M.L. Shah, R C Das, Vas Dev

Published Oct 24, 2017• 7 citations
Publisher

Abstract

The effect of an external magnetic field on the isotopic selectivity in a two-color three-step polarization-based isotope-selective photoionization of atomic samarium has been studied. Isotope-selective photoionization of samarium is realized through a level scheme having total angular momentum ( J ) sequence 0-1-1-continuum by using two lasers linearly polarized in the same direction and the isotopic selectivity is measured by varying the magnetic field strength perpendicular to the laser polarization axis while keeping the delay between the laser pulses fixed and vice versa. The experimental data have been analyzed and compared with the results of the numerical calculations. The magnetic field perpendicular to the laser polarization axis present in the laser-atom interaction region has been found to degrade the isotopic selectivity. Further, the Lande factor ( ) of the excited energy level 16 690.76 cm −1 ( J = 1) of samarium has been determined from the observed oscillation in the isotopic selectivity. The value of thus obtained, is in good agreement with that reported in the literature.

DOI
10.1088/2399-6528/aa959f

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The effect of an external magnetic field on the isotopic selectivity in a two-color three-step polarization-based isotope-selective photoionization of atomic samarium has been studied. Isotope-selective photoionization of samarium is realized through a level scheme having total angular momentum ( J ) sequence 0-1-1-continuum by using two lasers linearly polarized in the same direction and the isotopic selectivity is measured by varying the magnetic field strength perpendicular to the laser polarization axis while keeping the delay between the laser pulses fixed and vice versa. The experimental data have been analyzed and compared with the results of the numerical calculations. The magnetic field perpendicular to the laser polarization axis present in the laser-atom interaction region has been found to degrade the isotopic selectivity. Further, the Lande factor ( ) of the excited energy level 16 690.76 cm −1 ( J = 1) of samarium has been determined from the observed oscillation in the isotopic selectivity. The value of thus obtained, is in good agreement with that reported in the literature.
Excitation dynamics of atoms by linearly polarized lasers were analyzed for J=2→2→1→0 multistep transition under a magnetic field. The time evolution of the population in each magnetic substate was calculated by solving the rate equations under the condition where the atomic alignment is gradually destroyed by the precession motion of the angular momentum around the magnetic field. Isotopic selectivity in the laser isotope separation based on the polarization selection rules was discussed. It has been found that a relatively small magnetic field may cause a degradation of selectivity in the case of Gd and the magnetic field should be kept as small as the terrestrial magnetic field when the laser polarization is perpendicular to the magnetic field.
Isotope selective photoionization of 67 Zn is demonstrated with broadband excitation light (∼3.4GHz), temporally separated laser pulses in the presence of a weak external magnetic field. Starting with natural zinc metal (∼4% 67 Zn), the ion sample was enriched to ∼90% 67 Zn. Efficient suppression of the ionization of the even-mass isotopes was achieved by intermediate state alignment in the excitation scheme 4 s 2 1 S 0 →4 s 4 p 3 P 1 →4 s 4 d 3 D 1 , followed by nonresonant photoionization. The first excitation pulse was linearly polarized parallel to the transverse component of the external magnetic field and the linear polarization of the second excitation pulse was rotated to compensate for Larmor precession of the quantization axis during the temporal delay between the two excitations. This is the first systematic study, in the context of resonance ionization, investigating the independent and continuous rotation of the polarizations of the excitation pulses and demonstrating efficient compensation for the presence of an external magnetic field that has an arbitrary direction. Experimental results are supported by a vector model combined with rate equations.
Isotope selective photoionization of ⁶⁷Zn is demonstrated with broadband excitation light (∼ 3.4 GHz), temporally separated laser pulses in the presence of a weak external magnetic field. Starting with natural zinc metal (∼ 4% ⁶⁷Zn) the ion sample was enriched to ∼ 90% ⁶⁷Zn. Efficient suppression of the ionization of the even-mass isotopes was achieved by intermediate state alignment in the excitation scheme 4𝑠2 ¹𝑆₀ → 4𝑠4𝑝 ³𝑃₁ → 4𝑠4𝑑 ³𝐷₁, followed by nonresonant photoionization. The first excitation pulse was linearly polarized parallel to the transverse component of the external magnetic field and the linear polarization of the second excitation pulse was rotated to compensate for Larmor precession of the quantization axis during the temporal delay between the two excitations. This is the first systematic study, in the context of resonance ionization, investigating the independent and continuous rotation of the polarizations of the excitation pulses and demonstrating efficient compensation for the presence of an external magnetic field that has an arbitrary direction. Experimental results are supported by a vector model combined with rate equations.
Isotope selective photoionization of ⁶⁷Zn is demonstrated with broadband excitation light (∼ 3.4 GHz), temporally separated laser pulses in the presence of a weak external magnetic field. Starting with natural zinc metal (∼ 4% ⁶⁷Zn) the ion sample was enriched to ∼ 90% ⁶⁷Zn. Efficient suppression of the ionization of the even-mass isotopes was achieved by intermediate state alignment in the excitation scheme 4𝑠2 ¹𝑆₀ → 4𝑠4𝑝 ³𝑃₁ → 4𝑠4𝑑 ³𝐷₁, followed by nonresonant photoionization. The first excitation pulse was linearly polarized parallel to the transverse component of the external magnetic field and the linear polarization of the second excitation pulse was rotated to compensate for Larmor precession of the quantization axis during the temporal delay between the two excitations. This is the first systematic study, in the context of resonance ionization, investigating the independent and continuous rotation of the polarizations of the excitation pulses and demonstrating efficient compensation for the presence of an external magnetic field that has an arbitrary direction. Experimental results are supported by a vector model combined with rate equations.
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.
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.