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Physical Review A

Selective resonant ionization of zirconium isotopes using intermediate-state alignment

L. W. Green, G.A. McRae, Paul A. Rochefort

Published Jun 1, 1993• 26 citations
Publisher

Abstract

High selectivity (>10) for separation of $^{91}\mathrm{Zr}$ from other Zr isotopes was observed during triple resonant ionization of Zr vapors. Linearly polarized lasers were used to prepare aligned states, from which further excitation to a presently discovered J=0 level could be suppressed for even-mass isotopes, by a suitable choice of relative laser polarization. Photoionization of the odd-mass isotope $^{91}\mathrm{Zr}$ could be retained by use of transitions with relatively strong hyperfine interactions, because of associated population redistributions in the magnetic sublevels. The dependence of the even-mass-isotope signals on relative laser polarization followed sinusoidal forms, which are in excellent agreement with predictions derived using only geometric components of the transition dipole moment. The isotopic selectivity for $^{91}\mathrm{Zr}$ can be increased by use of saturating fluences, because of the effects of saturation, population trapping in the even-mass isotopes and strong hyperfine interactions in the odd-mass isotopes. For the same reasons, biases in even:odd isotope ratio measurements cannot always be eliminated by polarization scrambling or magic angles.

DOI
10.1103/physreva.47.4946

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High selectivity (>10) for separation of 91Zr^{91}\mathrm{Zr}91Zr from other Zr isotopes was observed during triple resonant ionization of Zr vapors. Linearly polarized lasers were used to prepare aligned states, from which further excitation to a presently discovered J=0 level could be suppressed for even-mass isotopes, by a suitable choice of relative laser polarization. Photoionization of the odd-mass isotope 91Zr^{91}\mathrm{Zr}91Zr could be retained by use of transitions with relatively strong hyperfine interactions, because of associated population redistributions in the magnetic sublevels. The dependence of the even-mass-isotope signals on relative laser polarization followed sinusoidal forms, which are in excellent agreement with predictions derived using only geometric components of the transition dipole moment. The isotopic selectivity for 91Zr^{91}\mathrm{Zr}91Zr can be increased by use of saturating fluences, because of the effects of saturation, population trapping in the even-mass isotopes and strong hyperfine interactions in the odd-mass isotopes. For the same reasons, biases in even:odd isotope ratio measurements cannot always be eliminated by polarization scrambling or magic angles.
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.
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.
We have examined the stepwise-resonant three-photon-ionization spectrum of neutral Zirconium atoms using three separately-tunable pulsed visible dye lasers. The ground-level (first-step) transitions were chosen on the basis of demonstrated 91Zr selectivity. Lifetimes of even-parity levels around 36000 cm-1, measured with the delayed- photoionization technique, range from 10 to 100 nsec. Direct ionization cross sections appear to be less than 10-17 cm2; newly-detected autoionizing levels give peak ionization cross sections (inferred from saturation fluences) up to 10-15 cm2. Portions of Rydberg series converging to the 315 and 763 cm-1 levels of Zr+ were identified. 'Clumps' of autoionizing levels are thought to be due to Rydberg-valence mixing.
Atomic selective ionization processes provide critical ionization probability and isotopic selectivity in laser isotope separation. Aimed at the general principles of transition cross sections and ionization probabilities under various atomic hyperfine structures and laser parameters, and the optimization of atomic multi-channel photo-excitation and ionization processes of non-zero nuclear spin isotopes, the semi-classical density matrix theory of an imaginary two-step photoionization process is established with the consideration of magnetic sublevels of near-resonantly excited hyperfine levels. For determining the dimensionless transition cross sections of hyperfine transition channels, two methods are proposed in this article, i.e. the saturated excitation line-shape comparison method and weak excitation signal strength method, and a good agreement is achieved between numerical dimensionless transition cross sections and experimental results for several non-zero nuclear spin isotopes. The dependence of saturated ionization probability on laser polarization is studied systematically, and the hyperfine transition of Δ F = +1 is found to be superior to Δ F = −1, 0 in terms of saturated ionization probability. A general formula of oversaturated ionization probability is summarized for different hyperfine transitions. An interesting phenomenon called “magnetic sublevel coherence trapping” is employed to explain the inadequate ionization probability of the Δ F = −1 hyperfine transition even under the oversaturated excitation condition, and its mechanism is discussed through the temporal evolution of the density matrix elements. These numerical studies can provide beneficial references for laser isotope separation of isotopes with non-zero nuclear spin, as well as atomic laser spectroscopy.
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.