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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Progress in zirconium resonance ionization spectroscopy

Ralph H. Page, S. C. Dropinski, Earl F. Worden, John A. D. Stockdale

Published May 28, 1993• 6 citations
Publisher

Abstract

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.

DOI
10.1117/12.145509

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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.
Two-step resonant photoionization of free Ni atoms is studied by one-color and two-color processes, using pulsed lasers with photon energies around half the ground-state ionization energy. Two-color two-step resonance ionization from the ground state a3{\mathrm{}}^{3}3F4{\mathit{F}}_{4}F4​ and the first metastable state a3{\mathrm{}}^{3}3D3{\mathit{D}}_{3}D3​ via the triplet states z3{\mathrm{}}^{3}3G3,4,50{\mathit{G}}_{3,4,5}^{0}G3,4,50​ was used to probe the autoionizing states just above the ionization limit. Several strong lines in the one-color ionization spectrum are explained as direct two-photon transitions to autoionizing states. Photoionization cross sections were deduced from saturation measurements. The large cross sections observed for specific one-color two-step transitions are due to the near double resonant excitation into autoionizing states. \textcopyright{} 1996 The American Physical Society.
Laser-induced photoionization and fluorescence signals were simultaneously observed in atomic uranium using a single Nd:YAG-pumped dye laser. These signals were recorded in two specific cases. In the first case, the dye laser was resonant to the first-step transition (0–16900.38 cm−1). In the second case, the laser was near-resonant to the first-step transition with a slight detuning (0.15 cm−1) so that it became two-photon resonant at 33801.06 cm−1. The uranium atoms in the ground state were ionized by a single-color, three-photon photoionization technique resulting in the photoionization signal, and the fluorescence signal was simultaneously obtained from the first excited state involved in the photoionization process. The photoionization and the fluorescence signals in the above-mentioned cases were also estimated theoretically for several values of the photoionization cross section for the transition between the second excited state at 33801.06 cm−1 and the autoionization state at 50701.59 cm−1 using density matrix formalism. From the comparison of theoretically calculated ratios of fluorescence signals in the two specific cases with the experimentally obtained values, the photoionization cross section for the 33801.06–50701.59 cm−1 transition has been obtained, which is found to be (5±1)×10−16 cm2.
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.
The two-photon excitation and three-photon ionization of atomic cesium is investigated over the 6550-6950-\AA{} wavelength region with a tunable-dye-laser source having a 0.06-0.08-\AA{} linewidth and a space-charge ionization detector sensitive to a few ions per second. An online data-acquisition computer provides significant signal-to-noise recovery. The resulting dispersion curve for photoionization is interpreted in terms of the two-photon transitions from the ground 62S126^{2}S_{\frac{1}{2}}62S21​​ level to resonant n2Dn^{2}Dn2D and n2Sn^{2}Sn2S intermediate states, and represents, to the authors' knowledge, the first such two-photon absorption spectrum of Cs. Transitions from n=9to13n=9\mathrm{to}13n=9to13 have been recorded for the $6^{2}S_{\frac{1}{2}}\ensuremath{-}n^{2}D_{\frac{5}{2},\frac{3}{2}}$ series and from n=11to14n=11\mathrm{to}14n=11to14 for the $6^{2}S_{\frac{1}{2}}\ensuremath{-}n^{2}S_{\frac{1}{2}}$ series.