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Soviet Journal of Quantum Electronics

Optimal scheme for multistage photoionization of lutetium atoms by laser radiation

G. I. Bekov, E. P. Vidolova-Angelova

Published Jan 31, 1981• 15 citations
Publisher

Abstract

An experimental investigation was made of the even Rydberg and autoionizing states of lutetium atoms. These atoms were excited and ionized in two stages by radiation from tunable dye lasers. A study was made of the dependence of the ion current on the laser wavelength in the second stage. The results were used to propose an effective ionization scheme involving an autoionizing state with an excitation cross section of 1.5 × 10 −15 cm 2 .

DOI
10.1070/qe1981v011n01abeh005395

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An experimental investigation was made of the even Rydberg and autoionizing states of lutetium atoms. These atoms were excited and ionized in two stages by radiation from tunable dye lasers. A study was made of the dependence of the ion current on the laser wavelength in the second stage. The results were used to propose an effective ionization scheme involving an autoionizing state with an excitation cross section of 1.5 × 10 −15 cm 2 .
Laser-induced autoionization, a form of forced autoionization, is discussed. Highly excited atomic Rydberg states generally have a small cross section for photoionization due to their small overlap with the nucleus. The cross section for photoionization can be strongly enhanced if the Rydberg series is perturbed by an atomic state with a relatively large cross section for photoionization. The photoionization dynamics of such a perturbed Rydberg system shows strong similarities to that of an autoionizing Rydberg series. For intense laser pulses, which strongly couple the perturber to the continuum, the photoionization rate is limited by the coupling between the Rydberg states and the perturber or, in other words, the autoionization rate. The model describing this ionization dynamics is applied to the barium 6snd(1,3{(}^{1,3}(1,3D2{\mathit{D}}_{2}D2​) state which is perturbed by the 5d7d(1{(}^{1}(1D2{\mathit{D}}_{2}D2​) state. The calculations show good agreement with experimental results of photoionization of barium Rydberg states with subpicosecond laser pulses of light of 620 nm in wavelength.
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.
An experimental investigation was made of the influence of a laser radiation field on the spectrum of barium atoms. The investigation was carried out by the method of three-photon ionization spectroscopy using dye laser radiation (ω = 14 800–18 700 cm − 1 ). The electric field intensity of the laser radiation was 10 3 –10 6 V/cm. This laser radiation field had a strong influence on a number of bound and autoionizing states. The nature of this influence depended on the ratio of the excitation frequencies of bound and autoionizing states.
Analytical solutions of the Schr?dinger equations for coherent excitation of an atom in the scheme of two- and three-stage photoionization are obtained as applied to the problem of laser isotope separation. A feature of the study is the introduction of laser excitation from an additional initial state to increase the fraction of the atoms involved in the separation process. The optimal from the points of view of photoion ratios of radiation intensities at individual photoionization stages are found. Key words: laser photoionization, laser isotope separation.
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.