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Journal of Physics B Atomic Molecular and Optical Physics

State-selective stepwise two-photon detachment study of the ion

H. H. Andersen, Péter Balling, V. V. Petrunin, T. Andersen

Published Jun 14, 1996• 21 citations
Publisher

Abstract

The presence of two metastable states in the negative beryllium ion, and , which are connected by an optical transition, offers a unique possibility for gaining new information about this ion. Utilizing resonant, two-photon detachment, combined with resonant-ionization detection of the resulting neutral atoms, we have obtained results yielding the lifetimes of the ( ) and ( ) levels, the fine-structure splittings of the levels, and an improved transition wavelength.

DOI
10.1088/0953-4075/29/11/005

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•Dec 1, 2013•0 citations
The presence of two metastable states in the negative beryllium ion, and , which are connected by an optical transition, offers a unique possibility for gaining new information about this ion. Utilizing resonant, two-photon detachment, combined with resonant-ionization detection of the resulting neutral atoms, we have obtained results yielding the lifetimes of the ( ) and ( ) levels, the fine-structure splittings of the levels, and an improved transition wavelength.
Simple ionic systems were studied, such as metastable autoionizing states of the negative He ion, two-photon decay spectrum of metastable He ion, optical excitation with low energy ions, and lifetime measurements of singly ionized Li and metastable He ion. Simple atomic systems were also investigated. Metastable autoionizing atomic energy levels in alkali elements were included, along with lifetime measurements of Cr-53, group 2A isotopes, and alkali metal atoms using level crossing and optical double resonance spectroscopy.
There have been relatively few investigations of multiphoton ionization from metastable helium. Of particular interest has been the work of Haberland et al. 1987 and Haberland and Oschwald 1988. In both the 1987 and 1988 papers they have described the two photon ionization of metastable helium. In each of these studies they have reported the occurrence of unexplained structure along the wings of their resonance profiles. Upon the performance of similar measurements by this study, the unexplained structure is not seen and the agreement of the experiment's measurements with the theoretical shape of the resonance curves has been good. To experimentally verify these resonance effects, we have used a tunable dye laser in conjunction with a time of flight mass spectrometer to create and detect ions from metastable helium by two-photon absorption. The use of a metastable state instead of the ground state is advantageous because of its proximity to the ionization continuum and its extended lifetime. Using a metastable state as a starting point for multiphoton absorption requires fewer photons to reach the ionization threshold. The extended lifetime of the state also makes it easy to access experimentally. For helium the singlet metastable state 2$\sp1$S lies at 20.61 eV above the ground level with a natural lifetime of close to a millisecond. Two photons of 501.7 nm and 504.35 nm are required for the ionization processes in resonance with the 3$\sp1$P and the 3$\sp1$D states. This thesis is the accounting of the experimental process involved in the measurement of the dipole and quadrupole resonances of two photon ionization from singlet metastable helium. The study includes the description of the laser, electron gun assembly for metastable helium creation, and the time of flight mass spectrometer. A discussion of the theory of multiphoton processes is included along with the discussion of the data, its reduction and analysis, and a comparison with theoretical prediction. This study has not only involved the acquisition of definitive atomic spectra data sets, but has also established a facility for resonant ionization spectroscopy at The American University.
The binding energy and fine-structure splitting of the ground state of the ${\mathrm{Sr}}^{\mathrm{\ensuremath{-}}}$n(${5\mathrm{s}}^{2}5p5p5p{\mathrm{}}^{2}$P) ion have been measured by means of the laser photodetachment threshold technique combined with state-selective detection of neutral atoms using resonant ionization spectroscopy. We find binding energies of 32.17\ifmmode\pm\else\textpm\fi{}0.03 meV (J=3/2) and 52.06\ifmmode\pm\else\textpm\fi{}0.06 meV (J=1/2), yielding a fine-structure splitting of 19.89\ifmmode\pm\else\textpm\fi{}0.07 meV. This represents an improvement of two orders of magnitude compared to the recent data based on a combination of laser photodetachment and accelerator mass spectroscopy [D. Berkovits et al., Phys. Rev. Lett. 75, 414 (1995)].
The aim of this thesis is to extend the study of multiphoton processes to the heavier negative ions of the alkali-metal atoms and thus, Na"- and K"- were natural candidates for this investigation. The main idea is to achieve an homogeneous description of negative alkaline ions and their response to an intense laser field. This idea has been suggested by recent experimental findings for negative alkaline ions which have revealed a rich structure of their scattering and photodetachment spectra, showing the existence of high-lying temporary negative ion states. Moreover, dramatic effects due to excess photon detachment have been observed yielding additional information on the dynamics of these highly correlated systems
Photoionisation des Beryllium atoms: Ziel dieser Arbeit war, eine Quelle für die Erzeugung von Beryllium-Photoionen zu konfigurieren und zu bauen um mit ihnen die sympathetische Laserkühlung hochgeladener Ionen in einer kryogenischen Paulfalle zu ermöglichen.Als atomphysikalischer Mechanismus bietet sich die Zwei-Photonen-Anregung neutraler Beryllium Atome an, bei der der resonante Übergang 1s 2 2s 2 1 S 0 → 1s 2 2s2p 1 P 1 des Atoms bei 235 nm von einer nichtresonanten Transfer des Elektrons ins Kontinuum gefolgt wird.Der apparative Aufbau, die Erzeugung und Anregung des Atomstrahls sowie die Ionenproduktion und Nachweis werden beschrieben.Die experimentellen Ergebnisse betreffen die Untersuchung der Doppler Verbreiterung und der intensitätsabhängigen Sättigung des Übergangs sowohl für die Fluoreszenz-als auch für die Ionenspektren.Schließlich wurden die Wirkungsquerschnitte für den Zwei-Photon-Prozess abgeschätzt.Der Aufbau erfüllt die an ihn gestellten Erwartungen und wird in das Hauptexperiment CryPTEx implementiert. Photoionization of the beryllium atom:The task of this thesis was to configure and build a source of beryllium photoions in order to sympathetically cool highly charged ions in a cryogenic Paul trap with them.A suitable process for this purpose is the two-photon ionization of neutral beryllium atoms that is achieved by a resonant excitation of the 1s 2 2s 2 1 S 0 → 1s 2 2s2p 1 P 1 transition at 235 nm of the atom followed by the non-resonant transfer of an electron to the continuum.The instrumental setup, the generation and excitation of the atomic beam and the production and detection of ions are described.The experimental results concern studies of the Doppler broadening and saturation intensity on both fluorescence and ion spectra.Finally the cross sections were estimated for the two photon process.The setup fulfills the required specifications and is being implemented in the main experiment CryPTEx.V VII 4 Experimental results 4.1 Characterization of the atomic beam . . . . . . . . . . . . . . . . .4.1.1Doppler shift and broadening calculation . . . . . . . . . . .4.1.2Results . . . . . . . . . .