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Surface ionization of metastable calcium and ytterbium atoms

Igal Bucay

Published Jul 31, 2019• 0 citations
Publisher

Abstract

This dissertation presents an experiment demonstrating the surface ionization of metastable atoms on a tungsten surface. Surface ionization refers to the thermionic ionization of adsorbates with low ionization potentials on hot, high-work function metal surfaces. The efficiency of surface ionization is exponentially dependent on the difference between the atom ionization energy and the surface work function. Since the work functions of metals are, at most, ~ 6 eV, surface ionization is limited to elements with low ionization energies and is frequency exploited for detecting alkali and alkaline earth metal beams. We use an electric discharge to excite a fraction of atoms in separate calcium and ytterbium beams to metastable states. In doing so, we effectively lower their ionization energy by about 2 eV. On a polycrystalline tungsten surface, with a work function measured to be 5.18 eV, surface ionization is theoretically more likely for metastable calcium and ytterbium atoms than for their ground state counterparts. However, the elementary theory of surface ionization does not take into account various de-excitation or neutralization processes that may preclude the formation of ions. We report findings that metastable calcium and ytterbium atoms undergo surface ionization at significantly higher rates than corresponding ground state atoms.

DOI
10.26153/tsw/3106

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This dissertation presents an experiment demonstrating the surface ionization of metastable atoms on a tungsten surface. Surface ionization refers to the thermionic ionization of adsorbates with low ionization potentials on hot, high-work function metal surfaces. The efficiency of surface ionization is exponentially dependent on the difference between the atom ionization energy and the surface work function. Since the work functions of metals are, at most, ~ 6 eV, surface ionization is limited to elements with low ionization energies and is frequency exploited for detecting alkali and alkaline earth metal beams. We use an electric discharge to excite a fraction of atoms in separate calcium and ytterbium beams to metastable states. In doing so, we effectively lower their ionization energy by about 2 eV. On a polycrystalline tungsten surface, with a work function measured to be 5.18 eV, surface ionization is theoretically more likely for metastable calcium and ytterbium atoms than for their ground state counterparts. However, the elementary theory of surface ionization does not take into account various de-excitation or neutralization processes that may preclude the formation of ions. We report findings that metastable calcium and ytterbium atoms undergo surface ionization at significantly higher rates than corresponding ground state atoms.
We report on an experimental study of surface ionization of metastable calcium atoms on a hot polycrystalline tungsten surface in vacuum. We implemented a hollow-cathode discharge to excite a fraction of calcium atoms in an atomic beam to metastable states and collected the resulting calcium ions. We observed that metastable calcium atoms are ionized with a significantly greater efficiency than ground-state atoms, and the results suggest that virtually every metastable atom impacting the hot surface is ionized. These results demonstrate the potential of metastable atom surface ionization as a means of enriching calcium isotopes for applications in medicine, metrology, and fundamental science.
Discrete atomic energy levels lying between the first and second ionization potentials which are metastable against both auto-ionization and radiative decay have been observed in the alkali elements. These levels arise from the excitation of an electron from the outermost closed shell of the atom and have characteristic lifetimes in the microsecond region. A beam of metastable alkali atoms is produced by electron bombardment of neutral ground-state atoms and is detected by collecting the charged products of the decay. Stern-Gerlach magnetic-deflection experiments have been performed to show unambiguously that the signals arise from paramagnetic atoms and not from possible stray-photon effects in the apparatus. The excitation energies, electron-bombardment production cross sections, natural lifetimes, and the tentative spectroscopic assignments of these atoms are listed below: The energy and lifetime of the metastable lithium atom are in good agreement with the available theoretical estimates. The metastable states in the heavier alkali atoms are classified in relation to the known Ib{\mathrm{I}}^{b}Ib spectral terms. Magnetic and electric fields have been found to be effective in reducing the lifetime of the metastable atoms. The Zeeman quenching is discussed in a separate paper. The Stark quenching rate has been found to be proportional to the square of the electric field and to increase rapidly with atomic number ZZZ. It is suggested that the Stark quenching results from a second-order interaction which involves the product of the electric field and the spin-orbit or other magnetic operators.
This thesis reports the first controlled study of the interactions of Rydberg atoms with a metal surface. In these experiments, a collimated beam of potassium Rydberg atoms is directed at a plane surface at near grazing incidence. Positive ions formed by surface ionization are attracted to the surface by their image charge, which is counterbalanced by an external electric field applied perpendicular to the surface. The ions are detected by a position-sensitive detector (PSD). At some critical value of the external field, the ion trajectories just miss the surface, suggesting that analysis of the dependence of the ion signals on external electric field can be used to determine the distance from the surface at which ionization occurs. This distance, and thus the corresponding critical electric field, is expected to be n-dependent. Experimentally, however, it was observed that the ion signal had a sudden n-independent onset when only a small positive perpendicular electric field was applied at the surface. This observation requires, surprisingly, that the ions produced by surface ionization can readily escape from the surface. The data do, however, show that Rydberg atoms are efficiently ionized in collisions with the surface. This process may provide a useful new detection technique for Rydberg atoms.
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.
The key features of the cross sections for electron-impact excitation out of the ground levels of the rare-gas atoms are summarized and application of these cross section data is shown. This is followed by discussions of recent experiments on determination of electron-impact cross sections for excitation out of the metastable levels of He and Ar. The metastiable atom targets are produced by two different methods, i.e., hollow-cathode discharges and charge-exchange collisions of rare-gas ions with Cs atoms. The former method is simple but is limited to low electron energies because of the very small metastable concentration, whereas the latter method yields a fast target beam with predominantly metastable atoms allowing measurements up to the keV range. The measured cross sections show features very different from those of excitation out of the ground levels. Another new development in electron-atom collision is the use of atom traps for measuring ionization cross sections. The experiment is conducted by passing an electron beam pulse through magneto-optically trapped Rb atoms while the trap is momentarily turned off. Turning the trap back on immediately after the electron beam pulse recaptures the unionized atoms allowing only the ions to escape. The ionization cross section is determined bymore » measuring the electron beam current density at the trapped atom region and the fractional loss of trapped atoms due to the electron beam. This method avoids measurements of the absolute number of the target atoms and the overlap between the atomic and electron beams and therefore eliminates the major sources of uncertainty associated with the crossed-beam method for measuring ionization cross section.« less