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Surface ionization of metastable calcium atoms

Igal Bucay, A. Helal, Mark G. Raizen

Published Jan 1, 2021• 3 citations
Publisher

Abstract

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.

DOI
10.1063/5.0005895

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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.
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.
A description is given of the method and the results are reported of an experiment in which quasi-cw lasing was obtained as a result of transitions from a resonance to a metastable level of the calcium atom without the use of any special impurities to quench the lower metastable state. Under these experimental conditions the metastable level was depopulated not as a result of collisions of the second kind or as a result of a chemical reaction, but because of the ionization, electron-impact excitation, and diffusion of metastable atoms to the discharge tube wall.
The ionization of calcium atoms resonantly excited to the 4s4p 3 P 1 metastable state is investigated. In particular, the influence of various experimental parameters on the total ionization yield and the rate of its growth are reported. Maximum ionization yield was obtained with the buffer gas (Ar) pressure in the range 100-650 mbar. Furthermore, the time history of depopulation of the metastable state 4s4p 3 P 1 monitored in a 'pump' and 'probe' experiment using a second dye laser tuned to the 4s4p 3 P 1 -4s5s 3 S 1 transition is presented. The effective lifetime of the 4s4p 3 P 1 state is shortened to 31+or-2 mu s at 850 degrees C and 800 mbar Ar pressure from the known value of about 350 mu s.
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