Field ionization of Rydberg atoms for high-brightness electron and ion beams

A. J. McCulloch, R. W. Speirs, J. Grimmel, B. M. Sparkes, D. Comparat, and R. E. Scholten

Phys. Rev. A 95, 063845 – Published 28 June 2017

Abstract

We present an ionization mechanism for use in a cold atom electron source with the goal of producing highly monochromatic electron beams. We experimentally produce a map of the Stark states of ${}^{85}{Rb}$ below the ionization threshold and identify states that undergo selective field ionization. The properties of an electron beam produced by field-assisted ionization of such states are quantified. A theoretical framework is established to predict the improvement to beam quality when ionization is conducted above the ionization threshold, where ionization conditions are typically more favorable than below the threshold. Calculations suggest that selective ionization of Rydberg states may offer a pathway to the production of high-brightness, highly monochromatic ion and electron beams.

Received 2 November 2016

DOI:https://doi.org/10.1103/PhysRevA.95.063845

Physics Subject Headings (PhySH)

Atomic & molecular processes in external fields Multiphoton or tunneling ionization & excitation Photoemission Stark effect

Electron sources Proton & ion sources Rydberg atoms & molecules

Accelerators & BeamsAtomic, Molecular & Optical

Authors & Affiliations

A. J. McCulloch¹, R. W. Speirs¹, J. Grimmel², B. M. Sparkes¹, D. Comparat³, and R. E. Scholten^1,*

¹School of Physics, The University of Melbourne, Victoria, 3010, Australia
²CQ Center for Collective Quantum Phenomena and their Applications, Physikalisches Institut, Eberhard-Karls-Universität Tübingen, Auf der Morgenstelle 14, D-72076 Tübingen, Germany
³Laboratoire Aimé Cotton, CNRS, Université Paris-Sud, ENS Cachan, Bât. 505, 91405 Orsay, France

^*scholten@unimelb.edu.au

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Issue

Vol. 95, Iss. 6 — June 2017

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Images

Figure 1
A schematic of the experiment to produce Stark maps of ${}^{85}{Rb}$ . (a) A neutral beam of rubidium propagates along $y$ , before entering a region of electric field produced between plates separated by $50 mm$ . Coupling to the Stark states is achieved with excitation and Rydberg laser beams, which are directed perpendicular to the field with polarizations parallel to the field. Ions are detected $680 mm$ downstream. Inset shows a plot of the electron potential versus position, showing the saddle-point energy $E_{s p}$ . (b) The energy-level diagram for ${}^{85}{Rb}$ .
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Figure 2
A map of the Stark states, presented as a waterfall diagram, produced near the ionization threshold at $F = 600 V / cm$ . The grayscale image shows the logarithm (base 10) of the rate of detected ionization events and the lines depict the calculated Stark states for both $m_{j} = 1 / 2$ [blue (dark gray)] and $m_{j} = 3 / 2$ [yellow (light gray)].
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Figure 3
Experimental Stark map in the region of $585 V / cm$ . The measured ion counts are shown in green (thick), and the calculated Stark states for $m_{j} = 1 / 2$ are shown in blue (dark gray) and for $m_{j} = 3 / 2$ are shown in yellow (light gray). State $| ψ_{i} 〉$ undergoes monotonic growth in the ionization rate with respect to the electric field whereas, in contrast, state $| ψ_{i i} 〉$ experiences a dramatic growth near a predicted anticrossing at $F = 585 V / cm$ . The measured ionization rates for $| ψ_{i} 〉$ and $| ψ_{i i} 〉$ are shown in yellow (light gray) with the blue line (dark gray, dashed) showing the interpolation of the linewidth as a function of field strength. As no peak is present near the anticrossing, the linewidth of $| ψ_{i i} 〉$ near $F = 586 V / cm$ is not well known; a value of $η = 10$ (see text) was used to estimate the linewidth.
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Figure 4
Ionization properties of a selectively ionizing Rydberg state in rubidium. (a) Predicted cumulative ionization probability of $| ψ_{i i} 〉$ as a function of electric-field strength around $F = 585 V / cm$ . The blue shaded region from $σ_{-} = 15.9 %$ to $σ_{+} = 84.1 %$ defines the region over which the beam ionizes. The color scale indicates the growth of ionization rate at the crossing, with $η = 1$ corresponding to $37 MHz$ . (b) Predicted ionization width as a function of the maximum ionization rate of $| ψ_{i i} 〉$ at the crossing. The parameters for excitation and ionization are given in the text.
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Figure 5
Cumulative ionization probability of the $| 17, 0, 13, 3 〉$ state for a rubidium beam (as detailed in Sec. 5) as a function of electric-field strength. The blue shaded region from $σ_{-} = 15.9 %$ to $σ_{+} = 84.1 %$ defines the region over which the beam ionizes. The inset shows a close up of the transition region, with the green area marking the range of the electric field over which the ionization occurs.
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