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The European Physical Journal D

Microwave ionization of Rydberg states of the barium ion

M. Seng, U. Eichmann, Volker Lange, Thomas F. Gallagher, Wolfgang Sandner

Published Jan 1, 1998• 3 citations
Publisher

Abstract

\n \nWe have investigated the electric field ionization of highly excited (ns)\nand (nd) Rydberg states of the barium ion by microwave fields. For states\nwith a principal quantum number $n\\leq 40$ the observed microwave ionization\nthreshold fields show the $Z^3/3n^5$ dependence expected on the basis of\npreviously observed ionization of neutral Rydberg states with Z=1. We show\nthat for states in this n regime microwave ionization is a state selective\ndetection technique. Threshold fields for states with n> 40, however,\ndepart from the expected behaviour, the thresholds become broader and shift\nto higher fields. For a total of 58 states we present the field strengths\nwhere 90% , 50% and 10% ionization occur.\n\n\n

DOI
10.1007/s100530050145

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We have measured the microwave ionization threshold electric fields for ns-state Na Rydberg atoms from n=24--33 as a function of the duration of the ionizing microwave pulse. Pulses as short as 2.8 ns were used, containing fewer than 25 cycles of 8.55-GHz radiation. While the fields required to produce the onset of ionization were independent of pulse duration and exhibited the ${\mathit{n}}^{\mathrm{\ensuremath{-}}5}$ scaling characteristic of ionization by diffusion through higher-energy states, the fields required to produce nearly complete ionization increased dramatically as the pulse was shortened. Additional measurements of the populations of bound states after the shortest microwave pulses showed that up to \ensuremath{\sim}10% of the atoms were excited to states above the initially populated ns state and the adjacent (n-1) manifold of higher angular momentum states, while most either ionized or remained in these states. From this observation we conclude that the rate-limiting process for ionization is the transition from the initially populated ns states or the adjacent manifold to the next higher manifold, not the diffusion through higher-lying states.
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