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Physical Review Letters

Triple Ionization of Lithium by Electron Impact

M.-T. Huang, W. W. Wong, Mitio Inokuti, Stephen H. Southworth, Linda Young

Published Apr 23, 2003• 10 citations
Publisher

Abstract

Ejection of the three electrons from lithium in a single electron collision has been observed for the first time. Triply charged lithium was observed in an ion time-of-flight spectrum following electron impact on a sample of ultracold, trapped lithium. The higher signal/background afforded by the trap environment made the observation of Li3+ possible. We measured the ratios of triple-to-double and double-to-single ionization at an impact energy of 1000 eV. The 3+/2+ ratio is approximately 0.001, a value 2 orders of magnitude lower than semiempirical predictions. We present a simple method that uses photoionization data combined with sum-rule analysis to predict the asymptotic charge-state ratios. The sum-rule predictions compare reasonably with experiment and shake calculations, but disagree sharply with the semiempirical estimates.

DOI
10.1103/physrevlett.90.163201

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Ejection of the three electrons from lithium in a single electron collision has been observed for the first time. Triply charged lithium was observed in an ion time-of-flight spectrum following electron impact on a sample of ultracold, trapped lithium. The higher signal/background afforded by the trap environment made the observation of Li3+ possible. We measured the ratios of triple-to-double and double-to-single ionization at an impact energy of 1000 eV. The 3+/2+ ratio is approximately 0.001, a value 2 orders of magnitude lower than semiempirical predictions. We present a simple method that uses photoionization data combined with sum-rule analysis to predict the asymptotic charge-state ratios. The sum-rule predictions compare reasonably with experiment and shake calculations, but disagree sharply with the semiempirical estimates.
Ejection of the three electrons from lithium in a single electron collision has been observed for the first time. Triply charged lithium was observed in an ion time-of-flight spectrum following electron impact on a sample of ultracold, trapped lithium. The higher signal/background afforded by the trap environment made the observation of Li3+{\mathrm{L}\mathrm{i}}^{3+}Li3+ possible. We measured the ratios of triple-to-double and double-to-single ionization at an impact energy of 1000 eV. The 3+/2+3+/2+3+/2+ ratio is $\ensuremath{\approx}0.001$, a value 2 orders of magnitude lower than semiempirical predictions. We present a simple method that uses photoionization data combined with sum-rule analysis to predict the asymptotic charge-state ratios. The sum-rule predictions compare reasonably with experiment and shake calculations, but disagree sharply with the semiempirical estimates.
In the course of this work dynamics in ionization processes of lithium by photon and electron impact were studied. Due to its electronic structure, lithium is an ideal candidate to examine electronic correlation effects and collision dynamics of simple few-body systems. The lithium target is cooled in a magneto-optical trap (MOT) down to the sub-millikelvin regime, which in combination with a reaction microscope (ReMi) allows for the determination of the momentum information of all collision products with unprecedented resolution. An electron gun serving as a new projectile source was implemented in the MOTReMi experiment. From first measurements, which showed that the developed setup is principally operational, time-of-flight and position spectra were analyzed, determining the ratio of Li2+=Li+ ion creation. In a second series of experiments, photoionization of laser-excited lithium has been investigated and fully differential cross sections (FDCS) were examined. As the target and also the laser beam were polarized, orientational effects could be studied. In some cases the electron emission characteristics could directly be deduced from dipole selection rules.
Reaction microscopes enable kinematically complete measurements of atomic and molecular fragmentation. An ultracold atomic target is usually provided by a supersonic gas jet. The apparatus developed in the course of this thesis for the first time combines the principle of the reaction microscope with a magneto-optical trap. This allows for the preparation of lithium atoms in the sub-mK range. Being a three-electron system, its simple atomic structure makes lithium a model system of great topical interest for all kinds of ionization reactions. In order not to deteriorate the determination of the momenta of the charged fragments by the magnetic field of the trap, a pulsed mode of operation is adopted, creating field-free conditions during data acquisition and making efficient use of the stored target. The novel apparatus provides a versatile target for investigations on fragmentation of lithium by electron, ion and photon impact. For the first time, momentum-resolved measurements on multiphoton ionization in intense laser fields with pulse durations of 25~fs and peak intensities in the range between 1011^{11}11~W/cm2^22 and 1016^{16}16~W/cm2^22 were performed. The acquired photoelectron spectra exhibit unexpected structures such as a preferred emission in the direction perpendicular to the laser polarization axis, which are not fully understood yet. Presently, corresponding calculations are being done in several theory groups.
We have observed electron impact ionization of highly excited sodium Rydberg atoms in nsnsns and ndndnd states, n=35−−51n=35--51n=35−−51, below E=2  eVE=2\text{ }\text{ }\mathrm{e}\mathrm{V}E=2  eV electron kinetic energy with energy resolution 0.25 eV. Measured absolute cross sections near 0 eV range from ${\ensuremath{\sigma}}_{35d}\ensuremath{\approx}7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ to ${\ensuremath{\sigma}}_{50d}\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\text{ }\text{ }{\mathrm{c}\mathrm{m}}^{2}$. The energy dependence is consistent with that of widely used binary encounter approximation cross sections, and ${\ensuremath{\sigma}}_{n}$ follows a power law in nnn. The measured cross sections are 14 to 24 times larger than theoretically predicted values. This enhancement may signal the effect of large polarizabilities of high Rydberg states not yet accounted for in ionization theories.
Measurements of the multiple ionization cross section ratios of Cesium were performed with ion time-of-flight (TOF) spectroscopy with a magneto-optical trap (MOT) apparatus, updating the previous measurement which dates back almost a century. Results are presented for collisions at energies of 50 eV to 120 eV. With a MOT, experiments can be performed with trapped, cold atomic targets which allow for unparalleled accuracy and experiments with signicant excited state target fractions above those achievable in most atomic beam experiments. A basic overview of optical cooling trapping, electron collision and atomic phenomena are presented. Experimental studies of electrons with Argon and Cesium targets were performed, measuring the multiple ionization ratios with ion TOF spectroscopy. The experimental apparatus and analysis methods are described in detail. Results are compared with previous measurements of multiple ionization ratios for both targets. Agreement within experimental error is found with the results of Tate and Smith across the energy range.