When an electron impacts an
ion, it can excite the electrons in that ion. The excited state thus formed
can decay in a variety of ways.
We have measured the cross sections
for this process in a variety of ions, from our very first publication on
neonlike barium to recent measurements in a number of lithiumlike ions.
Electron Impact Ionization
If an electron impacts an ion with enough energy, it can remove one of the electrons remaining in the
ion. This process is called ionization. At EBIT, we have studied ionization in a variety of systems, from lithiumlike low-atomic-number atoms to hydrogenic
Electron-Ion Recombination Processes
When an electron collides with an ion, it can be captured by the ion. This process is called recombination.
We have studied two forms of recombination: radiative recombination and dielectronic recombination.
In radiative recombination, an electron is captured into a vacant orbital of the ion, emitting a photon
whose energy is equal to the sum of the electron’s initial kinetic energy and the binding energy of the state to which it is captured.
In dielectronic recombination, an electron is captured, just as in radiative recombination, but instead
of emitting a photon, another electron already in the ion is excited. The resulting doubly-excited intermediate state decays radiatively; that is,
by emission of a photon. This process is resonant; that is, it only occurs if the kinetic energy of the beam electron is exactly right.
Here are level diagrams showing the two processes for a heliumlike ion:
We observe these processes by detecting the emitted photons. We quickly ramp the electron beam energy
over the range of energies to be studied. When a photon is detected, a fast
data-aquisition system records the photon energy, the electron beam energy,
and the time that the event occurred. We then plot the events on a two-dimensional
false-color scatter plot, like this one, for highly- charged uranium ions: