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.
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 electronís 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 higly- charged uranium ions:
The angled lines are the radiative recombination photons; horizontal lines correspond to bound-state transitions. The "bright" spots are the dielectronic recombination resonances.