Laser spectroscopic experiments often involve ionization through a photon energy resonance at an intermediate level, with an unbound final electron state and an ionic core.
On resonance for phototransitions permitted by selection rules, the intensity of the laser in combination with the excited state lifetime makes ionization an expected outcome.
(Compton and Johnson pioneered the development of REMPI[citation needed]) The same approach that produces an ionization event can be used to access the dense manifold of near-threshold Rydberg states with laser experiments.
The conversion between a highly elliptical and a nearly circular near-threshold Rydberg state might happen in several ways, including encountering small stray electric fields.
The technique involves waiting for a period of time after a resonance ionization experiment and then pulsing an electric field to collect the lowest energy photoelectrons in a detector.
The delay between the laser and the electric field pulse selected the longest lived and most circular Rydberg states closest to the energy of the ion core.
In REMPI photoelectron experiments, it was determined that a two-photon ionic core photoabsorption process (followed by prompt electronic autoionization) could dominate the direct single photon absorption in the ionization of some Rydberg states of carbon dioxide.
These sorts of two excited electron systems had already been under study in the atomic physics, but there the experiments involved high order Rydberg states.