The physical composition and intensity of ion beams, produced at ISOL facilities such as ISOLDE, depends strongly on the type of source used to ionize the isotope of interest.

The RILIS principle of multi-step resonance photo-ionization takes advantage of the element unique nature of the atomic (electron) energy level structure.  Multiple laser beams are used, with each laser wavelength precisely tuned so that the photon energies match a series of successive electronic transition energies, unique to that element.  This results in ionization of only the element of interest whilst the other elements that are present in the atomic vapour within the ion source are unaffected.

At ISOLDE the laser ionization takes place within a hot metal cavity, as shown above.  This high temperature (T~2000 K) cavity is an effective laser/atom interaction region for the ISOLDE RILIS because it combines a robust and reliable environment with spatial confinement of the atomic vapour to ensure that the atoms are illuminated by the focused laser beams.  The typical residence time of an atom within this cavity is 100 microseconds, thereby necessitating the use of a high repetition rate (10 kHz) laser system, to keep duty cycle losses to a minimum.

When used in conjunction with the ISOLDE mass separating magnets, the chemical element (proton number, Z) and the atomic mass (A) are both selected during the ion beam production process, thereby enabling a high degree of isobar-free isotope selectivity.