What is Resonance ionization?

Resonance ionization is a process in optical physics used to excite a specific atom beyond its ionization potential to form an ion using a beam of photons irradiated from a pulsed laser light. In resonance ionization, the absorption or emission properties of the emitted photons are not considered, rather only the resulting excited ions are mass-selected, detected and measured. Depending on the laser light source used, one electron can be removed from each atom so that resonance ionization produces an efficient selectivity in two ways: elemental selectivity in ionization and isotopic selectivity in measurement.

During resonance ionization, an ion gun creates a cloud of atoms and molecules from a gas-phase sample surface and a tunable laser is used to fire a beam of photons at the cloud of particles emanating from the sample.

An initial photon from this beam is absorbed by one of the sample atoms, exciting one of the atom's electrons to an intermediate excited state. A second photon then ionizes the same atom from the intermediate state such that its high energy level causes it to be ejected from its orbital; the result is a packet of positively charged ions which are then delivered to a mass analyzer.

Resonance ionization contrasts with resonance-enhanced multiphoton ionization in that the latter is neither selective nor efficient since resonances are seldom used to prevent interference. Also, resonance ionization is used for an atomic analyte, whereas REMPI is used for a molecular analyte.