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Resonance ionization is a selective mode of ionization that is based upon atomic vibrations as an atom is excited from the ground state. This ionization method is used for the spectroscopic analysis of inorganic, elemental species. Ionization is accomplished by use of a monochromatic tunable laser, which fires a beam of photons at a gas-phase sample (the analyte). 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 atom from the intermediate state such that its high energy level causes it to be ejected from its orbital; the result is a positively charged ion, which is then moved into a mass analyzer.

This method of ionizing atoms is analogous to REMPI; the difference being that resonance ionization is used for an atomic (elemental) analyte, whereas REMPI is used for a molecular analyte.

History
Resonance ionization was first used in a spectroscopy experiment in 1971 in the Institute for Spectroscopy Russian Academy of Sciences; in that experiment, ground state rubidium atoms were ionized using ruby lasers. In 1974, a need for a new spectroscopic method was realized for energy pathways research; this led to the development of resonance ionization spectroscopy, which coupled the resonance ionization source with a mass spectrometer. An experiment was conducted using this instrumentation to measure the quantity of a species of metastable helium atoms in a sample. This was an initial step in resonance ionization becoming more common for atomic analysis in a research setting.

Applications
A major advantage of using resonance ionization lies in the fact that it is highly selective ionization mode; it is able to target a single type of atom amongst a background of many types of atoms, even when said background atoms are much more abundant than the target atoms. In addition, resonance ionization incorporates the high selectivity that is desired in spectroscopy methods with ultrasensitivity. This makes resonance ionization useful when analyzing complex samples with many atomic components.

Due to the high sensitivity and its ability to target a single atom, Resonance Ionization coupled with mass spectrometry is a great technique for cosmochemistry which works with trace amounts of samples.