A spectroscopic technique in which single atoms in a gas are detected using a laser to ionize that atom. A sample containing the atoms to be excited is subjected to light from a laser, tuned so that only that type of atom is excited by the light. If the frequency of light at which the atom is excited is ν, the atoms in the excited state can be ionized if the ionization potential of the atom is less than 2ν. In contrast to other techniques of ionization, this type of ionization only occurs for atoms that are ‘in tune’ with the frequency of light. Because RIS is very selective in determining which atom is ionized for a given frequency it has many applications in chemistry.