Resonance-enhanced multiphoton ionization (REMPI) is a technique applied to the spectroscopy of atoms and small molecules. In practice, a tunable laser can be used to access an excited intermediate state. The selection rules associated with a two-photon or other multiphoton photoabsorption are different from the selection rules for a single photon transition. The REMPI technique typically involves a resonant single or multiple photon absorption to an electronically excited intermediate state followed by another photon which ionizes the atom or molecule. The light intensity to achieve a typical multiphoton transition is generally significantly larger than the light intensity to achieve a single photon photoabsorption. Because of this, subsequent photoabsorption is often very likely. An ion and a free electron will result if the photons have imparted enough energy to exceed the ionization threshold energy of the system. In many cases, REMPI provides spectroscopic information that can be unavailable to single photon spectroscopic methods, for example rotational structure in molecules is easily seen with this technique.
REMPI is usually generated by a focused frequency tunable laser beam to form a small-volume plasma.  In REMPI, first m photons are simultaneously absorbed by an atom or molecule in the sample to bring it to an excited state.  Other n photons are absorbed afterwards to generate an electron and ion pair.  The so-called m+n REMPI is a nonlinear optical process, which can only occur within the focus of the laser beam.  A small-volume plasma is formed near the laser focal region.  If the energy of m photons does not match any state, an off-resonant transition can occur with an energy defect ΔE, however, the electron is very unlikely to remain in that state.  For large detuning, it resides there only during the time Δt.  The uncertainty principle is satisfied for Δt, where ћ=h/2π and h is the Planck constant (6.6261×10^-34 J∙s).  Such transition and states are called virtual, unlike real transitions to states with long lifetimes.  The real transition probability is many orders of magnitude higher than the virtual transition one, which is called resonance enhanced effect.