Resonance-enhanced multiphoton ionization

Resonance Enhanced MultiPhoton Ionzation (R.E.M.P.I.)is a technique that has bee widely applied to study the spectoscopy of atoms and small molecules. In practice, a tunable laser can be used to access an excited intermediate state. In theory, 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 muliphoton photoabsorption to an electronically excited intermediate state. 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, a 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.

Low Rydberg Levels

High photon intensity experiments can involve multiphoton processes with the absorption of integer multiples of the photon energy. In experiments that involve a multiphoton resonance, the intermediate is often a Rydberg state, and the final state is often an ion. The initial state of the system, photon energy, angular momentum and other selection rules can help in determining the nature of the intermediate state. This approach is exploited in Resonance Enhanced Multiphoton Ionization Spectroscopy (REMPI). An advantage of this spectroscopic technique is that the ions can be detected with almost complete efficiency and even resolved for their mass. It is also possible to gain additional information by performing experiments to look at the energy of the liberated photoelectron in these experiments. (Robert N. Compton 1 formerly of O.R.N.L. and Philip M. Johnson 2 pioneered the development of REMPI.)