Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Resonance enhanced multiphoton ionization

Resonance Enhanced Multi-Photon 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 multi-photon 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, 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, for example rotational structure in molecules is easily seen with this technique.

Rydberg states, Rydberg molecules
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 low-lying 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 Multi-Photon Ionization Spectroscopy (REMPI). The technique is in wide use in both atomic and molecular spectroscopy. An advantage of the REMPI technique is that the ions can be detected with almost complete efficiency and even time 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 formerly of O.R.N.L. and Philip M. Johnson pioneered the development of REMPI. The technique was named by Johnson).

In "Applied Laser Spectroscopy, ed. D. Andrews, VCH (1992), Chapter 9 "Laser Mass Spectroscopy" by K.W. Ledingham and R.P. Singhal" it is stated, that R.N. Zare is the namegiving instance to the REMPI process.