REMPI
Encyclopedia
Resonance Enhanced Multi-Photon Ionization (REMPI) is a technique applied to the spectroscopy
of atom
s and small molecule
s. In practice, a tunable laser
can be used to access an excited intermediate state
. The selection rule
s 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
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.
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
of atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s and small molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s. In practice, a tunable laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
can be used to access an excited intermediate state
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
. The selection rule
Selection rule
In physics and chemistry a selection rule, or transition rule, formally constrains the possible transitions of a system from one state to another. Selection rules have been derived for electronic, vibrational, and rotational transitions...
s associated with a two-photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
or other multi-photon photoabsorption
Absorption spectroscopy
Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating field. The intensity of the absorption varies as a...
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
Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
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
Absorption spectroscopy
Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating field. The intensity of the absorption varies as a...
, for example rotational structure
Rotational spectroscopy
Rotational spectroscopy or microwave spectroscopy studies the absorption and emission of electromagnetic radiation by molecules associated with a corresponding change in the rotational quantum number of the molecule...
in molecules is easily seen with this technique.
Rydberg statesRydberg statesThe Rydberg states of an atom are electronically excited states with energies that follow the Rydberg formula as they converge on an ionic state with an ionization energy. Although the Rydberg formula was developed to describe atomic energy levels, it has been used to describe many other systems...
, Rydberg moleculesRydberg moleculesA Rydberg molecule is an electronically excited chemical species. Electronically excited molecular states are generally quite different in character from electronically excited atomic states...
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 stateRydberg states
The Rydberg states of an atom are electronically excited states with energies that follow the Rydberg formula as they converge on an ionic state with an ionization energy. Although the Rydberg formula was developed to describe atomic energy levels, it has been used to describe many other systems...
, 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
Atomic spectroscopy
Atomic spectroscopy is the determination of elemental composition by its electromagnetic or mass spectrum. Atomic spectroscopy is closely related to other forms of spectroscopy. It can be divided by atomization source or by the type of spectroscopy used. In the latter case, the main division is...
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
Time-of-flight
Time of flight describes a variety of methods that measure the time that it takes for an object, particle or acoustic, electromagnetic or other wave to travel a distance through a medium...
. 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.
Oak Ridge National Laboratory
Oak Ridge National Laboratory is a multiprogram science and technology national laboratory managed for the United States Department of Energy by UT-Battelle. ORNL is the DOE's largest science and energy laboratory. ORNL is located in Oak Ridge, Tennessee, near Knoxville...
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.
See also
- Rydberg Ionization SpectroscopyRydberg Ionization Spectroscopy-Rydberg states:Atoms and molecules have ionization energy thresholds associated with the energy required to remove an electron from the ionic core. Rydberg series describe the energy levels associated with almost removing an electron from the ionic core...
- Compare with Laser-induced fluorescenceLaser-induced fluorescenceLaser-induced fluorescence is a spectroscopic method used for studying structure of molecules, detection of selective species and flow visualization and measurements....
(LIF)