Interatomic Coulombic Decay
Encyclopedia
Interatomic Coulombic Decay (ICD) is a general, fundamental property of atom
s and molecule
s which have neighbors. The Interatomic (Intermolecular) Coulombic Decay is a very efficient interatomic (intermolecular) relaxation process of an electronically excited atom or molecule embedded in an environment. Without the environment the process cannot take place. Until now it has been mainly demonstrated for atomic and molecular cluster
s, independently of whether they are of van-der-Waals
or hydrogen bond
ed type.
The nature of the process can be depicted as follows: Consider a cluster with two subunits, A and B. Suppose an inner-valence
electron
is removed from subunit A. If the resulting (ionized) state is higher in energy than the double ionization threshold
of subunit A then an intraatomic (intramolecular) process (autoionization
, in the case of core ionization Auger decay) sets in. Even though the excitation is energetically not higher than the double ionization threshold of subunit A itself, it may be higher than the double ionization threshold of the cluster which is lowered due to charge
separation. If this is the case, an interatomic (intermolecular) process sets in which is called ICD. During the ICD the excess energy of subunit A is utilized to remove (due to electronic correlation
) an outer-valence electron from subunit B. As a result a doubly ionized cluster is formed with a single positive charge on A and B. Thus, charge separation in the final state is a fingerprint of ICD. As a consequence of the charge separation the cluster typically breaks apart via Coulomb explosion
.
ICD is characterized by its decay rate or the lifetime of the excited state. The decay rate depends on the interatomic (intermolecular) distance of A and B and its dependence allows to draw conclusions on the mechanism of ICD. Particularly important is the determination of the kinetic energy
spectrum of the electron emitted from subunit B which is denoted as ICD electron. ICD electrons are often measured in ICD experiments. Typically, ICD takes place on the femto second time scale, many orders of magnitude faster than those of the competing photon emission and other relaxation processes.
. There, ICD is faster than the competing proton
transfer that is usually the prominent pathway in the case of electronic excitation of water clusters.
The response of condensed water to electronic excitations is of utmost importance for biological systems. For instance, it was shown in experiments that low energy electrons do affect constituents of DNA
effectively. Furthermore, ICD was reported after core-electron excitations of hydroxide
in dissolved water.
as described above. Independent of what kind of electronic excitation is at hand, an interatomic (intermolecular) process can set in if an atom or molecule is in a state energetically higher than the ionization threshold of other atoms or molecules in the neighborhood. The following ICD related processes, which were for convenience considered below for clusters, are known:
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 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 which have neighbors. The Interatomic (Intermolecular) Coulombic Decay is a very efficient interatomic (intermolecular) relaxation process of an electronically excited atom or molecule embedded in an environment. Without the environment the process cannot take place. Until now it has been mainly demonstrated for atomic and molecular cluster
Cluster
-In science:* Cluster , a small group of atoms or molecules* Cluster chemistry, an array of bound atoms intermediate in character between a molecule and a solid...
s, independently of whether they are of van-der-Waals
Van der Waals force
In physical chemistry, the van der Waals force , named after Dutch scientist Johannes Diderik van der Waals, is the sum of the attractive or repulsive forces between molecules other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral...
or hydrogen bond
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
ed type.
The nature of the process can be depicted as follows: Consider a cluster with two subunits, A and B. Suppose an inner-valence
Electron shell
An electron shell may be thought of as an orbit followed by electrons around an atom's nucleus. The closest shell to the nucleus is called the "1 shell" , followed by the "2 shell" , then the "3 shell" , and so on further and further from the nucleus. The shell letters K,L,M,.....
electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
is removed from subunit A. If the resulting (ionized) state is higher in energy than the double ionization threshold
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...
of subunit A then an intraatomic (intramolecular) process (autoionization
Autoionization
Autoionization is a process by which atoms or molecules spontaneously emit one of the shell electrons, thus going from a state with charge Z to a state with charge Z + 1, for example from an electrically neutral state to a singly ionized state....
, in the case of core ionization Auger decay) sets in. Even though the excitation is energetically not higher than the double ionization threshold of subunit A itself, it may be higher than the double ionization threshold of the cluster which is lowered due to charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
separation. If this is the case, an interatomic (intermolecular) process sets in which is called ICD. During the ICD the excess energy of subunit A is utilized to remove (due to electronic correlation
Electronic correlation
Electronic correlation is the interaction between electrons in the electronic structure of a quantum system.- Atomic and molecular systems :...
) an outer-valence electron from subunit B. As a result a doubly ionized cluster is formed with a single positive charge on A and B. Thus, charge separation in the final state is a fingerprint of ICD. As a consequence of the charge separation the cluster typically breaks apart via Coulomb explosion
Coulomb explosion
A Coulomb explosion is a mechanism for coupling electronic excitation energy from intense electromagnetic fields into atomic motion. The atomic motion can break the bonds that hold solids together...
.
ICD is characterized by its decay rate or the lifetime of the excited state. The decay rate depends on the interatomic (intermolecular) distance of A and B and its dependence allows to draw conclusions on the mechanism of ICD. Particularly important is the determination of the kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
spectrum of the electron emitted from subunit B which is denoted as ICD electron. ICD electrons are often measured in ICD experiments. Typically, ICD takes place on the femto second time scale, many orders of magnitude faster than those of the competing photon emission and other relaxation processes.
ICD in water
Very recently, ICD has been identified to be an additional source of low energy electrons in waterWater
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
. There, ICD is faster than the competing proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
transfer that is usually the prominent pathway in the case of electronic excitation of water clusters.
The response of condensed water to electronic excitations is of utmost importance for biological systems. For instance, it was shown in experiments that low energy electrons do affect constituents of DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
effectively. Furthermore, ICD was reported after core-electron excitations of hydroxide
Hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...
in dissolved water.
Related Processes
Interatomic (Intermolecular) processes do not only occur after ionizationIonization
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...
as described above. Independent of what kind of electronic excitation is at hand, an interatomic (intermolecular) process can set in if an atom or molecule is in a state energetically higher than the ionization threshold of other atoms or molecules in the neighborhood. The following ICD related processes, which were for convenience considered below for clusters, are known:
- Resonant Interatomic Coulombic Deacy (RICD) was first validated experimentally. This process emanates from an inner-valenceElectron shellAn electron shell may be thought of as an orbit followed by electrons around an atom's nucleus. The closest shell to the nucleus is called the "1 shell" , followed by the "2 shell" , then the "3 shell" , and so on further and further from the nucleus. The shell letters K,L,M,.....
excitation where an inner-valence electron is promoted to a virtual orbitalMolecular orbitalIn chemistry, a molecular orbital is a mathematical function describing the wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding an electron in any specific region. The term "orbital" was first...
. During the process the vacant inner-valence spot is filled up by an outer-valence electron of the same subunit or by the electron in the virtual orbital. The following action is referred to as RICD if in the previous process generated excess energy removes an outer-valence electron from another cluster constituent. The excess energy can, on the other hand, also be used to remove an outer-valence electron from the same subunit (autoionizationAutoionizationAutoionization is a process by which atoms or molecules spontaneously emit one of the shell electrons, thus going from a state with charge Z to a state with charge Z + 1, for example from an electrically neutral state to a singly ionized state....
). Consequently, RICD competes not only with slow radiative decay as ICD, it competes also with the effective autoionization. Both experimental and theoretical evidence show that this competition does not lead to a suppression of the RICD. - Auger-ICD cascade has been first predicted theoretically. States with a vacancy in a core-shell usually undergo Auger decay. This decay often produces double ionized states which can sometimes decay by another Auger decay forming a so-called Auger cascadeAuger electron spectroscopyAuger electron spectroscopy is a common analytical technique used specifically in the study of surfaces and, more generally, in the area of materials science...
. However, often the double ionized state is not high enough in energy to decay intraatomically once more. Under such conditions, formation of a decay cascade is impossible in the isolated species, but can occur in clusters with the next step being ICD. Meanwhile the Auger-ICD cascade has been confirmed and studied experimentally. - Excitation-Transfer-Ionization (ETI) is a non-radiative decay pathway of outer-valence excitations in an environment. Assume that an outer-valence electron of a cluster subunit is promoted to a virtual orbital. On the isolated species this excitation can usually only decay slowly by photon emission. In the cluster there is an additional, much more efficient pathway if the ionization threshold of another cluster constituent is lower than the excitation energy. Then the excess energy of the excitation is transferred interatomically (intermolecularly) to remove an outer-valence electron from another cluster subunit with an ionization threshold lower than the excitation energy. Usually, this interatomic (intermolecular) process also takes place within a few femtoseconds.
- Electron-transfer-mediated decay (ETMD) is a non-radiative decay pathway where a vacancy in an atom or molecule is filled by an electron from a neighboring species; a secondary electron is emitted either by the first atom/molecule or by the neighboring species. The existence of this decay mechanism has been proven experimentally in Argon dimers and in mixed Argon - Krypton clusters .