Photoinduced charge separation
Encyclopedia
Photoinduced charge separation is the process of an electron
in an atom
being excited to a higher energy level by the absorption of a photon and then leaving the atom to a nearby electron acceptor
.
suggested that the electrons orbited the dense central nucleus in a manner analogous to planets orbiting the sun. The centripetal force
required to keep the electrons in orbit was provided by the Coulomb force of the protons in the nucleus acting upon the electrons; just like the gravitational force of the sun acting on a planet provides the centripetal force necessary to keep the planet in orbit.
This model, although appealing, doesn't hold true in the real world. Synchrotron radiation
would cause the orbiting electron to lose orbital energy and spiral inward since the vector quantity of acceleration of the particle multiplied by its mass (the value of the force required to keep the electron in circular motion) would be less than the electrical force the proton applied to the electron.
Once the electron spiralled into the nucleus the electron would combine with a proton to form a neutron, and the atom would cease to exist. This model is clearly wrong.
refined the Rutherford model by stating that the electrons existed in discrete quantized
states called energy levels. This meant that the electrons could only occupy orbits at certain energies. The laws of quantum physics apply here, and they don't comply with the laws of classical newtonian mechanics
.
An electron which is completely free from the atom has an energy of 0 joules (or 0 electronvolts). An electron which is described as being at the 'ground state' has an energy which is equal to the ionisation energy of the atom. The electron will reside in this energy level under normal circumstances.
If a photon of light hits the atom it will be absorbed if, and only if, energy of that photon is equal to the difference between the ground state and another energy level in that atom. This raises the electron to a higher energy level.
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...
in an 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...
being excited to a higher energy level by the absorption of a photon and then leaving the atom to a nearby electron acceptor
Electron acceptor
An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process....
.
Rutherford model
An atom consists of a positively charged nucleus orbited by electrons. The nucleus consists of uncharged neutrons and positively charged protons. Electrons are negatively charged. In the early part of the twentieth century Ernest RutherfordErnest Rutherford
Ernest Rutherford, 1st Baron Rutherford of Nelson OM, FRS was a New Zealand-born British chemist and physicist who became known as the father of nuclear physics...
suggested that the electrons orbited the dense central nucleus in a manner analogous to planets orbiting the sun. The centripetal force
Centripetal force
Centripetal force is a force that makes a body follow a curved path: it is always directed orthogonal to the velocity of the body, toward the instantaneous center of curvature of the path. The mathematical description was derived in 1659 by Dutch physicist Christiaan Huygens...
required to keep the electrons in orbit was provided by the Coulomb force of the protons in the nucleus acting upon the electrons; just like the gravitational force of the sun acting on a planet provides the centripetal force necessary to keep the planet in orbit.
This model, although appealing, doesn't hold true in the real world. Synchrotron radiation
Synchrotron radiation
The electromagnetic radiation emitted when charged particles are accelerated radially is called synchrotron radiation. It is produced in synchrotrons using bending magnets, undulators and/or wigglers...
would cause the orbiting electron to lose orbital energy and spiral inward since the vector quantity of acceleration of the particle multiplied by its mass (the value of the force required to keep the electron in circular motion) would be less than the electrical force the proton applied to the electron.
Once the electron spiralled into the nucleus the electron would combine with a proton to form a neutron, and the atom would cease to exist. This model is clearly wrong.
Bohr model
In 1913 Niels BohrNiels Bohr
Niels Henrik David Bohr was a Danish physicist who made foundational contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. Bohr mentored and collaborated with many of the top physicists of the century at his institute in...
refined the Rutherford model by stating that the electrons existed in discrete quantized
Quantization (physics)
In physics, quantization is the process of explaining a classical understanding of physical phenomena in terms of a newer understanding known as "quantum mechanics". It is a procedure for constructing a quantum field theory starting from a classical field theory. This is a generalization of the...
states called energy levels. This meant that the electrons could only occupy orbits at certain energies. The laws of quantum physics apply here, and they don't comply with the laws of classical newtonian mechanics
Mechanics
Mechanics is the branch of physics concerned with the behavior of physical bodies when subjected to forces or displacements, and the subsequent effects of the bodies on their environment....
.
An electron which is completely free from the atom has an energy of 0 joules (or 0 electronvolts). An electron which is described as being at the 'ground state' has an energy which is equal to the ionisation energy of the atom. The electron will reside in this energy level under normal circumstances.
If a photon of light hits the atom it will be absorbed if, and only if, energy of that photon is equal to the difference between the ground state and another energy level in that atom. This raises the electron to a higher energy level.