Collisional excitation
Encyclopedia
Collisional excitation is a process in which the translational energy of a collision partner is converted into the internal energy
of a reactant species.
s in the spectra
of astronomical object
s such as planetary nebula
e and H II region
s.
In these objects, most atom
s are ionised by photon
s from hot star
s embedded within the nebular gas
, stripping away electrons. The emitted electrons, (called photoelectrons), may collide with atoms or ions within the gas, and excite
them. When these excited atoms or ions revert to their ground state
, they will emit a photon. The spectral lines formed by these photons are called collisionally excited lines (often abbreviated to CELs).
CELs are only seen in gases at very low densities (typically less than a few thousand particles per cm³). At higher densities, the reverse process of collisional de-excitation suppresses the lines. Even the hardest vacuum
produced on earth is still too dense for CELs to be observed. For this reason, when CELs were first observed by William Huggins
in the spectrum of the Cat's Eye Nebula
, he did not know what they were, and attributed them to a hypothetical new element called Nebulium. However, the lines he observed were later found to be emitted by extremely rarefied oxygen
.
CELs are very important in the study of gaseous nebulae, because they can be used to determine the density and temperature of the gas.
is the process where an ion collides with an atom or molecule and leads to an increase in the internal energy of the ion. Molecular ions are accelerated to high kinetic energy
and then collide with neutral gas molecules (e.g. helium
, nitrogen
or argon
). In the collision some of the kinetic energy is converted into internal energy
which results in fragmentation in a process known as collision-induced dissociation
.
Internal energy
In thermodynamics, the internal energy is the total energy contained by a thermodynamic system. It is the energy needed to create the system, but excludes the energy to displace the system's surroundings, any energy associated with a move as a whole, or due to external force fields. Internal...
of a reactant species.
Astronomy
In astronomy, collisional excitation gives rise to spectral lineSpectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...
s in the spectra
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
of astronomical object
Astronomical object
Astronomical objects or celestial objects are naturally occurring physical entities, associations or structures that current science has demonstrated to exist in the observable universe. The term astronomical object is sometimes used interchangeably with astronomical body...
s such as planetary nebula
Planetary nebula
A planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of stars late in their life...
e and H II region
H II region
An H II region is a large, low-density cloud of partially ionized gas in which star formation has recently taken place. The short-lived, blue stars forged in these regions emit copious amounts of ultraviolet light, ionizing the surrounding gas...
s.
In these objects, most 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 are ionised by 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...
s from hot star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s embedded within the nebular gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
, stripping away electrons. The emitted electrons, (called photoelectrons), may collide with atoms or ions within the gas, and excite
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....
them. When these excited atoms or ions revert to their ground state
Ground state
The ground state of a quantum mechanical system is its lowest-energy state; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state...
, they will emit a photon. The spectral lines formed by these photons are called collisionally excited lines (often abbreviated to CELs).
CELs are only seen in gases at very low densities (typically less than a few thousand particles per cm³). At higher densities, the reverse process of collisional de-excitation suppresses the lines. Even the hardest vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
produced on earth is still too dense for CELs to be observed. For this reason, when CELs were first observed by William Huggins
William Huggins
Sir William Huggins, OM, KCB, FRS was an English amateur astronomer best known for his pioneering work in astronomical spectroscopy.-Biography:...
in the spectrum of the Cat's Eye Nebula
Cat's Eye Nebula
The Cat's Eye Nebula is a planetary nebula in the constellation of Draco. Structurally, it is one of the most complex nebulae known, with high-resolution Hubble Space Telescope observations revealing remarkable structures such as knots, jets, bubbles and sinewy arc-like features...
, he did not know what they were, and attributed them to a hypothetical new element called Nebulium. However, the lines he observed were later found to be emitted by extremely rarefied oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
.
CELs are very important in the study of gaseous nebulae, because they can be used to determine the density and temperature of the gas.
Mass spectrometry
Collisional excitation in mass spectrometryMass spectrometry
Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...
is the process where an ion collides with an atom or molecule and leads to an increase in the internal energy of the ion. Molecular ions are accelerated to high 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...
and then collide with neutral gas molecules (e.g. helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
, nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
or argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
). In the collision some of the kinetic energy is converted into internal energy
Internal energy
In thermodynamics, the internal energy is the total energy contained by a thermodynamic system. It is the energy needed to create the system, but excludes the energy to displace the system's surroundings, any energy associated with a move as a whole, or due to external force fields. Internal...
which results in fragmentation in a process known as collision-induced dissociation
Collision-induced dissociation
In Mass spectrometry, Collision-induced dissociation , referred to by some as collisionally activated dissociation , is a mechanism by which to fragment molecular ions in the gas phase. The molecular ions are usually accelerated by some electrical potential to high kinetic energy and then allowed...
.