Collisional excitation is a process in which the

kinetic energy In physics, the kinetic energy of an object is the energy that 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 accele ...

of a collision partner is converted into the

internal energy The internal energy of a thermodynamic system is the total energy contained within it. It is the energy necessary to create or prepare the system in its given internal state, and includes the contributions of potential energy and internal kinet ...

of a reactant species.

Astronomy

In astronomy, collisional excitation gives rise to

spectral line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...

s in the spectra of

astronomical object An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...

s such as

planetary nebula A planetary nebula (PN, plural PNe) is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The term "planetary nebula" is a misnomer because they are unrelated to ...

e and

H II region An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds ...

s. In these objects, most

atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, and ...

s are

ionised Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule ...

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...

s from hot

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

s embedded within the nebular

gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...

, stripping away

electrons The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...

. The emitted electrons, (called

photoelectron The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid sta ...

s), may collide with atoms or ions within the gas, and

excite Excitation, excite, exciting, or excitement may refer to: * Excitation (magnetic), provided with an electrical generator or alternator * Excite Ballpark, located in San Jose, California * Excite (web portal), web portal owned by IAC * Electron ex ...

them. When these excited atoms or ions revert to their

ground state The ground state of a quantum-mechanical system is its stationary state of lowest energy; 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) for forbidden transitions. For allowed transitions, the gas density can be substantially higher. At higher densities, the reverse process of collisional de-excitation suppresses the lines. Even the hardest

vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...

produced on earth is still too dense for CELs to be observed. For this reason, when CELs were first observed by

William Huggins Sir William Huggins (7 February 1824 – 12 May 1910) was an English astronomer best known for his pioneering work in astronomical spectroscopy together with his wife, Margaret. Biography William Huggins was born at Cornhill, Middlesex, in ...

in the spectrum of the

Cat's Eye Nebula The Cat's Eye Nebula (also known as NGC 6543 and Caldwell 6) is a planetary nebula in the northern constellation of Draco, discovered by William Herschel on February 15, 1786. It was the first planetary nebula whose spectrum was investigated by ...

, he did not know what they were, and attributed them to a hypothetical new element called

nebulium Nebulium was a proposed element found in astronomical observation of a nebula by William Huggins in 1864. The strong green emission lines of the Cat's Eye Nebula, discovered using spectroscopy, led to the postulation that an as yet unknown elem ...

. However, the lines he observed were later found to be emitted by extremely rarefied

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

. 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 spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...

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

accelerate In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by the ...

d to high

and then collide with neutral gas molecules (e.g.

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...

). In the collision some of the kinetic energy is converted into

which results in fragmentation in a process known as

collision-induced dissociation Collision-induced dissociation (CID), also known as collisionally activated dissociation (CAD), is a mass spectrometry technique to induce fragmentation of selected ions in the gas phase. The selected ions (typically molecular ions or protonated ...

References

{{Reflist Astronomical spectroscopy Mass spectrometry

Astronomy

Mass spectrometry

See also

References