Atomic Collision

an elementary collision event between two atomic particles, which may be atoms, molecules, electrons, or ions. Such a collision is either elastic or inelastic. In an elastic collision, the total kinetic energy of the colliding particles remains the same as before the collision: the kinetic energy is merely redistributed between the particles, and the directions of motion of the particles are altered. In an inelastic collision, the internal energy of the colliding particles changes—the particles undergo transitions to different energy levels. As a result, the total kinetic energy of the particles is not conserved. In such a collision, the electronic state of an atom or the vibrational or rotational state of a molecule is changed (seeMOLECULAR SPECTRA).

Elastic collisions account for transport phenomena in gases or weakly ionized plasmas. The free motion of the particles is hindered by the collisions the particles undergo. In these collisions the particles are scattered by other particles. Scattering events in which a particle’s direction of motion is markedly changed have the greatest effect on the motion of the particle. For this reason, the diffusivity (transport of particles), the coefficient of viscosity (transport of momentum), the thermal conductivity (transport of energy), and other gas transport coefficients can be expressed in terms of an effective cross section for the scattering of the atoms or molecules of the gas at large angles. In much the same way, the mobility of ions (seeMOBILITY OF IONS AND ELECTRONS) is connected with the effective cross section for the scattering of an ion by a gas atom or molecule at large angles. The mobility of electrons in a gas or the electrical conductivity of a weakly ionized plasma depends on the effective cross section for the scattering of an electron by a gas atom or molecule.

In the case where atoms or molecules with thermal energies are scattered at large angles, the elastic cross section is known as the gas-kinetic cross section. It has a value of the order of 10^–15 cm²and determines the mean free path of a particle in the medium.

Elastic scattering at small angles may affect the nature of the transfer of electromagnetic radiation in a gas. The energy of an electromagnetic wave passing through a gas is absorbed and then reemitted by the atoms or molecules of the gas. In this case, even a weak interaction of the emitting particle with the particles surrounding it “distorts” the emitted wave—that is, changes its

Table 1. Inelastic collision processes involving atomic particles and photons
Number	Type of collision	Symbolic representation ofprocess¹
¹ In the representations, A. B. and C stand for an atom or molecule, B* is an electronically excited atom or molecule, e is an electron, A⁺ is a positively charged ion, A^– is a negatively charged ion, is a photon, v is the vibrational quantum number of the molecule, and J is the rotational quantum number of the molecule; the arrows indicate the direction of the process
1	Collisional ionization of atoms and molecules	A + B → A + B⁺ + e
2	Transition between electronic states	A + B ⇆ A + B* e + B ⇆ e + B*
3	Transition between vibrational or rotational states of molecules	AB(v) + C → AB(v′) + C e + AB(v) → e + AB(v′) AB(J) + C → AB (J′) + C e + AB (J) → e + AB (J′)
4	Chemical reactions	A + BC ⇆ AB + C
5	Quenching of electronic excitation	A + BC ⇆ A + B + C
6	Transfer of electronic excitation	B* + AC(v) → B + AC (v′)
7	Spin exchange (the projection of the spin of each atom changes, while the projection of the total spin of the atoms is conserved)	A + B* → A* + B
8	Depolarization of an atom (the direction of the orbital moment of one of the colliding atoms Is changed)
9	Transitions between states of the fine and hyperfine structure of one of the colliding atoms or molecules
10	Ionization of an atom or molecule by electron impact	e + A → 2e + A⁺
11	Dissociation of a molecule by electron impact	e + BA → e + A + B
12	Recombination In triple collisions	e + B⁺ + B(e) → A + B(e) A^– + B⁺ + C → A + B +C
13	Dissociative recombination	e + AB⁺ → A + B
14	Dissociative electron capture by a molecule	e + AB → A^– + B
15	Electron capture by a molecule In triple collisions	e + A + B → A^– + B
16	Associative ionization	A + B → AB⁺ + e
17	Penning Ionization (atom A* is in a metastable state, and its excitation energy exceeds the ionization potential of atom B)	A* + B → A + B⁺ + e
18	Mutual neutralization of ions	A^– – B⁺ → A + B
19	Transfer of ionic charge	A – B⁺ → A⁺ + B
20	Ion-molecule reactions	A¹ + BC → AB⁺ + C A⁺ + BC → AB + C*
21	Destruction of a negative ion	A^– + B → A + B + e
22	Conversion of atomic ions into molecular Ions	A⁺ + B + C → AB^– + C
23	Photoexcitation of an atom or molecule (with subsequent spontaneous emission of the excited atom)	ħω + B → B*
24	Photorecombination and photolonization	e + A¹ ⇆ A + ħω
25	Photodissociation and photorecombination of atoms and radicals	ħω + AB ⇆ A +B
26	Radiative electron capture by an atom	e + A → A^– + ħω

phase or frequency. Under some conditions, the principal characteristics of an electromagnetic wave propagating through a gas are determined by the elastic scattering, by the surrounding particles, of the atoms or molecules that interact with the wave; here, scattering at small angles plays an important role.

Many different inelastic collision processes have been observed. A list of the inelastic processes that can occur in a gas or a weakly ionized plasma is given in Table 1. Specific types of inelastic particle collisions predominate under different laboratory conditions and in different natural phenomena. For example, the radiation from the surface of the sun results mainly from collisions between electrons and hydrogen atoms, in which negative hydrogen ions are formed (number 26 in Table 1). The basic process underlying the operation of a helium-neon laser is the transfer of the excitation of helium atoms in metastable states to neon atoms. The basic process in molecular gas lasers is the excitation of vibrational molecular states by electron impact (number 3); as a result of this process, the electrical energy of the gas discharge is partially transformed into laser emission energy. In gas-discharge lamps, the principal processes are the excitation of atoms by electron impact (number 2) in fluorescent lamps and the photorecombination of electrons and ions (number 24) in high-pressure lamps. Spin exchange (number 7) limits the parameters of quantum frequency standards, whose operation is based on transitions between hyperfine-structure states of the hydrogen atom or of alkali-metal atoms (number 9). The properties of the earth’s atmosphere are determined by various inelastic collision processes involving free radicals, ions, electrons, and excited atoms. Different processes predominate at different heights.

REFERENCES

McDaniel, E. Protsessy stolknovenii v ionizovannykh gazakh. Moscow, 1967. (Translated from English.)
Smirnov, B. M. Atomnye stolknoveniia i elementarnye protsessy v plazme. Moscow, 1968.
Smirnov, B. M. lony i vozbuzhdennye atomy v plazme. Moscow, 1974.
Hasted, J. Fizika atomnykh stolknovenii. Moscow, 1965. (Translated from English.)

B. M. SMIRNOV