Elementary Processes in Excited and Ionized Gases - Fundamentals of Ionized Gases - page 109

Ta b l e 2 . 18 Collision processes of atoms and molecules.

Process Scheme Comment number,

section of [146]

a ,Section2.1.9

Elastic collision of atoms and
molecules

A C B ! A C B

A C

b , Section 2.3.1

Excitation of electron levels in
collisions with atoms

A C B ! B

A C C

b , Section 2.3.1

Ionization in collisions of atoms A C B ! e C B

v 0 )

c

Transitions between vibrational levels
of molecules

A C BC ( v ) ! A C BC (

BC ( J 0 )

d

Transitions between rotational levels of
molecules

A C BC ( J ) ! A C

A C

b , Section 2.3.1

Quenching of excited electronic states
in atomic collisions

B ( BC ) ! A C B ( BC )

A C AB C C B ! e

Associative ionization Section 2.3.5

A C B C C B ! A C e

Penning process Section 2.3.5

A C B

B ! A C

Transfer of excitation Section 2.3.1

e

Spin exchange and transitions between
hyperfine structure states

A C B ( # ) ! A C B ( " )

B ( j 0 )

f

Transitions between fine structure
states

A C B ( j ) ! A C

g

Atom depolarization in collisions A C B ( # ) ! A C B ( " )

h ,Section2.2.3

Formation of molecules in three body
collisions

A C B C C ! AB C C

i

Chemical reactions A C BC ! AB C C

a For thermal energies of collision, the cross section has the gas-kinetic magnitude (approximately
10 15 cm 2 ).
b Under adiabatic conditions, these processes have a small transition probability for collision
energies of a few electronvolts, and the relative cross sections are smaller than typical atomic cross
sections by several orders of magnitude.
c At thermal collision energies, the cross section for the transition is smaller than the gas-kinetic
cross section by several orders of magnitude.
d At thermal collision energies, the cross section for the transition is of the order of a typical atomic
cross section.
e Arrows indicate spin orientation of valence electrons. This is a resonant process. Consider, as an
example, collision of two alkali metal atoms in the ground electronic state. Then, spin exchange
corresponds to exchange of valence electrons, so the spin transition causes the transition between
hyperfine structure levels of each atom.
f If the energy difference for fine structure levels is sufficiently small and the Massey
parameter (2.83) is small for the transition, this process is resonant and is characterized by large
cross sections.
g Arrows indicate directions of atomic angular momenta. This is a resonant process.
h For thermal energies, the rate constant for the process is of the order of 10 33 10 32 cm 6 /s.

i The cross section of the process is larger than the gas-kinetic process cross section.