chemical physics of discharges - Argonne National Laboratory

35
must be added to the Langevin cross section. Here is dipole moment and Etis the
translational energy of the reacting system in center of mass coordinates.
NO attempt will be made here to review all of the studies of the effect of energy
upon the rates of ion-molecule reactions. However, several investigators who have
made especially important contributions should be mentioned. In addition to the
work of Hamill discussed above, important studies have been made by Futrell and
Abramson (70), Giese and Maier (71), Friedman (72,73) and Light and Horrocks(74).
- -- --
(70) J. H. Futrell and F. P. Abramson, "Ion-Molecule Reactions in the Gas Phase,"
Adv. in Chem. Series 58, Amer. Chem. SOC., 1966, p. 107.
(71) C. F. Giese and W. B.xaier, J. Chem. Phys. 39, - 197, 739 (1963)
(72) T. F. Moran and L. Friedman, J. Chem. Phys. 39, 2491 (1963); 62, 2391 (1965) - -
(73) F. S. Klein and L. Friedman, J. Chem. Phys. 4l, 1789 (1964)
(74) J. C. Light and J. Horrocks, Proc. Phys. 527 (1964)
It should be pointed out that if the relative velocities of ions and neutrals
become sufficiently high other reactions begin to OCCUL as a result of the different
forces coming into play. Thus a fast moving ion passing a molecule with sufficient
velocity may simply strip off a peripheral atom, leaving the partially denuded entity
behind. Such stripping reactions have been studied by Henglein (75) and Koski
(76), who found that they obey quite different rules from those above. It thus
(75) A. Henglein, "Ion Molecule Reactions in the Gas Phase," &. in Chem. Series
- 58, Amer, Chem. SOC., Washington, D. C.,' p. 63
-
(76) M. A. Berta, B. Y. Ellis and W. S. Koski, ibid., p. 80
appears that a complete theory of the rates of ion-molecule reactions has not been
developed, but there is little doubt that to a first approximation the equation of
Gioumousis and Stevenson givsfairly good results. It is also true that the
reactions tend to be quite fast, and this becomes a matter of overriding importance
in many processes involving ions. Thus is has been shown by Lampe (77,78) and by
Futrell (79) that ion-molecule reactions are the controlling factor in certain
(77) F. W. Lampe, Radiation Research lo, 671 (1959)
(78)
(79) J. H. Futrell, J. &I. -m.
82, 1551 (1960)
=E
*. 8l, 5921 (1959)
-
F. W. Lampe, 2. &. k m . z.,
-
radiation induced processes involving paraffin and olefin hydrocarbons.
An electric discharge of course involves ions and electrons and the ions present
can be sampled and analyzed by mass spectrometry. Such studies have been undertaken
by a number of investigators and some progress is being made toward understanding
the chemical behavior of ions in various discharges. The problem is complicated by
the fact that there are several kinds of electric discharge, each of which has its
own physical and chemical characteristics. Of these, the type most often studied is
the direct current glow discharge, but corona and high frequency and micro-wave
discharges have also received some attention.
In order to analyze the ionic content of a discharge it is necessary to trans-
fer the ions from the discharge into the mass analyzer. While this is not a par-
ticularly serious problem at pressures below 10 microns, the difficulty becomes
more acute as the pressure increases. This is attributable to the fact that elec-
trons and ions diffuse to the walls at different rates, so that an electric gradient
is established which alters the distribution of energy of the various charged species.
Ordinarily a sheath of ions is formed at any surface, including that of a sampling
probe and ions or electrons must have, or be given enough energy to pass through
this sheath in order to be sampled. However, if the energy is sufficiently high
some ions may be decomposed by collision. The result then is that there is often
-