Fig. 1.23 Rate constant of
impact ionization in
X n **
X C C
SF 6 ,
SF 6 !
Na( n 2 P), 12
C SF 6 ! A C C SF 6 is inversely proportional to
the characteristic velocity of the electron in the excited orbit. When changing the
principal quantum number values from 23 to 106, the rate constant values of this
reaction lie in the range (4-4.5) 10 17
The reaction cross section A **
s 1 and are almost independent of n
(Beterov et al. 1987 ; Zollars et al. 1986 )(Fig. 1.23 ).
Large values of the rate constants of the reaction are connected here with the
fact that the excited ion SF 6 *
formed during reaction is a long-lived one with the
lifetime >10 s.
Another situation in collisions of the alkali atoms with the molecule of O 2 is
realized for the intermediate values of the principal quantum number n . In this case,
the lifetime of the excited ion O 2 * is small, i.e., it is of the order of 10 9 s, and
effective electron auto-detachment in the absence of a stabilizing mechanism takes
place. The interaction with the ionic core of the Rydberg atom, which is especially
effective for small n , can be the stabilizing mechanism. Therefore, the rate constant
with the participation of the O 2 molecule should decrease with the increase of n in
contrast to SF 6 . This change explains the small value of the constant (10 12 cm 3
s 1 )
and its decrease with increase of n .
Returning to the above diffusion approximation ( 1.41 )and( 1.42 ), we note the
following. For exothermic AI reactions at low relative kinetic energies of colliding
atoms, entry of the representation point to the spatial domain of the intermediate