Geoscience Reference
In-Depth Information
1.9.2
Asymmetrical Collisions with Molecules
In this case we can speak about the internal degrees of freedom of the complex
with a large number of possible reaction channels, respectively, and the number of
possible resonances with an energy defect
E
0. A model of the energy transfer
of the molecular rotation to the Rydberg electron used to explain the experimental
data was suggested by Matsuzawa (
1979
). The experimental values of the ionization
cross sections in collisions with the polar and the quadrupole molecules at
T
300
are of the order of 10
14
to 10
12
cm
2
. These cross sections are proportional to
n
3
and the square of the dipole moment of the polar molecules and weakly dependent
on the quantum numbers
n
and
l
.
Because the frequencies of transitions in the atom can be of the order of energies
of the vibrational transitions in the molecule, one can observe the resonant character
of the energy transfer of the excited atom to the vibrations of the molecule
Na
.n/
C
CH
4
!
Na
C
.CH
4
/
(1.74)
Collisions of the Rydberg atoms with the halogen molecules with the positive
electron affinity
PA
C
BC
!
A
C
C
BC
(1.75)
lead to negative ion formation. For example, the negative H
and He
ions appear
according to this scheme in the tandem (double) electrostatic charged-particle
accelerators using the same potential difference, when the ion A
C
changes its sign
in the result of the charge exchange.
The exothermic process cross sections of the ion pair formation depend on the
value of the vibrational excitation of the electronegative molecules. The influence
of the vibrational excitation has several reasons, such as a decrease in the energy
threshold of the reaction and the change in the vertical energy of the electron affinity.
Rise of vibrational excitation leads to a significant increase in the cross sections
and the corresponding rate constants. For example, the value of the rate constant is
approximately 10
7
cm
3
s
1
in the case of halogen-containing molecules.
The yield of the negative ions in thermal collisions with Rydberg atoms is
determined by the efficiency of the attachment process. This concept is confirmed by
the theory and measurements of the rate constants of the Rydberg atom ionization
with principal quantum numbers
n
D
20-100 in collisions with halide molecules.
For example, the slow Rydberg atom-molecule collisions with the formation of
negative ions and free electron capture by the molecules are similar in effectiveness,
as it follows from the Fermi quasi-free electron representation. Therefore, the cross
section of the collision of highly excited atoms A
**
with the halogen molecules,
calculated in the impulse approximation, is determined by the cross section of the
free electron scattering at the energies of about 3.10 eV.
