16 2. COLLISIONS IN PLASMAS
It should be noted that, in the case of molecules, the probability of resonant charge transfer
is low, due to the many open channels of vibrational and rotational excitation of the molecule
which violate the condition of resonant energy levels.
2.8 POLARIZATION SCATTERING
Polarization scattering is related to the scattering of a charged particle at a neutral gas atom, the
electron cloud of which polarizes when the charged particle is approaching (see Fig. 2.8).
q
1
R
+
+
–
Figure 2.8: Polarization of a neutral atom during the passage of a charged particle.
e electric field of the charged particle is
E
E D
q
1
4"
0
R
2
E
R
R
: (2.48)
With the polarizability ˛, the induced dipole moment of the atom becomes
Ep D ˛
E
E (2.49)
and the potential energy
V D Ep
E
E D ˛E
2
D
q
2
1
.
4"
0
/
2
˛
R
4
: (2.50)
e strong radial dependence of this potential results below a certain impact parameter p
L
in a
trajectory which spirals into the neutral, i.e., the charged particle is captured. is situation is
sketched in Fig. 2.9.
Calculations show for the corresponding “Langevin capture cross section”
L
D p
2
L
D
s
˛q
2
1
"
0
1
r
; (2.51)
where
r
is the relative velocity of the collision partners before the collision.
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