140
7.
THE OUTER PARTS OF THE ATOM
,_
j
where N,, is the original number of electrons entering the gas and e 55
is the base of the Naperian system of logarithms. The absorption
coefcient # is given thus as
.
1
.
u =
=
—(7—7)
-
x
«
It will be a comparatively large number whenever the electrons
_
are easily absorbed.
A comparison of effective-cross—sections, Â, determined experimentally by the use of equation 7—6, with those (Âk) calculated
,,
,
by the kinetic theory has shown agreement over a range of inter- '
mediate velocities of the electrons.
However, for electrons with
7
velocities less than a value of approximately 4 >< 108 centimeters
per second (=50 e.v.), there has not been agreement2 in many
cases, thus showing that for these collisions, the atom cannot be;
considered as a solid sphere. It may be seen in gure 7—1, that
cm./sec.
A
’
l
=
:
-
AK
--…____--.…-__
‘
'
2
3
4
s
e
v
s
"
Wous
oc
VELOCITY -—>
FIG. 7—1.
Effective-cross-section (A) of argon atoms for electrons of
\
'
Various velocities.
‘
:
,
forcertain velocities of the electrons, the argon atom offers a much
‘
to the electrons than that given by the k1netlc
;
dotted line). The decrease in 1 ”to the left ofth€
max1mum,forthe slower electrons, is true for a number of gases
andlsknownastheRam—mw eect.
,