User's Manual

SE-9639 Franck-Hertz Experiment
6
012-14264A
When an electron has an inelastic collision with an argon atom, the kinetic
energy lost to the atom causes one of the outer orbital electrons to be
pushed up to the next higher energy level. This excited electron will within
a very short time fall back into the ground state level, emitting energy in
the form of photons. The original bombarding electron is again accelerated
toward the grid anode. Therefore, the excitation energy can be measured in
two ways: by the method outlined above, or by spectral analysis of the
radiation emitted by the excited atom.
Figure 2 displays a typical measurement of the anode current, I
A
, as
a function of the accelerating voltage. As soon as V
G2K
> V
G2A
the
current increases with rising V
G2K
. Notice that the current sharply
decreases for a voltage U
1
and then increases up to U
2
, and then
this pattern recurs. The interpretation of these observations is suc-
cessful with the following assumptions:
• Having reached energy of about e•U
0
, electrons can transmit
their kinetic energy to a discrete excitement state of the argon
atoms.
• As a result of the inelastic collision, they pass the braking volt-
age.
• If their energy is twice the required value, or 2 e•U
0
, they can
collide two times inelastically and similarly for higher volt-
ages.
• As a matter of fact, a strong line can be found for emission and absorption corresponding to an energy of e•U
0
, the exci-
tation energy of argon, in the optical spectrum (108.1 nm).
In figure 2, the resonance voltage is denoted by U
0
.
e•U
0
= hƒ = hc/
or
where e is the charge on an electron, h is Planck’s Constant, and c is the speed of light.
Figure 1.1: Franck-Hertz tube
Figure 1.2: Anode current curve
he
U
0
c
------


=