1. Frank Hertz Experiment
Confirmation of energy levels in an atom
Elastic & Inelastic collisions

2. Setup of Frank-Hertz Experiment

3. Setup of Frank-Hertz Experiment

4. Result of Frank-Hertz Experiment

6. This original Franck-Hertz data shows electrons losing 4
This original Franck-Hertz data shows electrons losing 4.9 eV per collision with mercury atoms. It is possible to observe ten sequential bumps at intervals of 4.9 volts.

7. Collisions of electrons with Mercury atoms

8. Conclusions: direct evidence for the existence of energy level
electrons have collisions with the atoms of a gas at low
pressure
some electrons undergo elastic collisions, collide with massive
gas molecules
- some electrons undergo inelastic collisions, e- in gas molecules
gains exactly the amount of energy it requires to reach a higher
energy level.
Energy lost by colliding e- does not appear as K.E. of the atom
but is emitted as EM radiation, 10 –19 s
p.d. through which the bombarding e- has to be accelerated
from rest to cause excitation – excitation potential
ionization p.d.

9. The Frank-Hertz experiment shows that mercury has energy levels at E1, E2 = E eV and E3 = E eV. (a) Predict the wavelengths of the line spectrum of mercury. E1 E2 E3
E1+4.9
E1+6.7

10. Electrons are accelerated in the Franck-Hertz apparatus and the collected current rises with accelerated voltage. As the Franck-Hertz data shows, when the accelerating voltage reaches 4.9 volts, the current sharply drops, indicating the sharp onset of a new phenomenon which takes enough energy away from the electrons that they cannot reach the collector. This drop is attributed to inelastic collisions between the accelerated electrons and atomic electrons in the mercury atoms.
The sudden onset suggests that the mercury electrons cannot accept energy until it reaches the threshold for elevating them to an excited state. This 4.9 volt excited state corresponds to a strong line in the ultraviolet emission spectrum of mercury at 254 nm (a 4.9eV photon).
Drops in the collected current occur at multiples of 4.9 volts since an accelerated electron which has 4.9 eV of energy removed in a collision can be re-accelerated to produce other such collisions at multiples of 4.9 volts. This experiment was strong confirmation of the idea of quantized atomic energy levels.

11. In the Frank-Hertz apparatus, mercury is vaporized or neon is present in a specially designed vacuum tube, the atoms of the vapor providing targets for a scattering experiment. Electrons are accelerated through the vapor by an applied electric field. (In the discussion below, read neon or mercury wherever mercury is noted.
As soon as they have enough energy, about 5 eV for mercury, they excite the next mercury atom they hit to its first excited state. This is observed as a drop in the current of electrons passing through the tube. Several such drops in the beam current can be seen as the acceleration voltage is increased, corresponding to the electron exciting one, two, three,...atoms during its passage through the tube.
(Note: the conditions of the neon tube and spectrum are such that only 3 drops may be seen. If one is fortunate, as many as six drops in current may be observed in mercury.)
The excitation energy can be determined from the spacing of the peaks.

12. END