Transcript Frank Hertz Experiment

Frank Hertz
Experiment
Confirmation of energy levels in an atom
Elastic & Inelastic collisions
http://www.physics.gatech.edu/advancedlab/
http://www-hep.physics.uiowa.edu/~reno/p3/e14.html
Setup of Frank-Hertz Experiment
Setup of Frank-Hertz Experiment
Result of Frank-Hertz Experiment
This original FranckHertz 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.
Collisions of electrons with Mercury atoms
http://www.sc.ehu.es/sbweb/fisica/cuantica/frankHertz/frankHertz.htm
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.
The Frank-Hertz experiment shows that mercury has energy levels
at E1, E2 = E1 + 4.9 eV and E3 = E1 + 6.7 eV. (a) Predict the
wavelengths of the line spectrum of mercury.
E3
E1+6.7
1  2 E  4.9 eV   
E2
1.24
 0.253 m ultraviolet
4.9
E1+4.9
1  3 E  6.7 eV    0.185 ultraviolet
E1
E1
1  2 E  4.9 eV   
1.24
 0.253 ultraviolet
4.9
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.
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.
END