204 PHYWE Systeme GmbH & Co. KG · D-37070 GöttingenLaboratory Experiments Physics
5.1.03-15 Franck-Hertz experiment with Ne-tube
Principle:
Electrons are accelerated in a tube
filled with neon vapour.
The excitation energy of neon is determined
from the distance between
the equidistant minima of the electron
current in a variable opposing
electric field.
Example of a Franck-Hertz curve for Ne-gas.
Tasks:
1. To record the counter current
strength Is in a Franck-Hertz tube
as a function of the anode voltage
Ua.
2. To determine the excitation energy
Ea from the positions of the
current strength minima or maxima
by difference formation.
Franck-Hertz operating unit 09105.99 1
Franck-Hertz neon-tube with housing 09105.40 1
5-pin connecting cable, for Ne tube 09105.50 1
Shielded BNC-cable, l = 75 cm 07542.11 1
RS232 data cable 14602.00 1
Franck-Hertz software 14522.61 1
PC, Windows® 95 or higher
Optional equipment:
Oscilloscope, 30 MHz, 2 channels 11459.95 1
Adapter, BNC-socket/4mm plug pair 07542.27 2
Screened cable, BNC, l = 75 cm 07542.11 2
What you need:
Complete Equipment Set, Manual on CD-ROM included
Franck-Hertz experiment with Ne-tube P2510315
Physical Structure of Matter Physics of the Electron
What you can learn about …
Energy quantum
Quantum leap
Electron collision
Excitation energy
LEP
5.1.03
-15
Franck-Hertz experiment with Ne-tube
PHYWE series of publications • Laboratory Experiments • Physics • PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen 25103-15 1
Related topics
Energy quanta, quantum jumps, electron collision, excitation
energy, Bohr’s shell model.
Principle
Electrons are accelerated in a tube filled with neon gas. The
excitation energy of neon is determined from the distance between
the equidistant minima of the electron current in a variable
opposing electric field.
Equipment
Franck-Hertz operating unit 09105.99 1
Franck-Hertz neon-tube with housing 09105.40 1
5-pin connecting cable, for Ne tube 09105.50 1
Shielded BNC-cable, l = 75 cm 07542.11 1
RS 232 data cable 14602.00 1
Franck-Hertz software 14522.61 1
PC, Windows®
95 or higher
Optional equipment:
Oscilloscope, 30 MHz, 2 channels 11459.95 1
Adapter, BNC-socket/4mm plug pair 07542.27 2
Screened cable, BNC, l = 75 cm 07542.11 2
Tasks
Record the countercurrent strength I in a Franck-Hertz tube
as a function of the anode voltage U. Determine the excitation
energy E from the positions of the current strength minima
or maxima by difference formation.
Set-up and procedure
Set up the experiment as shown in Fig. 1. For details see the
operating instructions of the unit 09105.99. Connect the operating
unit to the computer port COM1, COM2 or to USB port
(use USB to RS232 Adapter Converter 14602.10). Start the
measure program and select Cobra3 Franck-Hertz experiment
Gauge. Select the parameters given in Fig. 2 and press continue
button.
Theory and evaluation
Niels Bohr introduced the planetary model of the atom in
1913: An isolated atom consists of a positively charged nucleus
about which electrons are distributed in successive orbits.
He also postulated that only those orbits occur for which the
angular momentum of the electron is an integral multiple of
h/2p, i.e. n*h/2p, where n is an integer and h is Planck’s con-
stant.
Bohr’s picture of electrons in discrete states with transitions
among those states producing radiation whose frequency is
determined by the energy differences between states can be
derived from the quantum mechanics which replaced classical
mechanics when dealing with structures as small as atoms. It
seems reasonable from the Bohr model that just as electrons
may make transitions down from allowed higher energy states
to lower ones, they may be excited up into higher energy states
by absorbing precisely the amount of energy representing
difference between the lower and higher states. James Franck
and Gustav Hertz showed that this was, indeed, the case in a
series of experiments reported in 1913, the same year that
Bohr presented his model. Franck and Hertz used a beam of
accelerated electrons to measure the energy required to lift
Fig.1: Set-up for the Franck-Hertz experiment with PC.
LEP
5.1.03
-15
Franck-Hertz experiment with Ne-tube
25103-15 PHYWE series of publications • Laboratory Experiments • Physics • PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen2
electrons in the ground state of a gas of mercury atoms to the
first excited state (see experiment 5.1.03-11). In present experiment
a tube filled with neon gas is used.
The electrons emitted by a thermionic cathode are accelerated
between cathode C and anode A in the tube filled with
neon gas (Fig. 3) and are scattered by elastic collision with
neon atoms.
From an anode voltage U1 of 16,8 V, however, the kinetic energy
of the electrons is sufficient to bring the valence electron of
the neon to the first excitation level by an inelastic collision.
Because of the accompanying loss of energy, the electron can
now no longer traverse the opposing field between anode A
and counter electrode S: the current I is at a minimum. If we
now increase the anode voltage further, the kinetic energy of
the electron is again sufficient to surmount the opposing field:
the current strength I increases. When U1 = 2x16.8 V the
kinetic energy is so high that two atoms in succession can be
excited by the same electron: we obtain a second minimum
(Fig. 4). The graph of I/U1 thus shows equidistant maxima and
minima.
These minima are not, however, very well-defined because of
the initial thermal distribution of the electron velocities.
The voltage U1 between anode and cathode is represented by
U1 = U + (␾A –␾C),
where U is the applied voltage, and ␾A and ␾C the work function
voltages of the anode and cathode respectively. As the
ecxitation energy E is determined from the voltage differences
at the minima, the work function voltages are of no significance
here.
According to the classical theory the energy levels to which
the mercury atoms are excited could be random. According to
the quantum theory, however, a definite energy level must
suddenly be assigned to the atom in an elementary process.
The course of the I/UA curve was first explained on the basis
of this view and thus represents a confirmation of the quantum
theory.
The excited neon atom again releases the energy it has absorbed,
with the emission of a photon. When the excitation energy
E is 16.8 eV, the wavelength of this photon is
= 73.8 nm, where
c = 2.9979 · 108
and
h = 4.136 · 10-15
eV.
For our evaluation we determine the voltage values of the
minima. From the differences between these values we obtain
the excitation energy E of the neon atom by taking an aver-
age.
By evaluating the measurements in Fig. 4 we obtained the
value
E = (17.4 ± 0.7) eV.
m
s
l ϭ
ch
E
Fig. 2: Measuring parameters.
Fig. 3: Principle of the measurement.
Fig. 4: Example of a Franck-Hertz curve recorded with Ne-
tube.