PHYSICS 320 LAB
THE FRANCK-HERTZ EXPERIMENT
To demonstrate, through the study of collisions between electrons and gas molecules, that the energy is indeed quantized in atomic interactions.
A mercury-filled Franck-Hertz tube, an electric oven, a neon-filled Franck-Hertz tube, a control unit providing various power supplies and a DC current amplifier, and digital oscilloscope.
Simplified Circuit for Franck-Hertz Experiment
In an oven-heated vacuum tube containing mercury
gas, electrons are emitted by a heated cathode, and then accelerated toward a grid that is
at a potential, Va, relative to the cathode. The anode (plate) is at a lower
potential, Vp = Va - DV. If electrons
have sufficient energy when they reach the
grid, some will pass through and reach the anode. They will be measured as current Ic
by the ammeter. If the electrons do not have sufficient energy when they reach the grid,
they will be slowed by DV, and will fall back to the
grid. As long as the
electron/molecule collisions are elastic, the collector current depends only on Va
since the electrons lose no energy. However, Franck and Hertz discovered that Ic
went through a series of maxima and minima as Va was varied. This implies that
the gas molecules absorb energy from the electrons only at specific electron energies
For example, the first excited state of mercury is 4.9 eV above the ground state. This is thus the minimum energy that mercury atoms can absorb from the accelerated electrons. Hence, if Va< 4.9 volts, any collisions are elastic and if Va >DV, many electrons pass through the grid and reach the anode, to be measured as Ic. If Va= 4.9 volts, the electrons gain enough kinetic energy to collide inelastically with the mercury atoms just when they reach the grid. In these interactions, the mercury atoms absorb 4.9 eV. Thus, the electrons lose the same amount and no longer have sufficient energy to overcome DV. They fall back to the grid and Ic is a minimum. As Va is raised beyond 4.9 volts, Ic increases again. However, when Va reaches 9.8 volts, the electrons can lose all their energy in two collisions with mercury atoms in two inelastic collisions between the cathode and grid. Again, these are pushed back onto the grid, and Ic falls to a minimum. Current minimum are found whenever Va is a multiple of 4.9 volts.
This simplified description neglects contact potentials. Therefore, Va will need to be somewhat higher than 4.9 volts when the first minimum occurs. Nevertheless, all successive current minima should differ by multiples of 4.9 volts from the first minimum. The spectral frequency corresponding to this energy is 1.18 x 10-15 Hz and the wavelength is 253.7 nm. In their original experiments, Franck and Hertz verified the presence of the ultraviolet radiation with the aid of a quartz spectrometer.
Neon has 10 energy states in the range between 18.3eV and 19.5eV. From these excited levels, the Ne atoms decay to other excited states. These intermediate states decay to the ground state by emitting visible radiation and can be seen in a tube with the room darkened.
|M = Plate or Anode||A = Grid (not Anode!)||H = Filament Heater||K = Cathode||Ub = Accelerating voltage Va|
Heater = a little less than midrange
Accelerating voltage = fully counterclockwise (Va)
Amplitude = nearly all the way counterclockwise
Reverse bias = fully counterclockwise
Switch Va = ramp (a sawtooth waveform voltage-60 Hz)
Note that on the oscilloscope trace the vertical deflection is proportional to the anode current Ic, and the input to the x-channel of the oscilloscope is equal to Va.
Use the digital oscilloscope to average traces for both channels. Observe and save the traces in both the dual trace and X-Y modes. Explain why the current peaks are not evenly spaced in the dual trace mode. Now transfer the two time traces to a computer. While you can observe on the oscilloscope the traces in X-Y mode, you will have to transfer the traces separately and then recombine them in other software (preferably Origin) to obtain Ic vs. Va.
Read the data into Origin and plot the anode signal vs. the ramp voltage. On the plot, use the cross-hairs tool to measure values of Va for which the collector current (Ic) is a minimum and compute the separation between adjacent dips. Then tabulate the voltage difference between adjacent maxima. Compute a 90% confidence interval for your results.
You should find that the current minima and maxima are spaced at intervals of ~4.9 volts, showing that the excitation energy of the mercury atom is ~ 4.9 eV. Compare this energy with your 90% confidence interval and explain any deviation.
Repeat the analysis for the Neon tube. You will need a higher Va and reverse bias for this tube.
Convert your excitation energy values for Hg and Ne in to nanometers and identify the EM spectral region in which they lie. Draw an energy level diagram for the two elements. How are the differences in them manifested in your Ic vs. Va data?
What effect does the spread in kinetic energy of the incident electrons affect your data?