UTModPhy: Experiments: Franck-Hertz Effect

	Franck-Hertz Experiment	FH
	The Bohr model states that electrons are only found at certain energy levels. Are intermediate energies forbidden – bound electrons are always in a Bohr level – or are they only unstable levels – bound electrons will always wind up in such a level? For instance, can you transfer some energy in, put an electron "halfway" between levels, and let it emit a photon to pop back down? Or does something new and interesting happen? We can generate free electrons by thermionic emission, and accelerate them through a tube filled with gas at low pressure. The electrons will collide with the gas atoms and transfer energy; we can control the energy of the electrons by varying the accelerating voltage. You will find that the electrons are indeed forbidden to be between levels. When an atom absorbs or emits a photon, the electron goes from the first energy to the second without being anywhere in between. When an electron collides with an atom, either no energy is transferred (perfectly elastic), or precisely the transition energy is transferred. This experiment has great historical importance as a touchstone highlighting these paradoxes.

	Pre-Lab questions
	Why is it important to use a monatomic gas? According to basic Classical Mechanics, when an electron collides elastically with a mercury atom, what will happen to the atom? To the electron? Should we expect the gas to heat up greatly? What is the first energy level of mercury? Of Neon? What accelerating voltages should we use? What causes the second peak in the graph? Briefly outline the procedure you will follow, including the ranges and variable you will record.

Procedure notes

Take current-voltage graphs at several different settings for both the neon and mercury tubes. Go slowly and be careful of hysteresis. A sudden increase in current indicates that ionization is occurring inside the tube. Promptly reduce the accelerating potential. To get started,

Start with the Neon tube.

Make a directory for your data files

Set gain at first tick mark; Counter voltage at second tick mark.

Load the file called "Start Here," in the Franck-Hertz directory.

Heater voltage in the middle.

Yes, you want to load the calibration.

For mercury oven, "2" is about 150�C.

Do a "save as" into your directory.

	Points to consider during your analysis
	There are many different parameters for you to control; you should demonstrate the effect of (or the irrelevance of) each parameter on your data. Which energy state of mercury did you observe? Why don’t other states come into play? (Historically, the Franck and Hertz experiment provided the first evidence for metastable states).

References

	Franck, J. and G. Hertz, 1916, Physik Zeitz 17, 409: the original paper. Also, see their Nobel Prize lectures.
	"Franck-Hertz Instruction Sheets" (knowledge of German may help :), available from the TA.
	Hanne, "What Really Happens in the Franck-Hertz Experiment", Am. J. Phys. 56 (1998) p.696, is almost as good as its title suggests.
	Mellisinos, A. Experiments in Modern Physics (Academic Press 1966), p. 8ff. An excellent reference for this experiment in general, but his apparatus differs from ours.
	Preston, D.W. and Dietz, E.R., Art of Experimental Physics (Wiley 1991), p 210. Advanced for this class but often contains useful information.
	Harnwell, G. P. and Livingood, J. J. Experimental Atomic Physics (McGraw-Hill, 1933), p. 314.
	Brehm, J.J. and W.J. Mullin. Introduction to the Structure of Matter (Wiley 1989), p.451, 475.
	Tipler, P. Modern Physics (Worth 1978), p 155. These titles discuss the physics of the experiment.

You will absolutely need to read the instruction sheets, Mellisinos and the Hanne paper. Try to do so before you start the lab.