To observe directly the excitation of the first excited state in mercury and to obtain a value for the excitation energy.
When an electron encounters an atom it will bounce off without losing any of its energy unless it has sufficient energy to cause a change in the internal energy of the atom. Since atomic energy levels are quantized, this means that electrons flowing through a gas of atoms with less energy than the first excited state of the atoms will not lose any energy as they travel. On the other hand, an electron with enough energy to cause a transition to an excited state of the atom may induce such a transition with subsequent loss of kinetic energy. Such an event is called an "inelastic scattering" of the electron by the atom.
In this experiment electrons are emitted from a hot cathode into a tube filled with mercury vapor. A constant electric field is established in the tube by a potential applied between the cathode and a grid so the electrons are constantly accelerated. An anode with a small retarding potential is located just past the grid and will collect electrons if they have sufficient energy to overcome the retarding potential. This will be the case unless the electrons have suffered an inelastic collision near the grid, have lost all their kinetic energy there, and have not been further accelerated enough to reach the anode. The circuit is shown in Fig. 1. The grid G1 near the cathode serves to set the total emission current in the tube. As the voltage on grid G2 is increased, the current to G2 will increase, as in an ordinary diode. Some electrons will pass through this grid and, after the potential on G2 exceeds the retarding potential on the anode, will register as (negative) anode current on the picoammeter. When the electrons have fallen through a potential of about 4.9 volts they will have enough energy to excite the first excited state of Mercury. Many electrons will give up their kinetic energy to the excitation of mercury atoms. They will then still be collected by the grid but those passing through the grid will not have enough energy to overcome the retarding potential on the anode and the anode current will drop. As the potential on G2 is increased further, the position where the electron energy loss occurs will be further from G2. The electrons will be accelerated to G2 and those that pass through will have enough energy to get to the anode. The anode current will increase again. At some point the electrons will lose their energy at some midpoint in the tube and be accelerated enough to have 4.9 eV of energy just in front of G2. The anode current will then show a second dip as before. This process can be continued to show three or four or more dips in the anode current as the voltage on G2 is increased.
In order to get well defined peaks and valleys in this experiment it is necessary that the mercury vapor in the tube have the proper pressure. This pressure is a well defined function of the temperature of the mercury. (See the graph posted near the experiment.) The temperature of the tube containing the mercury will be controlled by an oven which is powered through a constant-voltage transformer and a variable transformer. However, the mercury in the tube is heated also by the filament and by the passage of current through the tube, and so is not immediately subject to measurement. It is something of a matter of good or bad fortune to get the oven heater properly set. Generally, if the pressure is too high the emission current (to G2) will be small (less than a few nanoamps). Variation of the voltage on G1 will have no effect until it exceeds 5 volts or so. If the pressure is too low the emission current will be high and the minima will be hard to see. The experiment was recently performed successfully with the oven heater voltage at 38 VAC, Vg1 = 4 V, and Vanode = -0.6 V. A thermocouple inserted into the oven gave a temperature of 142 C. A 20 percent dip was observed at Vg2 = 9 V.
The initial setup of this experiment requires several hours to heat the tube and achieve thermal equilibrium. This will have been accomplished before the lab period begins.
Verify that a dip in the anode current can be seen as Vg2 is varied. Then take readings of anode current versus Vg2 in steps of about 0.5 volts from zero up to about 25 volts. Your data should show three or four dips. Repeat the measurements in the vicinity of the dips in steps of 0.2 volts.
When you are finished, remove the tube from the oven and let it sit for a few minutes to cool off before you turn off the filaments. Otherwise mercury vapor might condense on the filaments and present a damaging short circuit the next time the experiment is activated.
Prepare a graph of the anode current versus Vg2. Include all of your data, using different symbols for the different data runs. Locate as many minima as possible and find the voltage difference between them. Compare this with the expected difference (the first excited state in mercury decays with the emission of a line with a wavelength of 253.6 nm).