Quantum Mechanics Demonstration with your Cathode Ray Tube Video Monitor
You will need:
(1) A pocket spectrometer to disperse the visible spectrum into a rainbow.
(2) A video display that uses a cathode ray tube (that is, the only vacuum tube in your computer!)
(3) The demonstration will not work with a liquid crystal diaplay (LCD) or an active matrix display.
(4) Flat panel displays based on field emission from negative electron affinity cold cathodes may be suitable (I will check as soon as someone invents such a display).
(5) If you don't have a spectrometer, you can click on a button to make the spectrum appear.
(1) Follow the sequence of screens
(2) Look at each screen with your spectrometer
What do you see?
In a cathode ray tube video monitor, light is produced by the impact of a beam of fast electrons on special chemicals. These chemicals are called phosphors. The chemical charactersitics of the phosphors is such that some of the energy of the beam of electons is converted into visible light. The phosphor is typically a wide band gap semiconductor or insulating material that has been doped with a trace of either a transition metal or rare earth metal.
Following electron impact , electrons in the phosphor are excited across the band gap  from the valence band into the conduction band. As these excited electrons decay back to the valence band, they can pass though intermediate states  located within the band gap. These intermediate states are contributed by the dopant atoms. The spacing of these levels is such that the transitions produce light in the visible spectrum.
Some of these levels are exposed on the outer shell of the dopant atoms. Here they experience the effects of the crystal field. This is the electric field inside the material resulting from the superimposed effects of the charge inside the atoms of the crystal. This field is affected by the ambient temperature which causes the lattice atoms to oscillate. Consequently, the exposed levels are broadened as a result of pertubations from the crystal field. This mainly occurs in the transition metal dopants. This is seen in the green and blue phosphors.
With a rare earth dopant, the levels contributed to the band gap result from the unpaired electrons in the inner 4f shell. The outer shells are filled with paired electrons that do not contribute to the levels in the band gap. The perturbations from the crystal field do not perturb the 4f shell because it is shielded by the filled outer shells. This means that the transitions across the levels based on the screened 4f levels are sharp This is seen in the red phosphor.
An actual visible-light photograph of the luminescence of a small group of phosphors from a computer video display unit. In this case the luminescence was excited by bombardment from 3 MeV protons.
More details on this and other luminescent phenomena are to be found in my lecture notes.
Acknowledgements: I am grateful to Andrew Bettiol for providing the luminescence spectra of the video phosphors.
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