Some material consisting of a collection of microscopic systems is kept at a high temperature, so that all excited states are populated and can participate in emission of photons. A photon detector capable of detecting photon energies from infrared through ultraviolet observes photons emitted with energies of 0.3 eV, 0.5 eV, 0.8 eV, 2.0 eV, 2.5 eV, and 2.8 eV. These are the only photon energy observed.
a) Draw and label a possible energy-level diagram for one of the microscopic systems, which has 4 bound states. On the diagram, indicate the transitions corresponding to the emitted photons. Explain briefly.
b) The material is now cooled down to a very low temperature, and the photon detector stops detecting photon emissions. Next a beam of light with a continuous range of energies from infrared thorugh ultraviolet shines on the material, and the photon detector observes the beam of light after it passes through the material. What photon energies in this beam of light are observed to be significantly reduced in intensity ("dark absorption lines")? Explain briefly.