The bound electron waves surrounding atoms can have only discrete energies. When an isolated atom is disturbed in some way, for example by a collision with a high energy particle, an electron wave can have its energy increased to another higher energy discrete state. To be pedantic, the higher energy state need not always be discrete for if the energy is high enough we have a free electron and an ionised atom.
But back to bound electrons. The higher energy electron wave can relax back to a lower energy state by emitting light. The light is, in principle, monochromatic because only discrete electron wave energies exist. In reality, the emitted light is never completely monochromatic. One reason is fundamental, the Heisenberg Uncertainty Principle. The higher the probability of light emission (i.e. the greater the ability to predict the time of its emission) the greater is the light’s frequency spread. Another reason is prosaic. If the atoms are in a high pressure gas, frequent collisions perturb the electron energies and ‘pressure broadening’ spreads the emitted wavelengths.
The degree of monochromaticity is a key difference between street lamps and aurorae.
Street lamp tubes have reasonable pressures compared to the upper atmosphere and therefore have slightly radiation broadened light. Astronomer loathed high pressure sodium street lamps have such extensive pressure broadening that their sky polluting light cannot be filtered.
Auroral discharges are from (in Peter’s case) oxygen atoms in the near vacuum of the upper atmosphere disturbed by particles ejected from Earth’s magnetotail. Quantum mechanics rears its head in that the oxygen high energy state is awkward. Ordinary transitions back to the original low lower energy wave are not permitted because the electron spins of each condition are not compatible. But nearly all things happen in quantum mechanics. The electron energy eventually seeps away in the familiar auroral green glow. But ‘eventually’ is comparable to the age of the Universe in atomic terms. The Uncertainty Principle acts again. The time of the emission is grossly uncertain and its HUP pairing with the radiation bandwidth means that the green light has an extremely narrow wavelength spread. |
