Luminescence vs. X-ray emission

Question

When a target atom is struck by some kind of radiation (for example, a $\text{MeV}$ proton), electrons from lower shells are kicked off and replaced by electrons from higher shells, which in return emit electromagnetic radiation (photons). Energies of photons emitted during these transitions are specific for every element and they can help us identify elemental composition of the target. Energies corresponding to certain transitions ($\text{K}\alpha$, $\text{K}\beta$, $\text{L}\alpha$, $\text{L}\beta$ lines, etc.) can be found in this table, and they are (mostly) from the X-ray part of the spectrum.

As for luminescence, from what I read, it also takes place during transitions of electrons between shells. But, if energy gaps are the same, I can't understand how can energy ever be emitted in the visible part of the spectrum. I am obviously missing something here. Maybe luminescence occurs only for crystalline structures, but can never happen for one atom alone?

So, to sum up: When does luminescence occur? Why and when are electrons transitions followed by emission of photons in visible part of the spectrum instead of X-ray? Some examples could also be helpful. Thanks!

Answer 1

I'll try my best to answer; Luminescence would be what we call a "cold" emission of light, in opposition to incadescing.

Such emission are the result of electric interaction between charge particule such as electrons. When interacting or changing level (when they'll emit light, they might not drop the the inital energy level or fall from the highest energy level reached), they'll emit light. The type of emission depends on the source of the excitement (e.g. chemical vs electric).

As for the second part of when and why it the emitted light is in the visible spectrum, it is closely linked (of my understanding) to the level split between the interracting atoms, energy level mouvement and where/when does the electron jumps/emit the light.

An example of luminescence could be enzyme (like fireflies) on specific substrate, or chemical reactions. Another would be fluorescence. Electro luminescence (those pesky super bright blue light on cars) would be another example.

Hope it helps, cheers