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(3) Collisional excitation: Free electrons (and protons) collide with atoms of heavy elements (e.g. Oxygen), bumping them into a higher energy state. The blast wave of a SNR leads to a high rate of collisions between the atoms in the gas, and this leads to an enhanced rate of collisional excitations.
This type of excitation leads to the radiation of several prominent emission lines in the optical and UV regimes.
A good example of collisional excitation in SNRs is the [O III] doublet which is emitted in two different lines at wavelengths of 4959 and 5007 Angstroms. The Oxygen atoms are floating around (as a relatively small impurity in the gas!), and an oxygen atom can be hit by one of the many free electrons that exist in the ionized gas, in which case the atom picks up some of the kinetic energy of the electron. The excited Oxygen ion can decay radiatively to two (out of its three existing) lowest energy levels, thereby releasing an emission-line photon.
The symbol OIII actually refers (somewhat confusingly) to an Oxygen atom which is an ion that has lost two electrons, and which therefore has two units of positive charge. A much clearer and better Symbol to represent this ion is to use the letter O with a superscript of 2+ , because this symbol clearly shows that two negative charges have been lost by the atom. These two distinct [O III] spectral lines are emitted by the transition of one of these Oxygen ions from an excited energy level to its fine-split ground level (as can be appreciated by drawing a diagram of a simple Bohr Model of an Oxygen atom nucleus and its surrounding electrons that are able to move between different energy levels)
(Another emission line that is emitted by the OIII ion is at 4363 angstroms, This comes from a different transition between electronic energy levels.)
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Interesting discussion. Sorry for the following diversion.
OIII emissions are examples of "forbidden transitions". According to the selection rules for quantum mechanics electrons can carry off single units of orbital angular momentum when going from a ground to excited state and vice versa. The probability of an electron carrying anything other than a single unit of orbital angular momentum is very low.
In fact OIII emissions have never been observed in the laboratory. Excited O atoms in the laboratory lose energy through collisions making forbidden transitions extremely unlikely.
Not so in outer space. The density of your emission nebula, planetary nebula or SNR remnant is so low compared to a "laboratory sample" that the O atoms do not lose energy through collisions. Excited O atoms can exist in a an extended excited transition state, and the probability of a transition to a lower energy or ground state via a forbidden transition is greatly increased.
Before the advent of quantum mechanics the observation of forbidden lines in the spectra of emission nebulae led to the "discovery" of a new element, the appropriately named Nebulium.
Unfortunately Nebulium caused havoc amongst scientists of the time. It effectively threw the Periodic Table classification of elements out the window as Nebulium did not sit anywhere in the table.
Through quantum mechanics the problem was solved, Nebulium was nothing more than excited Oxygen.
Regards
Steven