Spectral Line Formation

• A given atom will Absorb and Emit the SAME frequencies of E-M radiation.
  • So a gas of hydrogen will produce an absorption line spectrum if it is between you (your spectrograph) and a continuum light source, and an emission line spectrum if viewed from a different angle.

  

• The E-M radiation frequencies absorbed/emitted map out the allowed energy levels in the atom.

• It turns out that the allowed energy levels in an atom depend mostly on the electric field configuration. So, Hydrogen, with one proton in the nucleus has a different field configuration than does Helium with two protons - this is why the two atoms have a different energy levels and different characteristic absorption and emission lines.

• Why do stars have absorption line spectra? We can think of stars as a hot continuum source with a "cool" atmosphere of absorbing gas. The wavelengths that get absorbed depend on the chemical make up of the gas in the atmosphere.

  Note! this is not quite right on two fronts (more later).

• A gas can also be collisionally excited. Imagine a hot gas. The atoms are flying around, bumping into one another and sometimes the energy of motion during the collision will bump an electron into a higher energy level (or completely ionize the atom). When this electron drops back down to lower energy, a photon is emitted.

  This conversion of kinetic energy into radiant energy cools the gas.

• Hydrogen "recombination" emission lines are very commonly observed. These occur when a H atom is ionized, then at recombination, the electron cascades down through the energy levels, emitting photons long the way. Within the atom, there are different probabilities for the different decay paths. It turns out that one of the most likely transitions is for an electron to drop from the 2nd excited state to the 1st excited state with a red photon emitted in the process. This is the source of the red glow of many astronomical clouds. Here is an example from the WWW and here is another.

  

• There is a connection between Emission lines from a gas and the Continuous spectrum from a solid

  As you crowd atoms together (as in a solid), the allowed energy levels in one atom start to become distorted due to the influence of the electric field of neighboring atoms. Distort an energy level difference a little bit and you get a slightly different frequency emission/absorption line. A distribution of distortions leads to a distribution of lines than eventually blend into a continuum.

  Note: There is much more to it than this. The various vibrational modes of the molecules and compounds formed in the solid begin to become important for example.