If an emission spectrum contains broad emission bands, what could be the origin of these bands?
If the emission bands are broad it could mean more than one thing. How broad are we talking? In a hot gas the particles move faster and in all directions, so emission lines get broadens by doppler shifts in both directions. So a moderately broad electronic transition emission line may be an indicator of high temperatures. Another option is that you are looking at an opticall thin expanding cloud of gas – in this case the material is expanding in all directions so you get a similar doppler shift effect. Alternatively, if there are molecular bands (in the infrared), these vibrational modes are broader than the electronic transitions – so it could be due to molecules instead of atoms.
