Question

Why do atomic emission lines have such narrow bandwidths relative to molecular emission lines?

Answer 1

Generally, in the case of molecules, absorption occurs from the lowest vibrational level of the electronic ground state (S=0, V0) to different vibration levels of the electronically excited state. This results in a broadening in the absorption spectra. Similarly, during emission also, it is coming from S1, V=0 state to various vibrational levels of the electronic ground state. In the case of atoms, vibrational levels are absent. Thus we are getting a sharp peak.