What is the excitation energy of the He atom? I imagine this is also the energy of the photon released by the electron as it deexcites from the p orbital to the s orbital. I guess this would be different to ionisation energy as that refers to taking the electron away completely from the atom. Is it explained here with 9 different excitation levels? http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/atspect.html It looks like going from 1s to 2p recquires 21eV. But from this graph http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/atspect.html it looks like the excitation energies are around 2.5eV corresponging to around 500nm wavelength photons. Is that because they only showed excitations not from 1s but from 2s and above? In fact the excitation from 1s would recquire 21eV or more corresponding to a photon of wavelength in the ultaviolet range so can't be seen. The site lists ones that can be seen so can't be the 21eV photons.