Some of these spectral lines can be forbidden on the Earth : in fact, collisions between particles can excite ions to metastable states; in a dense gas these ions immediately collide with other particles and so they de-excite with an allowed transition to an intermediate level, while in the corona it is more probable that this ion remains in its metastable state, until it encounters a photon of the same frequency of the forbidden transition to the lower state.
12.
Dioxygen is a special case as the molecule is paramagnetic so magnetic-dipole-allowed transitions can be observed in the infrared . so that each rotational level is split into three states with total angular momentum ( molecular rotation plus electron spin ) \ mathrm { J \ hbar } \,, J = N + 1, N, and N-1, each J state of this so-called p-type triplet arising from a different orientation of the spin with respect to the rotational motion of the molecule.
