Electronic absorption spectroscopy spin allowed transitions

The spin selection rule is a consequence of the fact that the electric dipole and quadrupole moment operators do not operate on spin. Integration over the spin variables then always yields zero if the spin functions of the two states 0 and are different, and an electronic transition is spin allowed only if the multiplicities of the two states involved are identical. As a result, singlet-triplet absorptions are practically inobservable in the absorption spectra of hydrocarbons, or for that matter, other organic compounds without heavy atoms. Singlet-triplet excitations are readily observed in electron energy loss spectroscopy (EELS), which obeys different selection rules (Kuppermann et al., 1979). 

As is well known from electronic spectroscopy [1], the probability of light absorption (and thus the intensity of the corresponding absorption band) is related to the characteristics of the states involved and particularly to their spin quantum number. Transitions from the ground state to excited states having the same spin value are allowed and give rise to intense bands, whereas transitions to excited states of different spin value are forbidden and... 

The transition type Because luminescence is a technique of electronic spectroscopy, the same selection rules apply for luminescence as those that apply for absorption. For organic compounds, the most common luminescence bands are due to jt - n and jt - n transitions. The a - a transitions, although strongly allowed, are not normally seen because of their high energies in most organic compounds. The spin... 

---