What is spin multiplicity?
Molecules have an even number of electrons and are in the ground state; all electrons are spin paired. The quantity 2S+1, where S is the total electron spin, is called the spin multiplicity of a state.
Spin orientation on the absorption of light photon
When the spin is paired (↑↓), the upward orientation of the electron spin is cancelled by the downward direction so that S=0, shown in figure(a)
Thus, the spin multiplicity of molecule 1. It is saying that the molecule is in the singlet ground state.
If the absorption of a photon of a suitable energy hv, one of the paired electrons jumps to the higher energy level, and the spin orientation of the two single electrons is either parallel (↑↑), as shown in figure 1(b) or anti-parallel (↑↓) as shown in figure(c).
Suppose spin parallel then,
So, the spin multiplicity of the molecule is 3. It is expressed that the molecule is in the triplet excited state. If spin is anti-parallel, then
The spin multiplicity of the molecule is 1. This is expressed as the molecule is in the singlet excited state.
Electrons can jump to any higher electronic states depending on the energy of the photon.
Singlet excited state, Sn, here n=1,2,3,4……..
S1= first singlet excited state
S2 = second singlet excited state
Same for triplet excited state, Tn Here, n=1,2,3,4….
T1 = first Triplet excited state
T2 = second Triplet excited state
