Why Is Spin-Orbit Coupling Selective in Phosphorescence Transitions?

[Gavroy]

hi

i have still some troubles to understand phosphorescence:

http://en.wikipedia.org/wiki/Phosphorescence"

wikipedia has a diagram, where an electron in an exicted state that is a singulet state falls on a second level that is a triplet state.

my problem is, that this first transition is explained by spin-orbit coupling. but is it no so, that the ground state and this triplet state should have spin-orbit coupling either?- but in this case, the transition from the triplet state to the ground state should also be possible.

so it is possible to have this coupling just between some states?

sorry about my english.

 

[Gavroy]

does nobody have an idea or is there anything wrong with my question?

 

[DrDu]

Of course there is also spin orbit (SO) coupling between S0 and T0, if not you would not observe phosphorescence.
The point is rather that for the transition from S1 to T0 you need both SO coupling and vibronic coupling while for the transition from T0 to S0 you need both SO coupling and coupling to the radiation field.
As in the case of fluorescence, the vibronic coupling between S0 and T0 is rather inefficient due to the large energy gap (Kasha's rule: http://en.wikipedia.org/wiki/Kasha's_rule )