[pm133]

The ground state for the OH radical is given as X ²Π_i.

Similarly for NO it is given as X ²Π_r.

Does anyone know what the 'i' and 'r' subscripts represent? I can only think of either "imaginary" or "real" but neither make sense.

Any help would be appreciated.

[pm133]

It's OK I have found the answer in a dusty 1950's spectroscopy book which I found by luck stored in the wrong place in our library.

For anyone who ever wants to know this here is the answer..

The "r" means the energy of the states follows the order of the multiplicity once degeneracy is broken (regular or normal multiplicity). "i" means the order of the states is inverted.

i.e. for a ²Π state you'll have Ω = 1/2 and 3/2.
Normal multiplicity would mean (I think) that Ω = 1/2 would be lower in energy with Inverted multiplicity being the other way round.