There is a higher energy 2Σ+ state with a bond length of 1.15 Å for CN molecule, explain this

[![Term symbol and bond lengths for CN](https://i.stack.imgur.com/1EFyF.png)](https://i.stack.imgur.com/1EFyF.png)


How can there be an excited state for CN where the bond length is lower (stronger bond) than the actual ground state? The molecular orbital diagram suggests that the single electron in the sigma orbital gets excited, but to which orbital? This was a question asked by one of my professors and I don't know where to start, I think that the ground state X 2Σ+ goes to the B 2Σ+ excited state and maybe the electron could go to the 3S orbital? but then how would this affect the bond long is beyond me. In addition, would the vibrational fine structure go from V'=0 to V'=2? Can anyone help?


