Comparing the Energy Levels of Diatomic Molecules

[sams]

Dear Everyone,

If we have a set of diatomic molecules consisting of a common element (F atom) and the other atoms belong to the same group; which are: BeF, MgF, CaF, SrF, and BaF. We want to study the electronic structure of these molecules by comparing their molecular electronic states. We notice that the energy levels of each molecule decrease from one molecule to another, in such a way, the energy is lowered from the highest energy separation: the BeF molecule, than from BeF to MgF, from MgF to CaF, ... , to the smallest energy separation the BaF molecule. This is due to the difference in electronegativity of the Alkaline atoms which lowers the energy of these diatomic molecules. Another important thing is noticed that the order of the electronic states or molecular states of the corresponding molecules are not exactly the same. There is an inversion of some states between these molecules (i,e. The first excited state of BeF molecule would be the second excited state of the MgF molecule and vice versa ...).
Kindly, may anyone please explain this to me what is the reason behind this inversion of these electronic states? Is this inversion is also due to the difference in electronegativity of the Alkaline atoms of these diatomic molecules? This question is a part of a PhD thesis and for research purposes as well, so I really appreciate if someone can help me with this query and if possible, can provide me with useful articles or published papers concerning this subject or matter.

Thank you very much for your precious time and for your support...

 

[DrClaude]

Another important thing is noticed that the order of the electronic states or molecular states of the corresponding molecules are not exactly the same. There is an inversion of some states between these molecules (i,e. The first excited state of BeF molecule would be the second excited state of the MgF molecule and vice versa ...).

Could you please expand on that? Because according to Herzberg, the first excited state for all the molecules you listed is the $^2\Pi$.

 

[sams]

Dear DrClaude,

Thank you very much for your reply and for your consideration. The electronic excited states for the discussed diatomic molecules are not all presented by Herzberg.
To be more precise, I have attached a small part of the energy levels of these diatomic molecules. This image shows the inversion that exists three times between the energy levels: The first between the BeF and the CaF, the second between the CaF and the SrF, and the third between the SrF and the BaF molecules. A circle has been added to these inverted states.

 

[DrDu]

This inversion certainly has also to do with electronegativity differences. But another important point is the presence of d-orbitals for all the metals heavier then Be.

 

[sams]

Thank you DrDu for your reply. But how can we prove that this inversion is due to the electronegativity differences?