How do we measure the energies of molecular orbitals? [closed]








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This question came on my mind after a sentence of a teacher who said during a lesson:



> 
> If you imagine hydrofluoric acid, to make the MO's diagram because the
>  energy gap between the $\text{2s}$ of fluorine and the $\text{1s}$ of
>  hydrogen is quite important you can make the hypothesis at first there
>  is no interaction between both. But experiments show that the bonding
>  MO's of $\ce{HF}$ has an energy a bit different than the energy of the
>  $\text{2s}$ of fluorine.
> 
> 
> 


At first it not supprised me that much, I knew it. But know there is one question. 




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**Question's':**


How is that possible to measure the energy of a specific MO? Is that also possible if there is no electron in the MO or if it is an anti-bonding MO? 


