When speaking about these quantum interactions, most of it is conceptual.
A massless photon doesn't "hit" an electron when it is absorbed. Because it is massless and basically made of energy, that energy is absorbed by the electron which has been discussed. There are quite large gaps between molecules, but there are electrical and weak nuclear forces which account for many of the interactions we are talking about here.
http://acept.la.asu.edu/PiN/rdg/visnxray/visnxray.shtml can explain better than I the basics of the process of photon absorbtion.
It's well understood what wavelengths excite certain types of electrons.
http://wine1.sb.fsu.edu/chm1045/note...m/Struct02.htm discusses how to calculate this.
Here is a study observing multiphon absorption mechanism of TMTTF (a polarized molecule)
http://www.edpsciences.org/articles/...jp4114024.html Click on the pdf link at the top for the full article
I apologize for all the links, but there is no easy way to summarize all this Quantum Theory in any concise form.