I was presenting my data on the Frank-Hertz lab to a lab class when I was asked by the professor why the interactions only happened in intervals of 4.9eV. Why weren't the free electrons exciting the mercury atoms to other energy levels? My answer was that I suspected at these temperatures and energies, this interaction was simply much more likely than other interactions, but I could not explain this physically. Researching online, the only information that I could find relevant to this was that the 4.9eV interactions occur up 100ev, after which other effects become significant. I was hoping someone here had a substantive answer why the electrons only seem to interact with mercury in intervals of 4.9ev. Thank you!