A physics textbook I am currently reading states that

"if an electron with kinetic energy equal to 1.12 aJ collides with a single mercury atom in its ground state, the atom may be excited and the electron will lose kinetic energy. If the same atom is hit by a photon with an energy of 1.12 aJ, the photon will not be absorbed, and the atom will not be excited by the photon."

I am a bit confused about this. Why is it that the electron may excite the atom, but the photon may not, when they both carry exactly the same amount of energy?

if anyone can explain the statement to me, then I would greatly appreciate it!

  • $\begingroup$ I suspect it may have to do with selection rules. $\endgroup$ Commented Feb 24, 2022 at 21:01

1 Answer 1


In photon absorption excitation, the photon needs to carry an energy that is exactly the difference between the atom's (Mercury here) ground state and a higher energy state (or an energy that is exactly the difference between any lower energy and a higher energy state).

This is different to collisional excitation where an electron collides with an atom, such that the atom will absorb some fraction of the electron's initial kinetic energy. This fraction of kinetic energy changes the atom's energy (causing excitation) and some kinetic energy is still left over in the scattered electron.

So in collisional excitation, the incoming electron will have an energy greater than that required to excite the atom. On the other hand, for photon absorption, the incident photon needs to have an exact energy, equal to the difference between two energy levels.

  • 1
    $\begingroup$ Awesome! Thanks a lot for a really great and informative answer! $\endgroup$
    – user12277
    Commented Feb 24, 2022 at 22:21
  • $\begingroup$ Good luck with your studies. Cheers $\endgroup$
    – joseph h
    Commented Feb 24, 2022 at 22:22

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