Concept Question about Excitation of Atoms

[LHC]

Let's say, for ease of reference, an electron strikes a mercury atom with just enough energy such that this mercury atom is excited to another energy level. What would happen if a photon struck this atom with the same amount of energy? I remember encourtering a True/False problem like this in a textbook, and the answer was that it won't be excited. How is this so?

 

[dynamicsolo]

It sounds like what the question intends is that the electron has a certain amount of kinetic energy, but only transferred a portion of it to an atomic electron to cause its upward "transition" (the colliding electron continues on with reduced KE). The photon's energy is being compared to the electron's initial kinetic energy, rather than the energy required for the transition. Anyway, that's how I'm understanding this.

I guess it isn't clear what is referred to in the question by the phrase "same amount of energy". The same as what?

 

[Moetasim]

This is because in order for an atom to be excited, the incoming energy (whether from an electron or a photon) must be equal to the energy difference between the current energy level and the higher energy level. If the incoming energy is not exactly equal to this difference, the atom will not be excited. Therefore, if the electron already provided the exact amount of energy needed for excitation, the photon with the same amount of energy will not have any additional effect on the atom. This is also known as the principle of conservation of energy, where the total energy of the system must remain constant.