I was recently taught that the probability of the photoelectric effect occurring was proportional to Z^3 and E^-3 (where Z is the atomic number, and E is the energy of the photon). My understanding is that the photon's energy must be close to the binding energy of the inner electron to be absorbed, increasing the energy decreases probability because you're getting further from the binding energy value. However, I don't understand how a higher atomic number would increase the probability; I've been told that more tightly bound electrons are more likely to be ejected. Why would that be? Wouldn't tightly bound electrons be difficult to remove?