If you hit a bound electron with a photon corresponding to that electron's ionization energy, you ionize the electron. What if you hit the electron with a photon with less energy? Will the photon still add its energy to the electron? Will the electron re-emit the photon with less energy? Do you always need the exact amount of energy in a photon to excite an electron to a higher orbital? Can you say hit an electron with 2 photons of less energy to raise it to the same orbital as would be done by a photon with the exact energy of the next orbital?