A photon colloids with a stationary hydrogen atom, in its ground state, inelastically. Energy of the colliding photon is 10.2 eV. After a time interval, of the order of a microsecond, another photon colloids with the same hydrogen atom, inelastically, with an energy of 15 eV. A detector observes:
(1) 2 photons of energy 10.2 eV
(2) 2 photons of energy 10.4 eV
(3) 1 photons of energy 10.2 eV and an electron of energy 1.4 eV
(4) 1 photons of energy 10.2 eV and another photon of energy 1.4 eV