A photon of energy 10.2 eV collides inelastically with a hydrogen atom in ground state. After a certain time interval of few microseconds another photon of energy 15.0 e V collides inelastically with the same hydrogen atom,then the observation made by a suitable detector is

10.2 eV photon on collision will excite hydrogen atom to first excited state but hydrogen atom will return to ground state before next collision. For second photon 15 eV > the ionization energy 13.6 ev' The electron in the ground state n = I) is ionised and the excess energy 15 eV - 13.6 eV = 1.4 eV appears as the kinetic energy of the electron.