De-excitation

The excited state of the electron is unstable.The electron moves down to a lower energy level (for example, a vacancy in an inner shell is filled by an outer shell electron).The electron emits a photon when it de-excites.The energy of the emitted photon

hf   =   E_higher - E_lower

De-excitation is the process by which an atom moves from a higher-energy state to a lower-energy state. An atom with an electron in a higher level is in an excited state; the lowest energy state is the ground state.De-excitation can occur by emitting the energy as a photon, or by transferring the energy to another particle in a collision. The electron may return to the ground state in one step, or in several smaller steps, emitting one photon at each step.Because the energy levels are discrete, only certain photon energies are possible. Each transition gives a photon of one frequency, so the light forms a line spectrum. Using c = fλ:λ = hc / (E_higher - E_lower)For example, in hydrogen the n = 3 level is at -1.51 eV and the n = 2 level is at -3.40 eV. The photon emitted in the n = 3 to n = 2 transition has energy 1.89 eV = 3.02 × 10^-19 J, so its frequency is 4.57 × 10¹⁴ Hz and its wavelength is 656 nm (red light).