In a photoemission experiment, the maximum kinetic energies of photoelectrons from metals P, Q and R are Eₚ, Eq and Eᵣ, respectively, and they are related by Eₚ = 2Eq = 2Eᵣ. In this experiment, the same source of monochromatic light is used for metals P and Q while a different source of monochromatic light is used for the metal R. The work functions for metals P, Q and R are 4.0 eV, 4.5 eV and 5.5 eV, respectively. The energy of the incident photon used for metal R, in eV, is ___.