In a photoemission experiment, the maximum kinetic energies of photoelectrons from metals 𝑃, 𝑄 and 𝑅 are $$E_P, E_Q$$ and $$E_R$$, respectively, and they are related by $$E_P = 2 E_Q = 2 E_R$$. 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 ___.