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 \( E_P = 2E_Q = 2E_R \). 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 \( 4.0 \,\text{eV} \), \( 4.5 \,\text{eV} \) and \( 5.5 \,\text{eV} \), respectively. The energy of the incident photon used for metal \( R \), in \( \text{eV} \), is _____.
