A 4000 Å photon is used to break the I₂ molecule. The bond dissociation energy of I₂ is 246.5 kJ/mol. The % of photon energy converted to kinetic energy of iodine atoms is

E_photon = hc/λ = 2×10⁻²⁵/(4000×10⁻¹⁰) = 0.5×10⁻¹⁸ J. E_bond/molecule = 246.5×10³/(6.023×10²³) = 40.92×10⁻²⁰ J. KE = 0.5×10⁻¹⁸ − 40.92×10⁻²⁰ = 9.08×10⁻²⁰ J. % = 9.08/50 × 100 ≈ 18%