We have investigated hydrogen plasma induced by intense extreme ultraviolet (EUV) radiation by measuring the plasma parameters and population density of radicals, which is expected to clean the tin contamination on the optics in EUV sources, using a time-resolved optical emission spectroscopy. The obtained electron density was n e = (2 ± 0.4) × 1013 cm-3, and the electron temperature was T e = 1 ± 0.2 eV. The electron density was five orders of magnitude higher than that of previous study. The radical population density determined by the experiment was consistent with that calculated with a collisional radiative model, showing excitation and recombination are dominant production processes.