Interband absorption of Ru studied by optical absorption of continuous thin films and small particles

For continuous thin Ru films and Ru island films consisting of Ru particles smaller than about 320 {} in diameter, optical absorption has been measured in the photon energy range of 0.5--6.5 eV. Absorption, found below about 1 eV and at about 2, 3, 3.6, and 4.5--5 eV for the continuous thin films, shifted to higher energies and became weaker with decreasing particle size. This change is accompanied by lattice contraction and is similar to the lattice-contraction-induced change in interband absorption of hcp Co. Thus, the absorption is interband absorption as for hcp Co, and is due to transitions from d states into empty states above the Fermi level. The absorption at about 2, 3, and 3.6 eV shifted almost equally (0.2--0.3 eV) and disappeared at almost the same time below about 70 {} in diameter, indicating that d states are close to each other. The absorption at about 4.5--5 eV was still well defined at about 25 {} in diameter. Cu, Ni, and Pd particles have also shown such well-defined interband absorption. Thus, as for Cu, Ni, and Pd, d states are presumably in flat or parallel bands.