For continuous thin Ir films and Ir island films consisting of Ir particles larger than about 25 Å in diameter, optical absorption has been measured in the photon energy range of 0.5–6.5 eV. Interband absorption does not exist at about 1.1 eV. Absorption at about 3.1 eV is identified as interband absorption because its particle-size dependent change is similar to that of interband absorption of Cu particles. Interband absorption at about 4.4 eV is suggested to arise from transitions between bands 2 and 5 near L in the direction Q. Absorption due to plasma oscillations of conduction electrons is absent in the island films, showing that the conduction electrons are localized by a strong correlation interaction between the conduction electrons having strong d character. Strong d character based on large hybridization of s and d bands is responsible for the strong correlation interaction.
Optical plasma-resonance absorption of Pt island films consisting of Pt particles larger than about 25 Å in diameter has been measured in the photon energy range of 0.5-6.5 eV. As in Rh and Pd island films reported previously, the broadening of the optical plasma-resonance absorption reflects a correlation interaction between conduction electrons. Comparison of the broadening for the Pt island films with that for the Rh island films shows that the correlation interaction is strong when the conduction-electron density n is low. In an electron-gas model, the correlation interaction between electrons becomes stronger with lowering electron density, because the magnitude ratio of the Coulomb to kinetic energy increases as the electron density lowers. Thus, the strong correlation-interaction at low n proves that the correlation interaction in transition metals becomes stronger with magnitude ratio. The magnitude ratio in transition metals is pointed out to increase with lowering n and/or with strengthening d character of conduction electrons. Based on the correlation interaction, reflected by the broadening for the Pt, Rh, and Pd island films, and on the strong correlation interaction, found previously for Ir, the order of magnitude ratio is IrϾPtϾPdϾRh.
Optical absorption of Al island films consisting of Al particles smaller than about 1200 Å in diameter has been measured in the energy range of 0.5-6.5 eV. Below about 200 Å in diameter, parallel band absorption, due to transitions between almost parallel bands, weakens and then disappears with decreasing particle size. Nguyen et al. ͓Phys. Rev. B 47, 3947 ͑1993͔͒ have reported that the parallel band absorption of Al particles is not visible when relaxation times for conduction electrons and the parallel band absorption are much smaller than those in the bulk. We investigate the relaxation times by simulating the optical plasma-resonance absorption of the Al island films with a Maxwell-Garnet-type effective medium theory. The very small relaxation times are pointed out to be a possible cause of the weakening and disappearance.
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