Abstract
Optical absorption of Al island films consisting of Al particles smaller than about 1200 \AA{} in diameter has been measured in the energy range of 0.5--6.5 eV. Below about 200 \AA{} 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 (1993)] 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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