Photon frequency dependent electron relaxation time in noble metals: Effect of voids

Optical properties of noble metals at frequencies below the interband transitions are well described by a simple Drude model of the free electron gas. However, a photon frequency dependent electron relaxation time tau(omega)(-1)=tau(0)(-1)+beta omega(2) has been reported frequently for experimental data. Also, the parameters of the Drude model of silver were found to be influenced by fluids (H. Gugger et al., Phys. Rev. B, 30 (1984) 4189). On the other hand, thin silver films condensed on cold substrates contain a large amount of voids with elongated shape, which cause a strong absorption around lambda approximate to 450 nm. Borensztein et al. (Surf. Sci., 211/212 (1989) 775) modelled this absorption band with an effective medium theory (EMT), taking into account the shape of the voids. In the present work it is shown that both the frequency-dependent relaxation time and the effects of fluids can be explained by this EMT.