the preparation method on the level, which cannot be ignored. For this reason $\varepsilon_2$ is essentially unknown.

The relative correction (\[deps\]) is not negligible at low and high frequencies. At low frequencies the ratio $h/\lambda$ is small but $ \langle \varepsilon \rangle $ is large as it is the case for all good conducting materials. At the largest investigated wavelength $\lambda=30\; \mu m$ and $h=1\; nm$, the correction is about 3%. However, for $\lambda>20\; \mu m $ the noise in the data becomes significant, and this correction is below the noise level. If $h$ is larger, we could in principle determine its value. This is because at low frequencies $\varepsilon_2$ can be described by the Drude model, and we can determine $h$ together with the Drude parameters $\omega_p$ and $\gamma$. This procedure was applied for all investigated $Au$ films but minimization gave unreasonable values of the parameters including a negative water layer thickness. Moreover, the target function used in the minimization is larger than that in the case $h=0$. This probably means that $h$ is small enough so that the correction $\delta \varepsilon $ is on the noise level.

In the high frequency limit the correction is defined by the 