then $\varepsilon_1(i\zeta)$ can be expressed as $\varepsilon_1(i\zeta)=\tilde{n}^2(i\zeta)$. This is true because both functions $\tilde{n}$ and $\varepsilon_1$ are analytical.

The resulting dielectric function of water calculated at the imaginary frequencies is presented in Fig. \[fig4\]. The inset shows the compiled data [@Seg81] for the absorption coefficient of water $\mu(\lambda)$. Our result is close to that in Ref. [@Dag00] and deviates from both in [@Per81] and [@Rot96]. It is interesting to note that $\varepsilon_1(i\zeta)$ is still far from its static value $\varepsilon_1(0)\approx 80$ even at $\zeta=0.01\; eV$. The static value is reached only at $\zeta
\sim 10^{-6}\; eV$. This is a specific property of water.

![The dielectric function of water at imaginary frequencies. The blue curve is for bulk water (no free volume, $f=0$). The red curve corresponds to water with 50% of free volume, $f=0.5$ (see Sec. \[Sec3.4\]). The inset shows the compiled data for absorption coefficient used as input data to calculate $\varepsilon_1(i\zeta)$.[]{data-label="fig4"}](fig4.eps){width="45.00000%"}

Water film and optical data {#Sec3.3}
---------------------------

The optical properties of our gold films were measured at ambient conditions [@Sve08]. An ultrathin film of water is already incorporated in the optical response of these films. It means that the force calculated with the dielectric function of nominal 