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Metal nanofilms thinner than the skin depth can be studied well with infrared (IR) spectroscopy. The in-plane film conductivity with its correlation to the film-growth process can be measured without electrical contacts and macroscopic surface defects do not disturb the results. In contrast to studies in the visible range, the analysis of IR spectra is straightforward since interband transitions start at much higher energies for most of the typical metals. The parameters of the Drude model can be determined with high accuracy. Extrapolation to the static limit gives the dc conductivity. In the IR range the low-frequency tails of metal-particle absorption as well as the dynamic conductivity of coalesced films give strong signals with typical spectral slope, which allows to obtain information on the morphology development of ultrathin metal films. The power of IR spectroscopy as an in situ control of film growth under UHV conditions will be demonstrated with several examples. |