Linear and non-linear optical spectroscopies are applied to monitor the formation of large, several 10-nm-sized alkali clusters near epitaxially grown Au films on mica substrates. The Au films are covered by ultrathin, self-assembled alkane thiol films, The growth and morphology of these clusters are initially monitored in ultrahigh vacuum by comparison of calculated with measured polarization-dependent extinction spectra. We find that at low surface temperatures (150 K) the cluster growth is very similar to growth directly on insulating substrates. With increasing surface temperature the size distribution of the clusters changes. A quantitative evaluation of ambient-air measurements with scanning force microscopy (SFM) supports the conclusions from optical spectroscopy. Field-enhancement effects at the surface of the clusters facilitate the observation of second harmonic (SH) light. From angular- and polarization-dependent SH-measurements we deduce ratios of chi((2))-tensor components for the cluster-films.