Light induced plasmon excitations of copper nano-clusters have been probed using spectroscopic ellipsometry. Copper clusters of typically 20 nm diameter were created on the surface of a silicon wafer by means of physical vapour deposition. The growth of the clusters as well as the chemical interaction with atmosphere are monitored. Clear plasmon excitations are observed with resonance positions in the range from 1.85 to 2.1 eV for copper in its metallic state. The resonance position and amplitude scale linear with deposited amounts. Changes in resonance position and amplitude are observed when copper nano-clusters are exposed to atmosphere and a sub-nanometer thin copper oxide is formed. The resonance position red-shifts 0.4 eV (180 nm wavelength) after one day of exposure with respect to its metallic state. Spectroscopic ellipsometry is shown to have good sensitivity to ultra-thin (down to sub-nanometer thick) metal-oxide layers as well as to nanometer size changes of the copper nano-clusters. This puts the technique in a position where real-time monitoring of plasmon active material in gaseous or liquid environments of, for example, (photo-) catalytic processes becomes accessible.