Spectroscopic ellipsometry was used for the generation and study of the hybrid TPP-SPP mode as a sensor probe for the real-time formation of amalgam structures on the surface of a plasmon active gold layer. The Au/Hg amalgam formation features and the mercury atoms' penetration into the gold layer were determined by means of the experimental TIRE data and a regression analysis of a multi-layer model containing the index-profile amalgam layer. The hybrid TPP-SPP mode behavior of the coupled excitations provided more information about the mercury atoms' penetration into the gold layer than the single TPP and SPP resonances did. The present study demonstrated the possibility of using the hybrid TPP-SPP mode to design advanced optical gas sensor technologies.