We propose optical spectroscopy for investigation of phase transitions of two-dimensional (2D) physisorbed matter. The usefulness is demonstrated by investigation of the physisorbed butadiene iron tricarbonyl [Fe${\mathrm{}\left(\mathrm{CO}\right)\mathrm{}}_3$${\mathrm{C}}_4$${\mathrm{H}}_6$] molecule as our model system. The remission spectrum reveals the melting transitions of this 2D system and its melting phase diagram. Melting of the film is indicated by distinct alteration of the Rayleigh scattering properties of these molecules. Besides, we obtain absorption bands of Fe${\mathrm{}\left(\mathrm{CO}\right)\mathrm{}}_3$${\mathrm{C}}_4$${\mathrm{H}}_6$ in its 2D state (i.e., physisorbed on BN), revealing interesting band splittings. These splittings can be correlated to condensation of these molecules into the 2D plane.

• Received 1 November 1983

DOI:https://doi.org/10.1103/PhysRevB.29.5063