INITIAL-STAGES OF MULTILAYER GROWTH AND STRUCTURAL PHASE-TRANSITIONS OF PHYSISORBED BENZENE ON RU(001)

The initial stages of the multilayer growth of a model system for mole cular solids, namely physisorbed benzene on Ru(001), have been studied in detail by infrared reflection absorption spectroscopy and thermal desorption spectroscopy. A variety of different phases have been discr iminated spectroscopically and characterized in situ: the parallel ori ented first physisorbed layer which is found to rearrange into a more crowded layer with a high tilt angle at slightly higher coverages; an amorphous layer which grows at low temperatures (T less than or equal to 55 K), and a crystalline layer to which the former converts at elev ated temperatures. Clear evidence for structural disorder of the upper most layer of the crystalline phase is found. The amorphous-crystallin e phase transformation is irreversible and the required temperatures v ary considerably with the layer thickness. This is attributed to two d ifferent processes: at high coverages (Theta greater than or equal to 10 ML) crystallization is possible at low T without mass transport and requires only a reorientation and minor rearrangement of the benzene molecules. Low initial coverages (Theta = 2.5-5 ML) require nucleation and diffusion of benzene molecules to form stable 3D crystallites wit h the former process acting as the kinetically limiting factor. Partic ular attention has been devoted to the unravelling of the nature of th e metastable state observed in thermal desorption spectroscopy and its transformation into the more stable crystalline phase. (C) 1996 Ameri can Institute of Physics.