Structural properties of tetracene films on Ag(1Â 1Â 1) investigated by SPA-LEED and TPD

The adsorption of tetracene (Tc) on Ag(1 1 1) was investigated by high resolution LEED (SPA-LEED) and temperature programmed desorption (TPD). Two long-range ordered monolayer phases (α, β) were found after Tc evaporation at sample temperatures of 300 K and 230 K, respectively, and subsequent cooling below 230 K. The metastable β phase exhibits a 44% larger coverage than the α phase, and can be transformed into the α phase by annealing. At room temperature the long-range order is lost. At submonolayer coverage, only short-range order was observed, even at low temperatures. The unit cell of the α phase contains one molecule in planar geometry. There are two molecules within the unit cell of the β phase, which are presumably tilted with respect to the surface. Multilayer growth proceeds in a pronounced Stranski-Krastanov mode, leaving a large part of the monolayer uncovered. The TPD peak of the monolayer is very broad (300-550 K). The zero-order multilayer TPD peak has its onset at about ∼280 K and yields a desorption energy of 120-130 kJ mol−1.