Scanning Tunneling Microscopy study of the dynamical processes of atoms and molecules on transition metal surfaces

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The thermal decomposition processes of acetylen (C_2H_2) on the Pt(111) surface have been investigated by the use of variable-temperature Scanning Tunneling Microscopy (STM). Generally, C_2H_2 is dehydrogenated to form ethylidyne on the Pt(111) surface. Since C_2H_2 has only two hydrogen atoms, ethylidyne formation cannot occur by an intramolecular hydrogen rearrangement and the several reaction steps are expected to occur simultaneously. High resolution electron energy loss spectroscopy studies indicate that C_2H_2 is stable until 300 K and furthermore heating to higher temperature of 370 K leads to the formation of ethylidyne. On the other hand, sum frequency generation study says that C_2H_2 is changed to vinylidene even at 120 K and is converted to ethlidyne by several steps at the temperature range of 120-350 K.Our STM investigation has been made in order to make clear about this point. At 30 K, C_2H_2 molecules are randomly distributed on the Pt(111) surface and do not give ordered structure for all the coverage regime below 1 monolayer. Heating the C_2H_2-saturated surface to 120 K, local 2x2 ordered regions are observed. Heating to 220 K, surface is covered with the 2x2 structure with the band-like structures which correspond to the boundaries between the patches. Larger protrusions in the 2x2 structure appear and the number of them is increased by furthermore annealing to higher temperatures. On the otherhand, no ordered structure is observed at 120 K for a low coverage regime. By heating 220 K, 2x2 islands are observed. Therefore, repulsive and attractive interactions exist between the species which are observed at 120 K and 220 K, respectively. This indicates that the species which are observed at 120 K are different from the ones at 220 K.Therefore, present STM results support the latter reaction model based on the sm frequency generation study.