Evidence for the importance of kink sites

As shown in Fig. 4 the initial adsorption probability, S0, for oxygen on Ag(l 1 0) decreases smoothly with T and goes to zero only when desorption of the atomic state becomes important. At room temperature the ratio between dissociation and desorption probability is accordingly 2 to 1 in favour of dissociation. Such ratio can only be justified if dissociation occurs also at flat terrace sites [44]. On Ag (100), on the contrary, S0 drops abruptly above T = 170 K, i.e. as soon as the lifetime of the molecular precursor becomes smaller than the typical time scale of KW experiments. Such behaviour indicates 

Kinks and open steps with (110) orientation were therefore strongly suspected to play an active role for 02 dissociation at otherwise much more inert Ag(l 0 0) surfaces. 

Ethylene molecules are known to physisorb at low crystal temperature. The binding energy in this state was estimated to be 0.25 eV from isothermal desorption experiments on Ag(l 0 0) [84]. Near edge X-rays absorption fine structure showed that the admolecules occupy the fourfold hollows on Ag(l 0 0) with the axis parallel to the surface [85, 86]. The sticking probability into the physisorption well is inhibited for rotationally excited gas-phase molecules [84]. 

In Fig. 5 we compare HREEL spectra recorded after exposing the flat and stepped Ag surfaces at T = 105 K to small amounts of 02 dosed with E[ = 0.39 eY and at a crystal temperature T = 105 K. The angle of incidence was chosen normal to the crystal for Ag(l 0 0) and nearly normal to the (1 1 0) nanofacets for Ag(4 1 0) and Ag(2 1 0). HREEL spectra indicate that at this temperature only ad-molecules are observed on Ag(l 00), at least for small exposures. This is witnessed in the HREEL spectra by the loss at 81 meV [55], corresponding to the internal stretch motion of adsorbed 02, and by the absence of intensity in the frequency region of the O/Ag stretch, between 30 meV and 40 meV [62]. On Ag(4 1 0) partial dissociation occurs since two Ag/O stretch losses are present, at 32 meV and at 40meV, while the internal 02 vibration is visible at 84meV [96]. On Ag(2 1 0), on the contrary, only the low frequency losses are present, indicating that the admolecules are unstable [97]. Our first conclusion is therefore that open steps cause 02 dissociation and that this mechanism is very effective on Ag(2 1 0) and less efficient on Ag(4 1 0) where the terraces have a finite width. Also in this latter case, 

At the largest exposures a small shoulder at 69 meV is observed for Ag(41 0), which is particularly evident in the middle spectrum of Fig. 6B (0 = 31°). Such peak persists up to temperatures at which 

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