No desorption from Pt 1 11 -Ge surface alloy

The Pt(l 11) surface alloyed by a small amount of Ge is a nice alloy substrate for studying the origin of the desorption activity in UV laser-induced desorption [87]. This surface alloy is prepared by repeated cycles of deposition of a few ML Ge and subsequent annealing to 1100°C until a constant Ge Auger electron signal was obtained. The total amount of Ge contained in several surface layers of this alloy is 0.1 ML and the Ge coverage in the top layer is 0.04 ML due to a 5 x 5 structure observed by STM [88]. Fukutani et al. call this surface the Pt(l 1 1)-Ge surface alloy. 

When ArF excimer laser irradiates the Pt( 111 )-Ge surface alloy saturated by NO or CO at 80 K, desorbed NO molecules are detected by the REMPI method, while no CO desorption is observed [87]. Only a little modification of the Pt(l 1 1) surface brings such a remarkable change of the desorption activity. TDS of NO from the alloy at various NO coverages is shown in Fig. 29. Every spectrum has a prominent peak at 220 K, and NO is saturated at 0.2 L exposure in contrast with Pt(l 1 1), on which NO is saturated at 2 L exposure and saturation coverage is 0.75 ML. 

NO desorption from the alloy surface saturated at 80 K is observed at X = 193 nm and is a singlephoton process. The rotational energy distributions in the Boltzmann plot are shown in Fig. 30 and satisfy an almost linear relation, the gradient of which gives Tt 350 K. The two spin-orbit states look 

No fee hollow species for NO and no bridge species for CO are found by RAIRS on the alloy. The absence of these peaks is not due to intensity transfer, but due to an absence of these species, because only one peak corresponding to the on-top species is observed at 1700 cm-1 for NO and at 2100cm-1 for CO even at very low coverages. That is, the on-top site is dominant for NO and CO adsorption on the alloy. This instability of the multi-coordination site in NO and CO adsorption is characteristic of the alloy and is interpreted as due to hybridization between the Ge 4s and the Pt 5d orbital [87, 89], i.e. s-d hybridization, in which the d hole of the metal is partially occupied by s electron of the impurity. 

The chemisorption bond between CO or NO and Pt is mainly attributed to the interaction of a electron of the adsorbate with the d hole of Pt, whereas the site selection of the adsorbate is assisted by the interaction between the adsorbate 2tt and the Pt 5d orbital. The shape of the d orbital is important for this site selection. The five d orbitals in the Oh space group are generally classified into eg (dx2 y2 and d3,2 r2) and r2g (dxy, dyz, and dM) orbitals with respect to the cubic coordinate. Of the two states of Pt, t2g has more d hole character [90], and furthermore, only the f2g state is upward-shifted and the eg is preferentially filled as a result of s-d hybridization [89]. The d band filling in the eg state due to the s-d hybridization for the alloy is supported by the band structure of this alloy in the Y-L direction observed by angle-resolved photoemission spectroscopy using synchrotron radiation [88]. 

---