Abstract
The coadsorption of CO and NH3 on Re(0001) was investigated utilizing the optical second harmonic generation (SHG) technique. The results were compared to temperature programmed desorption (TPD) and work function change measurements (Δφ{symbol}). It was found that the enhancement of the SHG due to ammonia is quenched very efficiently in the presence of small CO coverages. A quenching cross section by CO of the second harmonic response of the metal substrate due to ammonia adsorbates, is defined to be 100 ± 10 A ̊2. This extreme sensitivity of the SHG signal to the presence of CO adsorbed with NH3 is in sharp contrast to both Δφ{symbol} and TPD measurements. These results present only a minor effect of CO on the Δφ{symbol} and TPD of ammonia on Re(0001). It is concluded that unlike the local contribution to the SHG signal generally assumed for separate adsorbates, the quenching by CO is a nonlocal effect. Moreover, the electronic interactions which govern the SHG from the NH3-rhenium system, are very different from the interactions which lead to the TPD and Δφ{symbol} data. At ammonia preadsorption on the surface, the effect of the same amount of coadsorbed CO is largely reduced. This observation is discussed in terms of possible islanding mechanisms driven by the postadsorbed CO.
Original language | English |
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Pages (from-to) | L583-L590 |
Journal | Surface Science |
Volume | 240 |
Issue number | 1-3 |
DOIs | |
State | Published - 3 Dec 1990 |