Dissociative chemisorption, curve

The mechanism we believe is responsible for the large SiOj-to-Si etch-rate ratios which have been obtained in fluorine-deficient discharges is based on several experimental observations. First of all, it has been shown that there are several ways in which carbon can be deposited on surfaces exposed to CF, plasmas. One way is to subject the surface to bombardment with CF ions which are the dominant positive ionic species in a CF plasma. The extent to which this can occur is shown by the Auger spectra in Fig. 3.3. Curve (a) is the Auger spectrum of a clean silicon surface and curve (b) is the Auger spectrum of the same surface after bombardment with 500 eV CFj" ions. Note that the silicon peak at 92 eV is no longer visible after the CFj bombardment indicating the presence of at least two or three monolayers of carbon. Another way in which carbon can be deposited on surfaces is by dissociative chemisorption of CFj or other fluorocarbon radicals. 

The formation of surface hydrides, oxides, and nitrides is usually a result of the dissociative chemisorption of the molecules of these gases. As can be seen from Fig. 9, which gives the potential curves for such an adsorption, the heat of adsorption is given by the difference in energy level between A and E, this difference being given by... 

Fib. 9. Potential curves relating to the dissociative chemisorption of a molecule M (H2) on a metal Me without an activation energy. 

Fig. 1. Potential curves for the dissociative chemisorption on a catalyst surface.

After reduction (Fig. 4b curve 4), the absorption due to Cu (NO) species was enhanced and shifted at 1740 cm. Parallely, the bands due to Cu mononitrosyls, nitrites and N2O (2250 cm ) developed, accounting for the dissociative chemisorption of NO on metallic Cu , which was at least partially reoxidised. Nitrites and N2O arose therefore from the reaction of gaseous NO with N and O ad-atoms, respectively, formed upon NO dissociation. 

Pt(lOO) is observed at 0sb = 0.35. Sb causes significant up-shifting of the CO band formed from HCOOH oxidation, which suggests that dissociative chemisorption is triggered at adjacent sites, electronically modified by Sb. Figure 8 shows IR spectra for Pt(lOO) and Pt(lOO) with an Sb adlayer (0st = 0.25) and the corresponding oxidation current versus and Oqo curves. The... 

Figure 3.6 The Lennard-Jones curve-crossing model for dissociative chemisorption, left without and right with an energy barrier. The undissociated A2 molecule is physisorbed at the surface. The A atoms are chemisorbed. The energy of the two new metal—A bonds suffices to compensate for the A-A bond energy e and the depth of the physisorption well. Therefore the interaction potential of the undissociated A2 molecule with the surface is asymptotically lower, by the A—A bond energy. But near the surface this potential curve is crossed by the interaction of two A atoms with the surface. The limitation of the two-body point of view is evident in this plot. The A—A bond distance, that is surely a key variable, is not represented in this simple view. More on this topic in Chapter 12.

Dissociative chemisorption of a diatomic molecule can also happen through the dissociation in a gas phase and a creation of two gas phase atoms these two atomic species can be then adsorbed on the surface (this way is almost always non-activated). If the curves describing molecular and atomic adsorption intersect at or below the zero potential energy line, then the precursor physisorbed molecule can experience non-activated dissociation, followed by chemisorption (Fig. 4. la). In contrast, if the energetic for these two pathways are such that the intersection occurs above the zero eneigy plane, then chemisorption wiU be activated with activation energy, Ead, as indicated in Fig. 4. lb. 

The molecule surface interaction can occur in a similar fashion, especially for metals where the work function (ionization energy) is relatively small (a few eV). The interaction between two atoms and the surface as well as the molecule and the surface can be visualized by the curves on Figure 1.2. The two curves cross at a certain distance and the energy at that point is the activation barrier for dissociative chemisorption. At laige distances a charged particle interacts with the surface through a classical image potential... 

Figure 4.8. displays oscillograms of evolution of the electric conductivity of the ZnO film in the process of catalytic dehydration of isopropyl alcohol at various temperatures of the catalyzer and equal portions of alcohol (5-10-2 Torr) admitted into the reaction cell. Experimental curves 1-4 are bell-shaped. We suppose that this fact is associated with two circumstances. On one hand, alcohol vapors dissociate on the oxide film producing hydrogen atoms. The jump in electric conductivity is caused by chemisorption of these hydrogen atoms on the film which plays a part of the sensor in this case. Chi the other hand, the drop in electric conductivity is caused by complete dissociation of the admitted portion of alcohol ( depletion of the source of hydrogen atoms) and by... 

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