T min

Figure 1. The dependences of thermo electronic current (1) and degree of dissociation y of CsCl molecules on time, at high temperature adsorption of C6H6 molecules.

that the surface of a monolayer of graphite on Re is homogeneous on work function and had (p = (4.45 ± 0.05) eV, that well coincided with work function of the same surfaces.17'18

At formation of 2DGF on Re the degree of dissociation of CsCl molecules y reduces from 1 to 10-3-10-6. On Figure 2 it is shown the characteristic dependence course of change of y(T) for dissociation on pure Re and for dissociation on Re with graphite monolayer.

Apparently from this figure yx 10-5 over the region 800 K < T < 1,100 K. At temperature T x 1,950 K graphite collapses and dissociation of CsCl molecules on pure Re, y increases up to 1 (Figure 2). One monolayer of graphite on Re is sufficient, that the surface becomes catalytic passive for dissociation of CsCl molecules. Tontegode11 attributes this fact to saturation of valence bond saturation of a graphite layer, which inhibits electron change interaction with adsorbed particles. Because of its valence bonds are saturated, single crystal graphite has a layered structure with van der Waals forces coupling the layers. Valence bond saturation is responsible for specific features in the adsorption and catalytic processes occurring on the graphite layer. Indeed, if the quasilevel of V' of the valence electron of an ad particle lies below the Fermi level (V' > 9) then the ad particle maintains electrical neutrality and is coupled to the surface by van der Waals forces only (physisorption). If, however, V' < 9, then the ad atom is partially charged, and image forces are added to the van der Waals forces. As the ad particles have orbitals that are not used in bonds to the substrate, this may enhance lateral interaction between them resulting in the formation in the ad layer of clusters and islands, with the particles on the graphite layer being in a less perturbed state than those on the metal surface. Finally, the dissociation catalysis originating from the electron exchange between the adsorbed molecule and the surface will be inhibited.

Indeed, many molecular gases (I2, Br2, O2, CO, H2O),19 a number of molecules (C6H3Br3, GeJ2, FeCl3, As2O3)20'21 as well as the Cu and Au atoms on the basal plane of the single crystal graphite22 and Pt and Ni atoms on the graphite monolayer on Ir and Re15 undergo only physisorption.

Apparently from Figure 2, on 2DGF on Re the degree of dissociation of CsCl molecules considerably falls, but above zero. We attribute this fact with dissociation on defects of graphite layer.

Figure 2. The dissociation of CsCl molecules on (1) Re and (2) Re covered with graphite monolayer.


Previously adsorption of CsCl molecules on 2DGF is investigated separately for investigation of co-adsorption of Tm atoms and CsCl molecules on 2DGF surface on (1010)Re. Experiments have shown, that even at 300 K about one third of incident flow is desorbed with a time constant ~10-2 s, which coincides with the time of incident flow. Through the use of the Frenkel formula, T = T0 exp(E/kT), we have estimated the upper limit of the desorption energy for CsCl at the temperature of the adsorbent below 750 K. Assuming t0 = 10-13 s, we have obtained E < 0.65 eV.

In Figure 3 dependences of desorbing flows, vdes, versus time at opening (1) and closing (2) incident to 2DGF on Re flow, vinc, of CsCl molecules (Tadsorbent = 650 K, vinc = 6.3 * 1012 cm-2 s-1) are plotted. It is seen that initially vdes increases and quickly becomes equal to vinc. This means that in ad layer steady-state cover, by Ns molecules (vinc = vdes = Ns/xdes), is established quickly. Time-by-time vdes becomes lower than vinc and continues to decrease. This means that on the surface the condensation of particles begins, with condensation coefficient o = (vinc - vdes)/vinc.

TD spectra show that adsorption of CsCl molecules on 2DGF leads to island formation at the temperature of the adsorbent below 750 K. At temperatures of the adsorbent higher than 800 K creation of islands was not observed. It has been found that at wide range of surface concentrations 1 x 1010-5 x 1015 cm-2 of molecules from 2GDF only CsCl molecules are desorbed.

Figure 3. The dependences of desorbing flows vdes versus t at (1) opening and (2) closing incident to 2DGF on Re flow vinc of CsCl molecules; Tads = 650 K, vinc = 6.2x1012 cm-2 s-1.

Figure 3. The dependences of desorbing flows vdes versus t at (1) opening and (2) closing incident to 2DGF on Re flow vinc of CsCl molecules; Tads = 650 K, vinc = 6.2x1012 cm-2 s-1.


Experiments were hold at temperatures of substrate T = 840-1,100 K and at density of flows Vcsci = 1 x 1010-6 x 1013 cm-2 s-1 and Vrm = 2 x 10103.4 x 1013 cm-2 s-1. According to these circumstances Tm was on a surface only in the form of chemisorbed atoms and islands did not form. In the Figure 4 it is shown the dependence of Q on T. At 820 K < T < 875 K, Q = 3. This fact indicates to ongoing exchange reaction Tm + 3CsCl ^ TmCl3 + 3Cs. At T — 920 K, Q — 2, in this region of temperature from surface mainly TmCl2 desorbs. At T > 970 K, Q < 1. This indicates that in the reaction mainly TmCl forms and Tm atoms desorb effectively from the surface. The maximum quantity of the efficiency of reaction was determined as n — 0.6. This quantity is less than obtained for K + CsCl (n = 1) reaction on such a surface.

Under the conditions of experiments concentration of particles on the surface were so less, that simultaneous meetings of three or four particles were less probably. There for it is assumed that the reaction goes in three levels: in the first TmCl is got, in the second - TmCl2 and in the third TmCl3.

Thus, it is possible to expect, that on a surface of 2DGF at interaction with CsCl molecules valence of Tm atoms in these reactions is equal to 3.

Figure 4. The dependence of Q = f(T) at vGsci = 6.8 x 1012 cm 2 s 1 and different flows of Tm. vTm x 10-10 (cm-2 s-1): A, 1.8; 4.9; •, 10.2; +, 19.5; □, 32.9.
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