Field adsorption

Also so far, the new theory does not consider the possibility of field evaporation as multiply charged ions. The advantage of the new method lies in the possibility of investigating how electronic-charge distribution changes as the atoms are field evaporated from the surface, and also how the binding force changes as the atoms are removed from the surface. 

In an applied field of the order of a few V/A, gas atoms and molecules, which normally will not adsorb on a surface at a given temperature, may adsorb on the surface by an effect of the applied field. Field adsorption can occur at a temperature much higher than that in ordinary adsorption. For example, He and Ne can be field adsorbed on tungsten surfaces at a temperature as high as 100 K with a probability of near one, whereas in ordinary physisorption the probability is nearly zero at such a tempera- 

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The secondary peak structures are observed only if the field is at least 10% above the best image field. These structures are especially pronounced if ions are collected from the flat area of the tip surface, for example from the middle of the (110) surface of a tungsten tip. When ions are collected from a kink site atoms of the W (110) plane step, the secondary structures are washed out. It is particularly interesting that in field ionization of hydrogen, secondary peaks are very pronounced for H+ and Hj ions but not HJ ions, as is shown in Fig. 2.6. The H3 peak is very sharp, indicating that ions are produced only right at the surface.22 This can be understood from the fact that H3 molecules are unstable in free space. It is formed by field induced polymerization and exist only in the field adsorption state, as will be further discussed.33... 

Fig. 2.21 Binding energy of field adsorption, or the desorption energy, of He as a function of field derived by different investigators using various methods. The solid line is a theoretical curve given by Ernst et al.42.

The binding energy in field adsorption can be derived from consideration of the kinetics of field adsorption. Specifically, it can be determined from a temperature dependence of the probability of field adsorption on an adsorption site, or the degree of coverage of field adsorption on a plane. As will be shown, a consideration of the probability of field adsorption based on adsorption time and desorption time leads to an equation equivalent to the Langmuir adsorption isotherm, but specific to the problem of field adsorption.112115 Let us focus on one surface atom. The average time it takes to have an image gas atom field adsorbed on the surface atom, ra, is... 

Table 2.6 Primary field adsorption energy onaW (112) surface...

Let us represent the probability of field adsorption at any instant of time t by p(t). This function has to satisfy the difference equation,... 

The equilibrium probability of field adsorption is therefore given by... 

Fig. 2.22 In [UP - 1 )/T] vs. 1 IT plot for field adsorption of He on the W (112) surface, obtained from a pulsed-laser stimulated field desorption experiment. In the high temperature region, the data points fit well into a straight line of H = 0.17 eV, as expected from a theoretical analysis. In the low temperature region, the data points deviate significantly from the linear plot of the solid line. This deviation indicates that there may be another binding state with a much smaller binding energy of 0.08 eV.

In field ionization, hydrogen molecules near the tip region are attracted to the tip surface. They either hop around the tip surface or are field adsorbed on it. As the hopping motion and the field adsorption are dynamical phenomena, some of the ionic species detected may also come from field adsorbed states, not necessarily just from the gas phase. On the other hand, in pulsed-laser stimulated field desorption, where gas pressure is very low, of only 1 X 10-8 Torr, gas molecules are thermally desorbed by laser pulses from their field adsorbed and chemisorbed states. When they pass across the field ionization zone some of them are field ionized. The critical ion energy deficit in pulsed-laser stimulated field desorption of a gas is therefore found to be identical to that found in field ionization. In both pulsed-laser stimulated field desorption and field ionization of hydrogen, the majority of ions detected are H3 and H+. 

Fig. 4.56 In pulsed-laser stimulated field desorption with coadsorption of N2 and H2, if the temperature of the Pt tip is between 120 and 150 K, then reaction intermediates of NH3 can be detected as shown in (a). If the surface temperature of the tip is only —50 K lower, no reaction intermediates of NH3 can be detected. Instead, only field adsorption products, N2 and N2H+ are detected.

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