Trapping probability

Figure 23 The trapping probability (Ag/go) and variation in surface potential per incident charge AF/go, as functions of incident electron energy for the 115-mm-thick cross-linked polyethylene (XLPE) sample. (From Ref. 32.)...

Whereas, the trends are the same for different materials, subtle differences in the condition of the surface can substantially affect the absolute values. For example, the trapping probability for Ar in nickel can be increased by a factor of two by adsorbing a layer of oxygen upon the surface . ... 

Therefore, we are able to discuss the existence of defect states (recombination centers and traps) in the forbidden gap of solid organic dyes characterized by different trapping probabilities (ranging from 10-12 cm2 to 10-20 cm2) for electrons and holes. Hence, asymmetric trapping of electrons and holes leading to n- and -photoconductivity is very probable. 

Figure 3.13. Trapping probability a for Ar on Pt(lll) at Ts = 80 as a function of Et for various angles of incidence as labeled in the figure. Results are plotted vs. Ee = Et cos1-5 6t. From Ref. [139].

To solve the problem, one has to consider the infinite (due to (5.2.7)) hierarchy of the non-linear equations (5.2.6). The set of correlation functions is complete and equation (5.2.6) is exact. The quantities wm in (5.2.7) are no longer the effective trapping probabilities, since it is not self-evident that the integral term there is positively defined. 

There is a very simple model for estimating the trapping probability in atomic adsorption due to a phonon-excitation mechanism. In the hard-cube model (HCM) [6, 7], the impact of the atom on the surface is treated as a binary elastic collision between a gas phase atom (mass m) and a substrate atom (mass Mc) which is moving freely with a velocity distribution Pc(uc). This model is schematically illustrated in Fig. 1. If the depth of the adsorption well is denoted by Ead, the adsorbate will impinge... 

Assuming a weighted Maxwellian velocity distribution for uc, the trapping probability in the hard-cube model can be analytically expressed as [7]... 

The decrease of the sticking probability is typical for atomic or molecular adsorption where the molecule adsorbs non-dissociatively. Consequently, it was assumed that the hydrogen molecules do not directly dissociate on Pd(l 0 0). They are rather first trapped in a molecular precursor from which they then dissociate [25, 44], and it is the trapping probability into the precursor state that determines the dependence of the sticking probability on the kinetic energy. 

Figure 8 Trapping probability of 02/Pt(l 11) as a function of the kinetic energy for normal incidence. Results of molecular beam experiments for surface temperatures of 90 and 200 K (Luntz et al. [81]) and 77 K (Nolan et al. [87]) are compared to simulations in the hard-cube model (HCM).

Another study of Xe on the Pt(l 1 1), conducted by Rettner et al. [25], examined the relationship between the incident energy and incident angle on the trapping probability. Using these results, these... 

Figure 14 shows the initial chemisorption probability, Sa, of methane on Ir(l 1 0) at four different surface temperatures, Ts, as a function of incident kinetic energy. Also shown, is a plot of the trapping probability, a, measured at a surface temperature of Ts = 65 K. Figure 15 shows S as a function of incident kinetic energy for various incident angles, 0j, at a surface temperature of Ts = 1000 K. 

Figure 14 Data adapted from Seets et al. [18], Initial chemisorption probability, S0, vs. incident kinetic energy, E, for methane on Ir(l 1 0)-(l x 2) at 0 = 60° for several surface temperatures, Ts, (left ordinate). Also shown (on right ordinate) is the trapping probability at Ts = 65 K and 0 = 60°. Uncertainties are 20% for S0 and 0.03 for a.

Here kCil is the rate of dissociation from the molecular chemisorbed state and ac is the trapping probability into the molecular chemisorbed state. Assuming the rates kca and kd can be expressed in an Arrhenius form (e.g. kd = udexp (—Ed/kT )) and ac is independent of surface temperature, Eq. (4) can be written in terms of the ratio of pre-exponentials, ud/uca and the difference in barrier heights AE = (Ed — ECd) (Eq. (5)). 

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