Detachment rate coefficients

Hydrodynamic detachment rate coefficient 1.0x10- kg4Ms... 

Time Dependence of Deposition and Detachment Rate Coefficients... 

This process proceeds by the two-stage Bloch-Bradbury mechanism (Bloch Bradbury, 1935 Alexandrov, 1981) starting with the formation of a negative ion (rate coefficient att) in an unstable auto-ionization state (r is the time of collisionless detachment) ... 

This is the reverse process with respect to the dissociative attachment (2-66) and therefore it can also be illnstrated by Fig. 2-7. The associative detachment is a non-adiabatic process, which occnrs via intersection of electroiuc terms of a complex negative ion A -B and corresponding molecnle AB. Rate coefficients of the non-adiabatic reactions are qnite high, typically kd = 10 °-10 cm /s. The kinetic data and enthalpy of some associative detachment processes are presented in Table 2-7. 

The rate coefficient of the detachment is very high 2 10 ° cm /s at room temperature. Electron detachment can be also effective in collisions with vibrationally excited molecules, for example,... 

Rate coefficients of the detachment process, which are especially important in the quasi-equilibrium thermal systems, are presented in Table 2-8. The kinetics of the detachment process can be described in this case in the conventional manner for all reactions stimulated by the vibrational excitation of molecules. The traditional Arrhenius formula, kd a exp(—fa/ Tv), is applicable here. The activation energy of the detachment process can be taken in this case to be equal to the electron affinity to oxygen molecules (E 0.44 eV). 

Detachment process Reaction enthalpy Rate coefficient, 300 K Rate coefficient, 600 K... 

The parameter 5- = k /k shows the detachment ability that compensates for electron losses dne to attachment. If 5- 1, the attachment inflnence is negligible and kinetic equation (4-21) becomes eqnivalent to one for non-electronegative gases. The kinetic equation inclndes the effective rate coefficients of ionization, kf = kj + g, and recombination, k f = kf + gk. Eqnation (4-21) describes electron density evolution to the steady-state magnitnde of the recombination-controlled regime ... 

Electron detachment (5-126) together with dissociative attachment (5-125) creates a chain reaction of water decomposition. One electron is able to participate in the H2O dissociation process many times in this case, which makes the whole kinetic mechanism energy effective. Cross sections of the electron detachment by electron impact (5-126) are shown in Fig. 5-58 a typical value of rate coefficient for the detachment process is very high (kd = 10 cm /s Smirnov Chibisov, 1965 Tisone Branscome, 1968 Inocuti et al., 1967). The chain mechanism, (5-125) and (5-126), is initiated by ionization of H2O molecules. The chain termination is related to fast ion-ion recombination,... 

In this relation, Z = A.( ) A. is the Coulomb logarithm Atj and Zd are the rate coefficient and velocity distribution related to detachment (5-126) <7a and are cross sections of dissociative attachment and electron-ion recombination is the generation rate of plasma... 

The remainder of this chapter is devoted to a discussion of some of the results obtained from studies of negative-ion-neutral reactions. No attempt is made to include extensive discussion either of associative detachment processes (see Chapter 8) or of those many negative-ion reactions which have been studied only at relatively high energies, i.e., above a few hundred eV. Two-body rate coefficients here are invariably given in units of cm sec and cross sections in units of cm. Energies refer to kinetic energies of the reactant ion in the laboratory system, unless otherwise stated. 

Fig. 6.15. Photodetachment of electrons from OH trapped at 180 K in a 22-pole. The small loss of ions (time constant 133 s) is significantly increased, if a He-Ne laser is switched on (here at 10s). The photon energy (1.96eV) is sufficient to detach the electron from the anion OH (electron affinity 1.8 eV). The solid lines are exponential fits. Measurements performed at various temperatures of the trap allow state specific rate coefficients to be extracted.

At the end of this section we come back once again to the kinetical equation, Eq. (4.25), and consider for further clarification a inuch simplified, but explicit model. We analyse, in more detail, the movement of the lateral growth front of a crystallite which is composed of stretched sequences of n monomers, and introduce for the description two rate coefficients, j- and j. These give the detachment- and the attachment-rate respectively, referring to one site at the lateral growth face to be occupied by a sequence. 

If the step is initially prepared to be straight, it relaxes to its fluctuating shape in the due course of time. This time evolution of step width depends on the relaxation kinetics, and can be used to determine the values of various kinetic coefficients [3,16-18,64-66], For example, if the attachment and detachment kinetics of adsorbed atoms at a step is rate limiting, the step width increases as [65]... 

In the Mint model, we have to take into account the following considerations (i) the initial filtration coefficient Xq, which is a parameter, presents a constant value after time and position (ii) the detachment coefficient, which is another constant parameter (iii) the quantity of the suspension treated by deep filtration depends on the quantity of the deposited solid in the bed this dependency is the result of the definition of the filtration coefficient (iv) the start of the deep bed filtration is not accompanied by an increase in the filtration efficiency. These considerations stress the inconsistencies of the Mint model 1. valid especially when the saturation with retained microparticles of the fixed bed is slow 2. unfeasible to explain the situations where the detachment depends on the retained solid concentration and /or on the flowing velocity 3. unfeasible when the velocity of the mobile phase inside the filtration bed, varies with time this occurrence is due to the solid deposition in the bed or to an increasing pressure when the filtration occurs with constant flow rate. Here below we come back to the development of the stochastic model for the deep filtration process. 

It is well known that only experimental investigation can validate or invalidate a model of a process. For the validation of the model developed above, we use the experimental data of the filtration of a dilute Fe(OH)j suspension (the concentration is lower than 0.1 g Fe(OH)3 /I) in a sand bed with various heights and particle diameters. The experiments report the measurements at constant filtrate flow rate and give the evolution with time of the concentration of Fe(OH)3 at the bed output when we use a constant solid concentration at the feed. Figure 4.36 shows the form of the time response when deep bed filtration occurs. The concentration of the solid at the exit of the bed is measured by the relative turbidity (exit turbidity/ input turbidity 100). The small skips around the mean dependence, which appear when the clogging bed becomes important, characterize the duality between the retention and dislocation of the bed-retained solid. This dislocation shows that the Mint model consideration with respect to the detachment coefficient is not acceptable, especially when the concentration of the bed-retained solid is high. 

The specific rate constants of interest to the ECD and NIMS are dissociative and nondissociative electron attachment, electron detachment, unimolecular anion dissociation, and electron and ion recombination. The reactions that have been studied most frequently are electron attachment and electron and ion recombination. To measure recombination coefficients, the electron concentration is measured as a function of time. The values are dependent on the nature of the positive and negative ions and most important on the total pressure in the system. Thus far few experiments have been carried out under the conditions of the NIMS and ECD. However, the values obtained under other conditions suggest that there is a limit to the bimolecular rate constant, just as there is a limit to the value of the rate constant for electron attachment. The bimolecular rate constants for recombination are generally large, on the order of 10 7 to 10-6 cc/molecule-s or 1014 to 1015 1/mole-s at about 1 atm pressure. Since the pseudo-first-order rate constants are approximately 100 to 1,000 s 1, the positive-ion concentrations in the ECD and NIMS are about 109 ions/cc. 

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