Mode transitional

Variational RRKM theory is particularly important for imimolecular dissociation reactions, in which vibrational modes of the reactant molecule become translations and rotations in the products [22]. For CH —> CHg+H dissociation there are tlnee vibrational modes of this type, i.e. the C—H stretch which is the reaction coordinate and the two degenerate H—CH bends, which first transfomi from high-frequency to low-frequency vibrations and then hindered rotors as the H—C bond ruptures. These latter two degrees of freedom are called transitional modes [24,25]. C2Hg 2CH3 dissociation has five transitional modes, i.e. two pairs of degenerate CH rocking/rotational motions and the CH torsion. 

To calculate N (E-Eq), the non-torsional transitional modes have been treated as vibrations as well as rotations [26]. The fomier approach is invalid when the transitional mode s barrier for rotation is low, while the latter is inappropriate when the transitional mode is a vibration. Hamionic frequencies for the transitional modes may be obtained from a semi-empirical model [23] or by perfomiing an appropriate nomial mode analysis as a fiinction of the reaction path for the reaction s potential energy surface [26]. Semiclassical quantization may be used to detemiine anliamionic energy levels for die transitional modes [27]. 

The intennolecular Hamiltonian of the product fragments is used to calculate the sum of states of the transitional modes, when they are treated as rotations. The resulting model [28] is nearly identical to phase space theory [29],... 

Figure 4. Interpretation of the transition modes reflection, passing, and hopping along the adiabatic potential. Taken from Ref. [19].

Because T -> V energy transfer does not lead to complex formation and complexes are only formed by unoriented collisions, the Cl" + CH3C1 -4 Cl"—CH3C1 association rate constant calculated from the trajectories is less than that given by an ion-molecule capture model. This is shown in Table 8, where the trajectory association rate constant is compared with the predictions of various capture models.9 The microcanonical variational transition state theory (pCVTST) rate constants calculated for PES1, with the transitional modes treated as harmonic oscillators (ho) are nearly the same as the statistical adiabatic channel model (SACM),13 pCVTST,40 and trajectory capture14 rate constants based on the ion-di-pole/ion-induced dipole potential,... 

Canonical variational transition state theory, with transitional modes treated as harmonic oscillators refs. S... 

The complete W(E, J) is obtained by convolution of the contributions of conserved modes and transitional modes. For the charge-dipole potential, the transitional modes are the free-rotor modes of the ion and two perturbed rotor modes of the linear neutral fragment, only the latter being governed by... 

The cross section area of the collinear IR beam is -1 cm2 and thus sufficient to cover the entire area of the C face of the ATR crystal, and as a result, a complete coverage of faces A by the IR radiation is achieved. Hence, the measured absorbance should be proportional to the fraction of the total area of faces A and C covered by the monolayer. Of course, all six faces of the crystal are covered with the monolayer film. However, only the A faces contribute to the measured signal via internal reflection. This is because the area of the C faces is only 7% of the total area (A + C) in a typical ATR crystal, and the differences between transition mode (in the C faces) and ATR mode (in the A faces) are not very large. Therefore, it was suggested by Maoz and Sagiv that no corrections for this effect are needed (1). 

The transition mode is an unstable process regime for conventional deposition systems. Closed-loop control concepts or modified chamber designs outlined in Sect. 5.3.4.2 are necessary for transition mode process control. 

In transition mode, the oxygen to metal ratio on the substrate is a continuous function of the oxygen partial pressure. It is, therefore, possible to control the stoichiometry of film and thus to optimize doping, morphology, and phase composition. 

Therefore, considerable efforts were made on deposition in the transition mode of the reactive magnetron discharge, where nonstable conditions are used and where closed-loop feedback control is mandatory for process stabilization. 

This section is on the reactive MF magnetron sputtering of ZnO films in the transition mode. The first part is on the deposition of dielectric ZnO films. The reactive sputtering of ZnO Al is discussed in subsequent sections. 

The closed loop process control permits the process to be stabilized in the region of the unstable transition mode between B and C, allowing compensation for the dynamic behavior of the target oxidation by corresponding... 

Process Reactive MF magnetron sputtering. Sinusoidal plasma excitation (40 kHz, advanced energy PEII) Static deposition. Boxcoater Pfeiffer PLS 580. Transition mode process control via A-probe measurement of 02-partial pressure. ... 

Figure 5.15 summarizes the analyses of the XRD measurements for the ZnO series deposited at different substrate temperature. The structural properties depend considerably on the substrate temperature and the reactive gas partial pressure. Films deposited at Ts = 200°C in transition mode reveal the optimum properties. 

The transition mode process described here differs fundamentally from conventional reactive sputtering processes for ZnO coatings which do not permit optimization of the film characteristics since they are hysteresis-based. An example of this is the work of Jacobson et al. [96] on reactive DC-Magnetron sputtering of ZnO coatings. 

The transition mode process control described above is the key to reactive magnetron sputtering of ZnO Al films. Several approaches have proven to be useful, either adjusting the reactive gas flow or the discharge power as a function of appropriate process variables. 

A compilation of the literature on transition mode process control for ZnO deposition is given within Table 5.1. 

Fig. 5.18. Transition mode process stabilization and film resistivity for ZnO Al films by reactive MF magnetron sputtering at different substrate temperature (reprinted from [103])...

The combination of magnetron sputtering and inductively coupled plasma excitation (ICP) is a technique which allows enhanced ionization of the sputtered material. The combination of transition mode process control of the reactive sputtering and ICP plasma excitation is described in [112]. However, the resistivity of ZnO Al films sputtered from a Zn 1.5 wt% A1 target is in the of 1,000 gH cm at T = 150 °C, which is inferior to results from conventional sputter processes under similar conditions. 

Fig. 5.22. Resistivity (a) and dynamic deposition rate (b) for large area reactive MF transition-mode magnetron sputtering at different total pressures and substrate temperatures. The power density is 4Wcm 2 (from [89])...

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