Thermal Dissociation of Diatomic Molecules

This reaction is peculiar in that a diatomic molecule possessing vibrational energy higher than the dissociation energy dissociates during one period of vibration (10 —10 s). Since under the conditions of gas kinetics the collision frequency Zq[M] virtually never exceeds 10 —10 s , the dissociation of diatomic molecules may be always considered as instantaneous (compared to the time lag between successive collisions). Thus, the thermal dissociation of diatomic molecules 

As follows from theory and experiment, in the high-pressure domain, a simple unimolecular reaction uncomplicated by secondary processes is first-order (rate constant k o) whereas in the low-pressure domain it is second order (rate constant ko). The object of experiment is to determine k and ko, for a general case expressed by 

In the general case, the effective rate constant k is a function of pressure (Eq. (17.6)) and the boundary between the high- and low-pressure domains lies at a certain pressure pi g which kj/2 = koo/2. 

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E.E.Nikitin, On a perturbation of the thermal equilibrium in thermal dissociation of diatomic molecules, Zhum.Fiz.Khim. 33, 519 (1959)... 

Osipov, A. (1966). Thermal dissociation of diatomic molecules at high temperatures, Teor. Exp. Khim. 2(11) 649. 

As a simple example take the thermal dissociation of diatomic molecules (forward reaction) and atomic recombination (reverse reaction) in the heat bath... 

Non-Thermal and Thermal Plasma Dissociation of Diatomic Molecules... 

II. NON-THERMAL AND THERMAL PLASMA DISSOCIATION OF DIATOMIC MOLECULES... 

For much of the discussion in this chapter, the BOA is assumed valid so that the bond making/breaking is simply described by motion of nuclei on a multidimensional ground state PES. For example, dissociation of a molecule from the gas phase is described as motion on the PES from a region of phase space where the molecule is far from the surface to one with the adsorbed atoms on the surface. Conversely, the time-reversed process of associative desorption is described as motion on the PES from a region of phase space with the adsorbed atoms on the surface to one where the intact molecule is far from the surface. For diatomic dissociation/associative desorption, this PES is given as V(Z, R, X, Y, ft, cp, < ), where Z is the distance of the diatomic to the surface, R is the distance between atoms in the molecule, X and Y are the location of the center of mass of the molecule within the surface unit cell, ft and cp are the orientation of the diatomic relative to the surface normal and represent the thermal distortions of the hh metal lattice atom... 

There is a possibility that an FC state will react before complete thermal equilibration. In the case of diatomic molecules, the process is usually known as predissociation — a dissociative state crosses the excited state potential surface. The situation is more complicated in the case of a coordination compound, but one can imagine an FC state relaxing along some nuclear coordinate leading to bond breaking. A state capable of such a process has been called a DOSENCO state, an acronym for Decay On SElected Nuclear Coordinates .21 The same authors use the term DERCOS (DEcay via Random Coordinate Selection) for a thexi state. 

The thermal dissociation and recombination reactions of diatomic molecules in the gas phase are important for several reasons (1) they may be the initiation and termination steps for complex reaction mechanisms,... 

The reactions that have been discussed so far in this section occur adia-batically on their ground state PES. However, there are thermal reactions which are electronically nonadiabatic the total electron spin changes going from reactants to products. The thermal dissociation of N2O—and of the electronically similar molecules CO2, OCS and CS2—have been much studied. The ground states of these molecules are singlets i.e. the total spin quantum number S=0), but the lowest energy products are a singlet diatomic molecule and an atom in a triplet state S = 1), for example ... 

Chain reactions such as those described above, in which atomic species or radicals play a rate-determining part in a series of sequential reactions, are nearly always present in processes for the preparation of thin films by die decomposition of gaseous molecules. This may be achieved by thermal dissociation, by radiation decomposition (photochemical decomposition), or by electron bombardment, either by beams of elecuons or in plasmas. The molecules involved cover a wide range from simple diatomic molecules which dissociate to atoms, to organometallic species with complex dissociation patterns. The... 

Figure 15 shows the CI2 bond distance for two different trajectories for which the initial conditions of the Ari25Cl2 cluster are the same, i.e. the configuration and the center of mass velocity of the clusters at the beginning of each trajectory (before the collision with the surface) are identical. The only differences between the two trajectories are the velocities (randomly chosen from a one-dimensional thermal distribution at 30 K) of the hard cubes that mimic the surface. Despite the rather low temperature of the surface, one of the trajectories results in the dissociation of the diatomic molecule while the other one ends with a vibrationally excited reactant molecule. The effect of the hard cube velocity on the energy of the atom scattering from the surface is negligible but the history of a single trajectory is extremely sensitive to the details of the collisions with the surface, as shown in Fig. 15. This is a characteristic of so called chaotic systems. In...

For a diatomic close to thermal equilibrium the assumption of impulsive collisions with the bath molecules is realistic only when the vibrational jjeriod is longer than the duration of a collision, but this is rarely the case for diatomics vibrating near the potential minimum. However, for realistic potentials the vibrational period lengthens near the dissociation threshold, and it is not so clear that an impulsive model will be quantitatively inaccurate in modeling dissociation, even though it may fail badly in describing vibrational relaxation deep with the well. A stochastic impulsive model of dissociation of a diatomic AB, which uses the zero-frequency frictions describ-... 

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