Direct dissociation

The distinction between direct dissociation processes discussed in the present section and indirect dissociation or predissociation processes discussed in Section 7.3 to Section 7.14 is that in a direct process photoexcitation occurs from a bound state (typically v = 0 of the electronic ground state) directly to a repulsive state (or to an energy region above the dissociation asymptote of a bound state) whereas in an indirect process the photoexcitation is to a nominally bound vibration-rotation level of one electronically excited state which in turn is predissociated by perturbative interaction with the continuum of another electronic state. Direct dissociation, often termed a half collision is much faster and dynamically simpler than indirect dissociation. In a direct dissociation process the distance between atoms increases monotonically and the time required for the two atoms to separate is shorter than a typical vibrational or rotational period (Beswick and Jortner, 1990). 

The photodissociation cross sections and angular distributions can be obtained experimentally by different methods for example, time of flight (TOF) photofragment translational spectroscopy (Busch, et al., 1969), Doppler analysis (Xu, et al., 1986), and ion imaging. 

Multi-dimensional DFT calculations show that the PES actually varies considerably with molecular orientation and impact point on the surface [21,23]. Universally, barriers for dissociation are substantially lower with molecular bonds nearly parallel to the surface. V is also laterally corrugated in the unit cell, both in magnitude and in location relative to the surface plane. The variation in the magnitude of V with X,Y has been labeled energetic corrugation, while the variation in its position 

One attempt to remedy the limitations of the 2D model, and yet retain its simplicity, is the so-called hole model [25] which represents a simple static way to average over this distribution of barriers. If the variation of barrier height with these spectator variables is given as (X, Y, i), ), then the 6D dissociation probability S6D is approximated in terms of the 2D S2D as 

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The direct dissociation of diatomic molecules is the most well studied process in gas-surface dynamics, the one for which the combination of surface science and molecular beam teclmiques allied to the computation of total energies and detailed and painstaking solution of the molecular dynamics has been most successful. The result is a substantial body of knowledge concerning the importance of the various degrees of freedom (e.g. molecular rotation) to the reaction dynamics, the details of which are contained in a number of review articles [2, 36, 37, 38, 39, 40 and 41]. 

Direct dissociation reactions are affected by surface temperature largely tlirough the motion of the substrate atoms [72]. Motion of the surface atom towards the incoming molecule mcreases the likelihood of (activated) dissociation, while motion away decreases the dissociation probability. For low dissociation probabilities, the net effect is an enliancement of the dissociation by increasing surface temperature, as observed in the system 02/Pt 100]-hex-R0.7° [73]. 

The co-existence of tetrameric and non-associated species in a system raises the question whether the following equilibrium is maintained by a direct dissociation of tetramers ... 

Figure 12. Two-dimensional cut through the potential surface for fragmentation of the transition state [OH - -CH3 F] complex as a function of the C—F bond length and the FCO angle. All other coordinates are optimized at each point of this PES. Pathway 1 is the direct dissociation, while pathway 2 leads to the hydrogen-bonded [CH3OH F ] structure. The letter symbols correspond to conhgurations shown in Fig. 11. Reprinted from [63] with permission from the American Association for the Advancement of Science. (See color insert.)...

Direct Dissociation above the Rg + XYfBV) Dissociation Limit... 

This theory is associated in its early protonic form with Franklin (1905, 1924). Later it was extended by Germaim (1925a,b) and then by Cady Elsey (1922,1928) to a more general form to include aprotic solvents. Cady Elsey describe an acid as a solute that, either by direct dissociation or by reaction with an ionizing solvent, increases the concentration of the solvent cation. In a similar fashion, a base increases the concentration of the solvent anion. Cady Elsey, in order to emphasize the importance of the solvent, modified the above defining equation to ... 

For a structureless continuum (i.e., in the absence of resonances), assuming that the scattering projection of the potential can only induce elastic scattering, the channel phase vanishes. The simplest model of this scenario is depicted schematically in Fig. 5a. Here we consider direct dissociation of a diatomic molecule, assuming that there are no nonadiabatic couplings, hence no inelastic scattering. This limit was observed experimentally (e.g., in ionization of H2S). 

C6H5CD3 — C6H5CD2 + D (direct dissociation after internal conversion)... 

Engels, H. et al., Direct dissociation of hydrogen iodide—an alternative to the General Atomic proposal, in Proc. 6th World Hydrogen Energy Conf., 2, 657-662, Vienna, Austria, July 1986. 

When an excited molecule also consists of a definite vibrational level as shown in Fig. 5.1(a), there shall be no direct dissociation of molecule and a fine structure in the electronic band spectra of the molecule will be observed. The excess energy, in usual course may be dissipated as heat or may give rise to fluorescence. But the molecule may retain its energy until it has not reacted with another molecule or transfer its energy to another molecule, e.g. decomposition of NOC1 as follows ... 

Another possibility is that the direct dissociation of the oxygen is more sensitive to strain than in the carbon monoxide complexes and is simply more efficient and competitively dominant over the pseudo four-coordinate pathway. 

Figure 2.1 Schematic representation of the ground and electronic excited potential energy surfaces (PESs) and the corresponding absorption spectra of the parent molecule, resulting from the reflection of different initial wavefunctions on a directly dissociative PES (a) absorption from a vibrationless ground state consists of a broad continuum and (b) absorption from a vibrationally excited state shows that extended regions are accessed, leading to a structured spectrum with intensities of the features being dependent on the Franck-Condon factors. Reproduced with permission from Ref. [34]. Reproduced by permission of lOP Publishing.

Figure 3.3. Schematic of direct and precursor-mediated dissociation processes on a typical adiabatic PES (given by the solid line). Solid arrow labeled S represents direct dissociation and that labeled a represents trapping into a molecular adsorption well. Dashed arrows represent competing thermal (Arrhenius) rates for desorption (kd) and dissociation (kc) from the molecular well.

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