Thermochemical excitation energy

Figure 3. Approximate lower-bound thermochemical excitation energy distribution for F-for-F activated CFs F (49)...

Significance of Thermochemical Excitation Energy Distributions. Two fundamentally different approaches have been used for the elucidation of primary product excitation distributions for hot atom activated species. Many workers have utilized the RRKM unimolecular rate theory to invert pressure falloff data measured for the products from energetic H-for-H (19,64,65,74,83,84,85) or F-for-X (19,27) substitution reactions. More complete citations to the early literature are given elsewhere (57,58,59,77,80). This inversion procedure is computationally straightforward. However, it involves several a priori untested and... 

Figure 8. Approximate thermochemical excitation energy distributions for the F-for-k alkyl replacement channels in CHfiHF (28, 77)...

The results of elctron-impact studies of phosphine by Halmann et al. are given in Table 3a. The authors used the appearance potentials, in conjunction with thermochemical data, to choose the probable reaction processes. In many simple cases the observed appearance potential A (Z) for an ion fragment Z from a molecule RZ is related to its ionisation potential 7(Z) and to the energy of dissociation 7)(R—Z) of the bond by the expression A (Z) = /(Z) + D (R—Z). This assumes that the dissociation products are formed with little, if any, excitation energy, and that /(Z) < /(R). The most abundant ion species in the usual mass spectrum of phosphine is PH, which is probably formed according to the following mechanism... 

The reported values of quantum yields for each of the above processes are 0.61, 0.18, 0.21, respectively. Of course these quantum yields are not necessarily applicable if excitation energy is different. For example, according to the recent theoretical thermochemical data given by Ho and co-workers (14,15) the reaction enthalpy for the formation of a silylsilylene and two hydrogen atoms from disilane (decomposition (b)) is about 166 kcal/mol, which is equivalent to the photon energy at 172 nm. On the other hand, the dissociative excitation of disilane can be effected with even longer wavelength of up to... 

The fractional product yields and decomposition threshold energies corresponding to the mechanism shown in Reactions 77-84 have been shown in Table XIII. On simple thermochemical grounds the detection of CClg F product shows that the nascent CFs F excitation energy distribution extends to at least 15 eV. 

Striking similarities exist between the product decomposition phenomena and the derived thermochemical excitation distributions for particular classes of hot H or F reactions occurring in closely related reactants (16,25). Although further measurements would be useful, this evidence suggests that hot reactions related in this fashion may occur upon topologically similar potential energy hypersurfaces (77,81,82). 

The search for a phenomenological alternative to RRKM inversion distribution mapping does not represent a novel idea. The first step in the RRKM modeling procedure for a chemically activated species involves the a priori characterization of its initial excitation energy distribution (70,89,90). For species produced from exoergic reactions this information is normally obtained from thermochemical data. A correspondingly simple direct method has not yet emerged for hot atom activation processes, because the associated dynamics are incompletely imderstood. 

The latter process is not competitive with the former above the thermodynamic threshold. Furthermore, as long as the available energy was kept below the thermochemical threshold for the production of vibrationally excited CN radicals, it was possible to fit the observed rotational distributions with phase space theory. The upper electronic state that is involved in the two-photon dissociation was shown to originate below 22,000 cm l and is thought to be repulsive. It could be the same state that has its absorption maximum at 270 nm. 

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