Fluorine excitation distributions

Figure 6 shows the uncorrected thermochemical excitation distributions corresponding to energetic substitution reactions at the fluorinated alpha carbon atom positions in CH3CHF2 and CH3CF3. From the left the respective fractional yield segments include the undecomposed primary species and the decomposition products from HF-eliminations, from secondary carbon-carbon scissions to alkyl radicals, and from tertiary radical dissociations. Following these alpha substitution processes product decomposition occurs principally via HF-elimination. 

Because of the stability of HF, the atmospheric densities of F and FO are very small and the effect of fluorine on odd oxygen is insignificant. The reaction of HF with 0(1D) is chemically possible, but is negligible due to the low abundance of this excited atom. The distribution of HF is therefore largely determined by the rates of the surface emission of fluorine containing gases, of photochemical destruction of these gases, and atmospheric dynamics. 

The chemically activated molecules are formed by reaction of CH with the appropriate fluorinated alkene. In all these cases apparent non-RRKM behaviour was observed. As displayed in figure A3.12.il the measured unimolecular rate constants are strongly dependent on pressure. The large rate constant at high pressure reflects an initial excitation of only a fraction of the total number of vibrational modes, i.e. initially the molecule behaves smaller than its total size. However, as the pressure is decreased, there is time for IVR to compete with dissociation and energy is distributed between a larger fraction of the vibrational modes and the rate constant decreases. At low pressures each rate constant approaches the RRKM value. 

Here M is any component of the reaction mixture that stabilizes vibra-tionally excited products on one or more collisions. Thermodynamic energy limits for each process were estabhshed. Thus, the integral of the CF4 excitation function could be estimated and used to obtain the initial internal energy distribution of the CFa F formed in Equation 1. The qualitative features of both results (5,21) are the same, and, as expected, the total energy deposited by hot fluorine atoms (22) is somewhat greater than by hot tritium in these replacement reactions. 

Qearly, these distributions are characteristic of systems where there are fluorine atoms present, but there has to be an alternative explanation to that suggested by Davis et al [14,15] [Reactions (3-5), where IF(B) is produced by colhsional excitation of IF(X) by 02( A)) in order to rationalise our observation that 02( A) is not necessary to produce such distributions. We have suggested [26] that in addition to F atoms, the other key species responsible for IF(B) production is electronically-excited iodine atoms I (2P y2) These can be produced in such systems by a reaction of the form... 

Such a process would favour the formation of high vibrational levels of IF(B) which when partially relaxed, might give a vibrational distribution of the form B. In general, the rates of two-body or three-body recombination reactions would not be sufficient to account for the observed yields of EF(B). One possibility is that an exciplex of the excited iodine atom with the iodide, (I. ..RI), might be formed [32] and the subsequent reaction of this exciplex with a fluorine atom would enhance the overall rate for IF (B)... 

We have shown that for low pressure gas systems (< 2 mbar), it is possible to categorise the mechanism of IF(B) formation by inspecting the form of the IF(B) vibrational state distribution. At these pressures, the effects of vibrational relaxation are minimised and the measured distributions are near to their nascent forms and hence characteristic of their mode of formation. We find that all the systems studied so far can be divided into three broad categories. These are bimolecular chemiluminescent reactions of Fj, creation of IF(B) by fluorine atoms and spin-oibit excited iodine atoms and direct coUisional excitation of BF(X) to IF(B) by an energy rich electronically or vibrationaUy excited species. Such a categorisation should assist the development and identification of promising schemes for a visible chemical IF (B-X) laser. 

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