Dissociation energy photoionization reactions

Photoionization of the hydrocarbon followed by dissociative electron attachment (Reaction 1) should be considered since the ionization potential of a molecule is less in the liquid phase than it is in the gas phase. For hydrocarbons the ionization potential is 1 to 1.5 e.v. less in the liquid phase (24). The photon energy at 1470 A. is about 1.4 e.v. below the gas-phase ionization potentials of cyclohexane and 2,2,4-trimethylpentane (14). Some ionization may therefore occur, but the efficiency of this process is expected to be low. Photoionization is eliminated as a source of N2 for the following reasons. (1) If photoionization occurred and the electron reacted with nitrous oxide, then O" would be formed. It has been shown in the radiolysis of cyclohexane-nitrous oxide solutions that subsequent reactions of O result in the formation of cyclohexene and dicyclohexyl (I, 16, 17) and very little cyclohexanol (16, Table III). In the photolysis nitrous oxide reduces the yield of cyclohexene and does not affect the yield of dicyclohexyl. This indicates that O is not formed in the photolysis, and consequently N2 does not result from electron capture. (2) A further argument against photoionization is that cyclohexane and 2,2,4-trimethylpentane have comparable gas-phase ionization potentials but exhibit quite different behavior with respect to N2 formation. 

To date, by far the best quantitative studies of unimolecular dissociations of ions with a well-defined energy distribution have been those using the photoelectron photoionization technique (PEPICO). These reactions have been discussed in detail elsewhere, most notably by Baer. ... 

Photodissociation is far more likely to occur than photoionization, however, because the energy needed to bring about dissociation is much less than that required for photoionization. In fact, once a water molecule escapes from the comet nucleus into the coma, the average time in which it is likely to he broken apart by solar radiation is about 25 seconds. By contrast, the average amount of time during which a water molecule is ionized by solar radiation is likely to be a few hours. Such reactions are more likely to occur when water molecules have trailed off into the comet s tail at distances of about 1 AU. Consequently, the coma of a comet can he expected to consist largely of neutral remnants of the photolysis of water molecules, such as H and OH, while ionized species, such as H+ and free electrons, are more likely to be found in the comet s tail. 

Rate constants measured for the reactions in 1,3-butadiene are given in Table I and compared with those measured by Barber et for reactant ions produced by electron impact. It is seen that the agreement between the two sets of rate constants is only fair, even if a correction factor is applied so that both quantities are made equal for one reaction. If the discrepancy is outside experimental error, it may have a very interesting origin. It should be remembered that the internal-energy distribution of the reactant ions produced by photoionization extends only to 10.19 eV - /, whereas the internal-energy distribution of ions produced by electron impact can extend up to the dissociation limit of the reactant... 

Figure 1 outlines various padiways can lead to the deactivation of die excited pesticide molecule (P ) throu luminescence, physical quenching, or by collisionally transferring energy to other gaseous molecules (M). This figure also illustrates electron transfer, photoionization, or direct chemical reaction processes of the excited state that can lead to dissociation and subsequent product formation. 

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