Excited ions collisional deactivation

Excited states can be formed by a variety of processes, of which the important ones are photolysis (light absorption), impact of electrons or heavy particles (radiolysis), and, especially in the condensed phase, ion neutralization. To these may be added processes such as energy transfer, dissociation from super-excited and ionized states, thermal processes, and chemical reaction. Following Brocklehurst [14], it is instructive to consider some of the direct processes giving excited states and their respective inverses. Thus luminescence is the inverse of light absorption, super-elastic collision is the inverse of charged particle impact excitation, and collisional deactivation is the inverse of the thermal process, etc. 

Several cases in which excited ions are deactivated on collision have already been discussed in previous sections and are included in Tables I and II. Collisional deactivation of electronically excited ions (i.e., quenching of metastable states) quite likely occurs in competition with many of the reactive channels shown in Table I, although it has been specifically studied or discussed for only a few systems. Collisional deactivation is at east partly responsible for attenuation of the ion beam when the attenuation technique (described earlier) is employed to determine the abundance of electronically excited ionic states. 

Photofragmentation in the liquid phase. Phorodissociation reactions in liquid phase occur at much reduced quantum yields because of the possibility of recombination within the solvent cage. Furthermore the product formation and distribution also differ because of collisional deactivation of initially produced vibrationally excited hot molecules. Where CT absorption produces radical ions, the solvent may react with the ionic species. 

Figure 32. Rate constants for collisional deactivation of electronically excited ions on collision with 02 as function of kinetic energy in center-of-mass system.46 ...

A somewhat more systematic study has been made of collisional deactivation of vibrationally excited ions. Some diatomic and triatomic systems that have been investigated are included in Table II. Vibrational-to-rotational transfer has been demonstrated95 for vibrationally excited H2+ colliding with helium ... 

Up to this point we have discussed collisional deactivation of vibration-ally excited ions formed by ionization or as products of exoergic particle-transfer ion-molecule reactions. A somewhat different situation prevails with larger vibrationally excited ions, such as those formed as intermediates in ion-molecule association reactions. Reactions in which such excited intermediates are formed generally demonstrate a third-order dependence of the rate on the concentrations of the reactants at relatively low pressures. The general reaction mechanism may be represented as... 

Ion-molecule association reactions and the collisional deactivation of excited ions have been the subjects of recent reviews.244-246 Several systematic studies have been performed in which the relative deactivating efficiencies of various Mf species have been determined. By applying the usual kinetic formulations for the generalized reaction scheme of equation (11.31), and assuming steady-state conditions for (AB+), an expression for the low-pressure third-order rate coefficient can be derived ... 

This effect of N08 ion is quantitatively consistent with a reaction mechanism (43) in which N08 interacts with an electronically excited water molecule before it undergoes collisional deactivation by a pseudo-unimolecular process (the NOs effect is temperature independent (45) and not proportional to T/tj (37)). Equation 1, according to this mechanism, yields a lifetime for H20 of 4 X 10 10 sec., based on a diffusion-controlled rate constant of 6 X 109 for reaction with N08 Dependence of Gh, on Solute Concentration. Another effect of NOa in aqueous solutions is a decrease in GH, with increase in N08 concentration (5, 25, 26, 38, 39). This decrease in Gh, is generally believed to result from reaction of N08 with reducing species before they combine to form H2. These effects of N08 on G(Ce+3) and Gh, raise the question as to whether or not they are both caused by reaction of N08 with the same intermediate. 

The yields calculated using these assumptions are in satisfactory agreement with the experimental values, as illustrated in Table 18. The observation that, among all the fragmentation processes induced by the decay, only reaction (27) is prevented by collisional deactivation, supports the view that the excitation level of the propyl ions that dissociate at 10 torr into allyl ions is low indeed, and that such decomposition is possible for its low energetic requirements. The situation is completely different in the liquid phase. This is indicated, in the first place, by the substantial decrease of the yield of HT, which provides a rough indication... 

In its simplest form, LEI is a two-step process (see Fig. 3). It involves three quantum states the atomic ground state, an atomic excited state, and an ionic ground state. For excited levels very near the ionization potential, ionization rates approach collision rates, giving ion yields near unity. The essential steps for LEI, photoexcitation and thermal ionization, are not the only processes occurring in an atmospheric pressure flame. An excited atom can also be collisionally deactivated or fluoresce. A detailed description of signal production requires a complex expression involving several competing rate constants 25). 

In the oxygen system at approximately 50 mm. pressure (collision frequency — 109 sec. 1) half of the 02 " ions are stabilized before emission can take place (13). In the condensed phase, therefore, deactivation should compete to the exclusion of electron emission. The much higher probability of collisional deactivation in liquids may explain why compounds such as C02 and CH3C1, for which attachment is very inefficient in the gas phase, are often effective electron scavengers in liquid systems. One must be wary, therefore, of using even relative gas-phase electron attachment coefficients in liquid-phase studies. For molecules with very small electron affinities (< 0.1 e.v.) the reversibility of Reaction 3 may have to be considered even after the excitation energy of the negative ion has been removed by collision. 

The excess electrons sooner or later react with the positive ions. If the parent ion has not fragmented or reacted in an ion-molecule reaction, excited states of the parent molecule will be formed. In the gas phase fragmentation of the parent ion is important, in the condensed phase less so, because of collisional deactivation and the cage effect. 

A significant characteristic of the radiation chemistry in the gas phase is the importance of the fragmentation of the excited molecules and ions. Ion fragmentation competes with collisional deactivation, but also with ion-molecule reactions and charge neutralization, the relative importance of which is dependent on the pressure and on the radiation intensity (dose rate). Dissociation of neutral excited states is of course also affected by the pressure. As a result the final product distribution will in general be dependent on the pressure as well as on the dose rate. 

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