Reactions of electronically excited ions

Participation of electronically excited ions in ion—molecule reactions has been most extensively studied for excited ions. 

Rutherford and Vroom [224] and Kaneko and Kobayashi [225, 226] have investigated the effect of excited ions in the primary ion beam on the measured cross-section of reaction (127). The role played by these excited ions in producing the peculiar data of this reaction (see Section 5.4) has been open to question. 

Using the fractional abundances obtained from the analysis of the above data and the data obtained using their previous method [221], they were able to determine the cross-sections for the separate ionic states for both reactions (127) and (133). The cross-sections for reaction (127) with only 0 ( S) ions and for reaction (133) with only 0 CD) ions are shown in Fig. 25 (curves denoted by RV), together with the results of other authors which are not necessarily for a single electronic state. It was concluded from their results that the contribution of the state to reaction (127) is negligible, at least in the eV region. An indication that this is true has also been reported by Stebbings et al. [227]. 

Kaneko and Kobayashi [225, 226], on the other hand, studied reactions (127) and (133) in the collision energy range from 0.04 to 3 eV using their injected-ion drift-tube mass spectrometer, and, surprisingly, found that the state has substantial cross-sections for reaction (127) in this energy range. Below 0.4 eV, the contribution from this state far exceeded that from the S state. Their results are also given in Fig. 25. KK—1 and KK—2 are the cross-section curves for reaction (127) for pure 0 ( S) and ions, respectively, obtained from the analysis of 

Their results clearly show that the cross-section for reaction (127) with 0 ( S) has a sharp minimum at 0.12 eV. This is consistent with the fact that the flowing afterglow experiments [228] show a decreasing cross-section at thermal energy while beam experiments [227] show an 

Reactions carried out at high pressures in several instances have shown that certain electronically excited species can undergo reaction. In each case, the appearance potential of the product ion did not correspond to that of any known fragment of the parent molecule. Thus, the ion is observed in nitrogen at an appearance potential of 20-22 eV. Kaul and Fuchs S found a value of 20 eV, Saporoschenko found 22.1 eV, Cermak and Herman found 21.1 eV and Munson et found 21.7 eV. Since the ionization potential of N2 is 15.567 eV and the appearance potential of N from N2 is 24.3 eV, the N3 must result from an excited nitrogen molecule ion. Cermak and Herman have shown the N2 to be in the long-lived state so that the reaction is 

In the mass spectrometer source, then, the process leading to reaction 

For a single excited ionic state, rate treatment of this process gives  

Kaul and Fuchs and Munson et observed, in addition, the reaction of excited with Ar, Kr, and Xe at electron energies of 21-22 eV 

Curran and Franklin and Munson have studied the formation of Oj and O in oxygen and found them to be second and third order in pressure, respectively. Both have appearance potentials of 17.0 eV indicating 02 ( n ) to be the reactant ion and the reactions to be 

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The study of reactions of electronically excited ions is not particularly new. For instance, the appearance potential method with conventional electron beam sources was used in early days to recognize the participation of excited ions in certain ion—molecule reactions. In 1960, Cermak and Herman [230] and Dong and Cottin [231] measured appearance potentials of a number of secondary ions in some simple systems, such as N2, O2, CO, CO2, SO2, CS2 and COS, and concluded that specific excited states of reactant ions are involved in these reactions. Kaul and Fuchs [232] were able to show that N3 ions are formed by the reaction of the excited molecular ions, viz. 

Though pure preparations, or known distributions, of electronically excited states are possible only in a few cases, the fine energy resolution of photoionization makes it a powerful tool for the study of reactions of electronically excited ions. This resolution usually makes possible the precise identification of an excited state when its reaction probability is significantly different from that of the lower states. In the reaction of Ar with H2 to form ArH, for example, the reaction cross section for Ar in its Pi/2 excited state is only about 30% larger than that for its P3/2 ground state but this difference makes the threshold for formation of the excited state clearly apparent in the photoionization efficiency curve for ArH formed in a mixture of Ar and When the ratio of the reactivi-... 

The inverted region in electron transfer reactions is studied for the reaction of electronically-excited ruthenium(II) tris-bipyridyl ions with various metal(III) tris-bipyridyl complexes. Numerical calculations for the diffusion-reaction equation are summarized for the case where electron transfer occurs over a range of distances. Comparison is made with the experimental data and with a simple approximation. The analysis reveals some of the factors which can cause a flattening of the In k versus AG curve in the inverted... 

In this chapter we have described the photophysics and photochemistry of C6o/C70 and of fullerene derivatives. On the one hand, C6o and C70 show quite similar photophysical properties. On the other hand, fullerene derivatives show partly different photophysical properties compared to pristine C6o and C70 caused by pertuba-tion of the fullerene s TT-electron system. These properties are influenced by (1) the electronic structure of the functionalizing group, (2) the number of addends, and (3) in case of multiple adducts by the addition pattern. As shown in the last part of this chapter, photochemical reactions of C60/C70 are very useful to obtain fullerene derivatives. In general, the photoinduced functionalization methods of C60/C70 are based on electron transfer activation leading to radical ions or energy transfer processes either by direct excitation of the fullerenes or the reaction partner. In the latter case, both singlet and triplet species are involved whereas most of the reactions of electronically excited fullerenes proceed via the triplet states due to their efficient intersystem crossing. 

The similar system K -)- H2 KH + H has also been studied, with similar information on the reaction dynamics [155, 156] the reaction of electronically excited potassium follows a collinear geometry, producing KH via the ion-pair intermediate K" H H. Very interestingly, this reaction is state specific K(7s S) leads to reaction, whereas K(5d D) does not. We shall come back to this important question later. 

Type C comprises a large number of dye-photosensitized reactions and usually involves a radiationless transition from the excited singlet to a triplet state prior to free radical formation in subsequent reactions. They are not discussed in detail here, although the similarity of the dye-photosensitized reaction with the reactions photosensitized by uranyl ion is noteworthy. Attention must also be drawn to Simons excellent review 18) of reactions of electronically excited molecules in solution, in which photochemical reactions of type C, including those involving energy transfer, are dealt with thoroughly. 

Metastable oxygen ions 02(a ITu) react fast with atoms or molecules (Ar, N2, CO, H2) in reactions that are endothermic with ground-state oxygen ions the respective rate constants do not depend much on translational energy. An interesting case is the charge-transfer reaction of electronically excited NO+(a S" ) with Ar The reaction is endothermic by 0.09 eV, and its rate constant increases with translational energy from the thermal value of 3 x 10 " cm s to a value of about 9 x 10 cm s at 3 eV in a way typical of... 

The ion chemistry of the upper TA (summarized in Figure 4) proceeds to convert the energetic precursor ions to the less energetic, predominantly diatomic molecular ions, which can be neutralized by the ambient free electrons via the process of dissociative recombination as shown. It was also realized that metastable electronically excited ions of 0+ and O 2 are produced in the upper ionosphere and so it became important to study the reactions of these excited ions. The SIFT is well suited to these studies (excited ions are quenched in flowing afterglow plasmas). The chemistry of these excited species is represented in Figure 4 by the thick lines. 

Reactions of Complex Ions. For reactions of systems containing H2 or HD the failure to observe an E 1/2 dependence of reaction cross-section was probably the result of the failure to include all products of ion-molecule reaction in the calculation of the experimental cross-sections. For reactions of complex molecule ions where electron impact ionization probably produces a distribution of vibrationally excited states, kinetic energy transfer can readily open channels which yield products obscured by primary ionization processes. In such cases an E n dependence of cross-section may be determined frequently n = 1 has been found. 

Section III.C). Using a rotational temperature to characterize an ion source can be misleading, as the reactions used to form the ions of interest can be quite exothermic, producing vibrationally and even electronically excited ions. These degrees of freedom are more difficult to cool than rotations. Transitions from vibrationally excited molecules provide very useful information, if they can be identified and analyzed. Hot FeO (produced using 3% N2O in helium) has a... 

Examination of the reaction kinetics of the M+ + H2S reactions show that these reactions are not simple first-order reactions, that is, nonlinear slope for the rate of disappearance of M+ shown in Fig. 7 for Pt+. The non-first-order rate of disappearance of M+ suggests that there is more than one intermediate, possibly due to the presence of electronic excited states of the metal ions or intermediates with different interactions between the metal and H2S. The addition of H2S to Au+ is similar to the reaction of H2S with Ag+ and Cu+ (M+ — [MH2S]+ — [M(H2S)2]+), but is dissimilar to most of the second- and third-row transition metal ions. 

Several books and review chapters devoted to the field of ion-neutral reactions in the gas phase have appeared in recent years, la 8, j,k some of which are concerned at least in part with the special topic of interest for the present review chapter—namely, the role of excited states in such interactions. The present review attempts to present a comprehensive survey of the latter subject, and the processes to be discussed include those in which an excited ion interacts with a ground-state neutral, interaction of an excited neutral with a ground-state ion, and on-neutral interactions that produce excited ionic products or excited neutral products. Reactions in which ions are produced by reaction of an excited neutral species with another neutral, for example, Penning ionization, are not included in the present chapter. For a recent review of this topic, the reader is referred to the article by Rundel and Stebbings.1 Electron-molecule interactions and photon-molecule interactions are discussed here only as they relate to the production of ions in excited states, which can then be reacted with neutral species. 

Recent advances in experimental techniques, particularly photoionization methods, have made it relatively easy to prepare reactant ions in well-defined states of internal excitation (electronic, vibrational, and even rotational). This has made possible extensive studies of the effects of internal energy on the cross sections of ion-neutral interactions, which have contributed significantly to our understanding of the general areas of reaction kinetics and dynamics. Other important theoretical implications derive from investigations of the role of internally excited states in ion-neutral processes, such as the effect of electronically excited states in nonadiabatic transitions between two potential-energy surfaces for the simplest ion-molecule interaction, H+(H2,H)H2+, which has been discussed by Preston and Tully.2 This role has no counterpart in analogous neutral-neutral interactions. 

Several examples of exoergic charge-transfer reactions that proceed at different rates with ground-state and electronically excited ions are listed in Table I. In some cases the cross section for the excited-state reaction may be smaller than that for the ground state, as is the case for the reactions Xe+(02, Xe)02+ Kr+(N20, Kr)N20+ Kr + (C02, Kr)COz+, whereas in other instances the excited state is more reactive, as for the processes N+(Kr,N)Kr+, N+(CO,N)CO+, 02+(Na,02)Na and 0/ (NO, 02)N0 +. The differences in reactivity are often more pronounced in the region of low ion translational energies1 lb (Fig. 10). The role of excited-state ions in charge-transfer reactions was reviewed by Hasted some time ago,175 but much more experimental data has been obtained recently, as indicated by the data shown in Table I. 

The effects of electronically excited reactant ions (AB+) on collision-induced dissociations have been studied in a number of systems, including those for which AB+ =0, N, N0+,C0+, and for which the neutral target is either a rare-gas atom or another neutral molecule. Various reactions of this type that have been investigated are included in Table I. 

Figure 22. Cross section for collisional dissociation reaction, 02 (02 02,0)0+, as function of energy of impacting electrons used to produce reactant. Solid curve represents cross section, bars indicate relative cross sections for various excited states, and arrows designate threshold energies for production of electronically excited states of O. Ion-beam energy in these experiments was 1.6 keV,38a...

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