Photoionization charge-transfer

What role does the dopant play in APPI (Ionization of molecules having a low photoionization cross section (probability) has been shown to be enhanced by the use of a dopant that is introduced into the vaporized plume of analyte molecules the dopant is selected on the basis of its high UV absorptivity and serves as a charge transfer reagent). 

From the steady state fluorescence spectrum of indole in water a fluorescence quantum yield of about 0.09 is determined. Since the cation appears in less than 80 fs a branching of the excited state population has to occur immediately after photo excitation. We propose the model shown in Fig. 3a). A fraction of 45 % experiences photoionization, whereas the rest of the population relaxes to a fluorescing state, which can not ionize any more. A charge transfer to solvent state (CITS), that was also introduced by other authors [4,7], is created within 80 fs. The presolvated electrons, also known as wet or hot electrons, form solvated electrons with a time constant of 350 fs. Afterwards the solvated electrons show no recombination within the next 160 ps contrary to solvated electrons in pure water as is shown in Fig. 3b). 

When an electron acceptor is in the system, the ionization is greatly facilitated either by the contribution of charge transfer excitation or by the interaction of excited donor with acceptor to release an electron. These photoionizations occur by a uniphotonic process, whereas a biphotonic process obtains in the absence of an electron acceptor (9). Charge transfer complex and ionization are thus closely related. 

Figure 11. Photoion photoelectron coincidence studies of charge-transfer reactions of state-selected ions. Cross sections for nitric oxide symmetric charge-transfer reaction are plotted as function of reactant-ion kinetic energy and reactant-ion vibrational state (o = 0,1,2,3,4,5). Solid lines are linear least-squares fits to experimental data (not shown).86c...

In a study on the 193-nm photoionization of a series of dsODNs, it has been shown that the charge transfer to G/GG sites occurs predominantly via intra- and not interstrand charge migration (O Neill et al. 2001). This observation could be simulated by quantum-mechanical calculations. 

More recently, photoionization following the same mechanism as described for 132 has been reported for 1-naphthylmethyl radical (154) [147] and 4-biphenyl-methyl radical (140) [ 148] (Scheme 21). Thus, the lowest excited state of the radical forms a chloro-adduct in carbon tetrachloride, presumably as a result of charge transfer followed by trapping, while an upper excited state produced by biphotonic excitation photoionizes and the carbocation product is quenched by alcohol. 

Nonsymmetric charge transfer Symmetric (resonance) charge transfer Dissociative charge transfer Associative ionization Photoionization Cumulative ionization... 

Two general mechanisms are usually advanced to explain ionization of molecules in flames direct ionization by thermoionization, photoionization, or chemiionization and indirect ionization by charge transfer with other ions. The assessment of both mechanisms requires knowledge of the ionization potential of molecules. In the following discussion, computations developed in the Appendix are used to estimate approximate ionization potentials of polynuclear aromatic hydrocarbons. 

From determination of the charge transfer equilibrium constant in the C-C3H6 C F mixture, the long questioned value for the 300 K ionization energy of cyclopropane of 9.86 eV has been obtained in satisfying agreement with the results of ah initio calculations (IE = 9.8eV) and photoionization and photoelectron experiments (9.8-10.06 Moreover, the characteristics of the ion/molecule reactions of the so... 

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