Electron dynamics, photodetachment

The behavior of CTTS states is dependent on energy levels of the ion-solvent molecular couphngs. These levels can lead to internal relaxation and/or complete electron detachment via adiabatic or nonadiabatic electron transfer. The ultrafast spectroscopic investigations of electronic dynamics in ionic solutions would permit us to learn more about the primary steps of an electron-transfer reaction within a cationic atmosphere. The influence of counterions on early electron photodetachment trajectories from a hahde ion can be considered as prereactive steps of an electron transfer. 

Figure 4. Comparative analysis of H-D isotope effects on elementary charge transfer, including electron photodetachment and localization, and electron-pro-tonated radical couplings in pure water at 294 K The dotted line represents the characteristic limit for which the electronic dynamics are independent of H-D...

Following the above-mentioned spectroscopic study by Johnson and co-workers [55], Neumark and co-workers [56] explored the ultrafast real-time dynamics that occur after excitation into the CTTS precursor states of I (water) [n — 4-6) by applying a recently developed novel method with ultimate time resolution, i.e., femtosecond photoelectron spectroscopy (FPES). In anion FPES, a size-selected anion is electronically excited with a femtosecond laser pulse (the pump), and a second femtosecond laser pulse (the probe) induces photodetachment of the excess electron, the kinetic energy of which is determined. The time-ordered series of the resultant PE spectra represents the time evolution of the anion excited state projected on to the neutral ground state. In the study of 1 -(water), 263 nm (4.71 eV) and 790 nm (1.57 eV) pulses of 100 fs duration were used as pump and probe pulses, respectively. The pump pulse is resonant with the CTTS bands for all the clusters examined. 

We present the results of experimental studies of photon-negative ion interactions involving the dynamics of two electrons. Resonances associated with doubly excited states of Li and He" have been observed using laser photodetachment spectroscopy. Total and partial photodetachment cross sections have been investigated. In the former case, the residual atoms are detected irrespective of their excitation state, while in the latter case only those atoms in specific states are detected. This was achieved by the use of a state selective detection scheme based on the resonant ionization of the residual atoms. In addition, in the case of Li-photodetachment, the threshold behavior of the Li(2 P)+e-(ks) partial cross section has been used to accurately measure the electron affinity of Li. 

An analj is of the interactions of ions with ligands leads to valuable information which is difficult to obtain otherwise. Important data for resolving the dynamical structure of the protonated methane cation CHs arises from the infra-red spectra of CH5 (H2)n [45,61]. The topology of shells is projected on the properties of bare ions. The process of the consecutive electron photodetachment from the central anion indicates the existence of the well developed shell structure of complexes. The theoretically predicted electronic affinities for O AXn clusters calculated for theoretical structures of complexes agree with the known measured values (Table 3) [62,63]. The ionization potential of CHa Arn clusters is little... 

Figure 6. Dynamics ofprimary electron-transfer processes triggered hy the femtosecond UV excitation of an aqueous sodium chloride solution ([H20]/[NaCl] = 55). The different steps of an electron photodetachment from the halide ion (Cl ) involve charge transfer to the solvent state (1,2), transient electron-atom couplings (4, 5), and the nonequilibrium state of excess electrons (3). The final steps of the multiple electron photodetachment trajectories (6, 7) are also reported. These data are obtained from time-resolved UV-IR femtosecond spectroscopic data published in references 85 and 86.

Figure 8. Energy-level diagram of ultrafast electron-transfer processes in aqueous sodium chloride solution. Transitions (eV) correspond to experimental spectroscopic data obtained for different test wavelengths. The abscissa represents the appearance and relaxation dynamics of nonequilibrium electronic populations (CTTS ", CTTS, (e hyd) fCl e pairs). The two channels involved in the formation of an s-like ground hydrated electron state (e hyd, c hyd ) (dso reported in the figure. From these data, it is clear that the high excited CTTS state (CTTS ) corresponds to an ultrashort-lived excited state of aqueous chloride ions preceding an electron photodetachment process.

In this chapter we will highlight recent experimental data on the picosecond dynamics of electron localization and solvation in polar liquids and on the ultrafast radiationless transitions that accompany laser excitation of e in the same systems. The specific issues we address concern (1) the mechanism for electron localization in polar liquids, (2) the molecular description of the solvation process in forming the cluster, and (3) the dynamics of electron transfer following photodetachment of an electron from its cluster. 

In order to study the viscosity effect on the quenching of triplet excited state of (53) by TEMPO, chemically induced dynamic electron polarization and transient absorption spectra have been measured in ethylene glycol, 1,2-propanol and their mixtures. The results indicate that the quenching rate constant is viscosity-dependent and decreases linearly with the increase in solvent viscosity. The spectroscopy and dynamics of near-threshold excited states of the isolated chloranil radical anion have been studied using photoelectron imaging taken at 480 nm, which clearly indicates resonance-enhanced photodetachment via a bound electronic excited state. Time-resolved photoelectron imaging reveals that the excited state rapidly decays on a timescale of 130 fs via internal conversion. ... 

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