Electron photodetachment channel

The direct photoexcitation of water molecules by ultrashort laser pulses is used for the investigation of primary events occurring from 10 s (thermal orientation of water molecules and ultrafast proton transfer) to 10" s (primary reactions of a solvated electron with protic species) (57,58,61-65). The nonlinear interaction of ultrashort UV pulses (typically less than 100 fs in duration and having a power of 10 W cm" ) with water molecules triggers multiple electron photodetachment channels within a hydrogen bond network (see equations 4-7). An initial energy deposition via a two-photon absorption process (2 X 4 eV) leads to the formation of nonequilibrium states of an excess electron... 

The second electron photodetachment channel occurring from excited electronic states of an ionic solute has been discriminated by femtosecond near-infrared spectroscopy (1.24-1.41 eV). This channel is characterized by the presence of two transient subbands peaking around 1.41 eV and wholly... 

Figure 9. Influence of ionic strength, < R)) (molecular ratio), on the relative spectral contributions of femtosecond photoinduced electron-transfer processes in aqueous sodium chloride solutions. The ionic strength is dj ined by the molecular ratio, R, which equals [H20]/[XC1], The different test wavelenghs (0.99, 1.24, 1.72, 1.77, and 1.88 eV) permit the discrimination of transient electronic states (CTTS, e m, electron-atom pairs Cl e Na ) and two configurations of the hydrated electron ground state ( e hyd)- concentrated ionic solution (R = 9), an electron photodetachment channel favors the formation of polaron-like states ( a e hyd) (see reference 86).

Figure 3. Time dependence of different electron-transfer trajectories in molecules of pure liquid water at room temperature. The femtosecond UV excitation of water molecules (2X4 eV) triggers either an ultrafast electron photodetachment with the formation of hydronium ions and a nonadiabatic relaxation of excited p-like hydrated electrons (high photochemical channel), or concerted electron-proton transfer (low photochemical channel) (56, 72). The characteristic time of each trajectory is reported on the curve.

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.

Figure 4.8 In (a) the principle oftransition state spectroscopy is shown. A continuous wave UV laser pulse with a frequency exceeding the energy gap between anionic potential (XHXq and neutral potential (XHX) is employed. After photodetachment of the electron, dissociation will occur equally in the two possible channels XH-tX and Xt-HX. In (b)...

The total photodetachment cross section describes the probability that an electron is detached from a negative ion following the absorption of a photon, regardless the excitation state of the residual atom or the energy or direction of the emitted electron. A total cross section is the sum of partial cross sections for detachment into each of the energetically allowed continua. This is illustrated in Fig. 1. Here we show the three possible channels accessible to a doubly excited state of Li" that lies just below the Li(32P) detachment threshold. The total cross section may be determined by... 

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