Photodetachment of electrons

Photodetachment (of electrons) Ejection of an electron from a negative ion upon photoexcitation. 

Note that solvated electrons can also be produced by photodetachment of electrons from certain anions or photoionization of molecules using UV excitation.Then the mechanism of metal ion reduction is expected to be quite similar to the radiolytic processes of Figures 2a, 2b and 2c. However, the matrices used as cluster hosts are generally not transparent to light, and the reduction is often restricted to the surface. 

Buben and co-workers (32, 41,42) investigated RPL excitation spectra, and they observed in aromatic matrices a similar concomitant decrease of two types of bands (42). They attributed one of these to anions and the other to trapped holes. In our case at least, this hypothesis does not apply because it would be incompatible with the observed restoration of the infrared response and of the first glow peak which can be fully explained by the photodetachment of electrons from biphenyl anions and their retrapping in the matrix. 

SOa is discussed in the last section of this chapter. The dye-laser photodetachment of electrons from SH has a threshold at 538.7 nm, indicating an electron affinity for SH of 2.301 eV.110 The reaction rate between CS and O (61)... 

Photodetachment of electrons from negative ions is a fundamental destruction process in chemical environments such as the upper atmosphere or interstellar space. In comparison with previous results, the measured cross sections seems to indicate that the cross section is proportional to... 

Fig. 6.15. Photodetachment of electrons from OH trapped at 180 K in a 22-pole. The small loss of ions (time constant 133 s) is significantly increased, if a He-Ne laser is switched on (here at 10s). The photon energy (1.96eV) is sufficient to detach the electron from the anion OH (electron affinity 1.8 eV). The solid lines are exponential fits. Measurements performed at various temperatures of the trap allow state specific rate coefficients to be extracted.

An important feature of photodetachment methods is that the selection rule for electronic transition is A5 = +1. Therefore, photodetachment of (doublet) negative ions... 

We have recently described another spectroscopic rnethod for observing IM reactions at atmospheric pressure that utilizes the photodetachment-modulated electron capture detector (PDM-ECD) as a means of monitoring the negative ions either consumed or produced in an IM reaction. The reaction of interest is made to occiu in a steady-state flow-through reactor in which ionization of the buffer gas is continuously caused by a Ni-on-Pt foil beta emitter. A chopped light beam of... 

Figure 4, Structure of a series of alkyl bromides and the rate constants (1 O " cm s ) for the IM reaction of each compound with the chloride ion, determined by the photodetachment-modulated electron capture detector (PDM-ECD) in 10% argon-inmethane buffer gas at atmospheric pressure and 125 °C. ...

Figure 5. The photodetachment spectra of Az measured by the photodetachment-modulated electron capture detector (PDM-ECD) in 10% argon-in-methane buffer gas at three different temperatures.

This step generates the TMM radical anion 27, which is selected for electron photodetachment of the mass 54 (TMM) anions from the beam. Two PE band... 

Photoionization of neutral atoms and molecules and electron-ion collisions, for example, are rich in infinite Rydberg series of Feshbach resonances. On the other hand, only a finite number of Feshbach (and possibly shape) resonances occur in electron-neutral collisions and photodetachment of an electron attached to a neutral species, with an exception of the following cases. 

The HSCC equations have been solved for various Coulomb three-body processes, such as photoionization and photodetachment of two-electron systems and positronium negative ions [51, 105-111], electron or positron collisions [52, 112-115], ion-atom collisions [116-119], and muon-involving collision systems [103, 114, 120-125]. Figures 4.6, 4.7, 4.8, 4.9, and 4.10 are all due to HSCC calculations. Figure 4.12 illustrates the good agreement between the results of HSCC calculations [51] and the high-resolution photoionization experiment on helium [126]. See Ref. [127] for further detailed account of the comparison between the theory and experiment on QBSs of helium up to the threshold of He+(n = 9). 

To focus the review, it is necessary to pass over some topics which are timely, relevant, and could be included were it not for space limitations. One of these is the area of electron photodetachment as applied to the study of the Franck-Condon region of the neutral PES accessed from the corresponding anion. This approach has been shown to provide information about the transition state region of several bimolecular reactions. This work was pioneered by Neumark and coworkers and an excellent review is already available (Manolopoulos et al. 1993 Neumark 1992). Another noteworthy area is state-to-state studies of vibrational predissociation in weakly bound complexes. Miller and coworkers have made impressive advances in which fully state and angle-resolved product distributions have been obtained (Bemish et al. 1994 Block et al. 1992 Bohac et al. 1986, 1992a,b Bohac and Miller 1993a,b Dayton et al. 1989), and these results have been used to bring theory and experiment into accord. The present review is limited to cases in which ultraviolet photodissociation of a complexed moiety initiates reaction. 

The co-existence of several isomers allows one, in principle, to check possible differences in their photophysical properties. In a recent paper, two-color mass-selective MPI was used to estimate the relative efficiency of the ionization of isomeric forms of DMOT-anthracene adducts [24]. It was found that photodetachment of an electron from the exciplex excited state is about 10 times more efficient than that from LE excited states. A similar observation was made in the case of Nal... 

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. 

Fluorescence (, 3, 247 nm in neat p-dioxane) occurs with a quantum yield of 0.029 (184) in the liquid state only (183). Addition of water effects a diminution and a red shift while isooctane causes a blue shift, as well as a reduction in intensity. O2 (183) and N2O (187) are quenchers. It is thought that the fluorescence is from some form of excited aggregate whose composition varies with dilution, and that monomeric p-dioxane does not fluoresce (183). It has also been suggested that photodetachment of an electron might occur, and fluorescence might be a consequence of charge recombination (187). 

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