Photo-detachment

It should be noted that the spectrum of the anion radicals is bleached by visible light. The photo-detachment of electrons from the anion radicals occurs. It is also interesting to note, in connection with the polymerization in the glass matrix mentioned in the following section, that the anion radicals disappear when the temperature of the glass is raised to about 133° K. 

Sofar the imaging results of Fig. 3.1 were discussed in very classical terms, using the notion of a set of trajectories that take the electron from the atom to the detector. However, this description does not do justice to the fact that atomic photoionization is a quantum mechanical proces. Similar to the interference between light beams that is observed in Young s double slit experiment, we may expect to see the effects of interference if many different quantum paths exist that connect the atom to a particular point on the detector. Indeed this interference was previously observed in photodetachment experiments by Blondel and co-workers, which revealed the interference between two trajectories by means of which a photo-detached electron can be transported between the atom and the detector [33]. The current case of atomic photoionization is more complicated, since classical theory predicts that there are an infinite number of trajectories along which the electron can move from the atom to a particular point on the detector [32,34], Nevertheless, as Fig. 3.2 shows, the interference between trajectories is observable [35] when the resolution of the experiment is improved [36], The number of interference fringes smoothly increases with the photoelectron energy. 

Absorption of UV or visible light from a flash lamp or a laser is used to produce the solvated electron, too. Two photolytic processes may occur (1) the photon energy is sufficient to ionize a solute and produce the electron, for instance, photo-detachment from an anion (Cl , Fe(CN)g , Na ...) (2) the laser intensity is high enough to induce ionization of the solvent via multiphoton absorption (e.g. HjO -I- hv 112 + e ). For water, at least 8 eV is needed to ionize the solvent. 

In photodetachment studies we need not be concerned with the coulombic interaction that dominates the neutralization channel in MPI at times <10 ° s, but since the threshold for ionization of an anion is higher in the liquid than in the gas phase, we obtain once again information on the polarization energy of the ion in the liquid prior to photo-detachment. The laser photodetachment mechanism has been used with inorganic ions, such as Fe(CN)g or organic ions such as phenolate, supported in a wide range of aqueous or organic liquids. 

First, let s note that experimentally the EA stands as a wild quantity being mostly indirect measured, for instance trough the Haber-Bom cycle and only recently improved since the advent of the laser photo-detachment experiments with negative ions (Hotop Lineberger, 1985) and by the electron transmission spectroscopy (Jordan Burrow, 1987). 

If the binding energy differs for different isobars of a species, photo-detachment can be used for efficient selection of the wanted isobar. 

The photo-detachment probability might depend on the polarization state of the laser. Detachment with linear polarized lasers can lead to interference effects between electrons ejected into different directions but collected again by the electric extraction field. This effect differs for circular polarization. 

With this technique the photo-detachment spectrum of NO ions has been measured [493]. Here the laser beam is reflected by a gold mirror into the anti-collinear direction of the ion beam (Fig. 4.36). The assembly is located in an electrode arrangement of three electrodes creating an electric field that bumps the ion packet over the mirror and returns it to the incident beam axis. 

Electron photo-detachment Ion intensities vs. photon energy (1) 1987WET/BRA (2) 1989CHE/ALB... 

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