Photoionization radical ion pairs

Although the effects of spin coherence have been mainly studied using radiation-chemical processes as an example, published are the first works on the MARY spectra of radical ion pairs produced in solutions by photoionization. Probably, there are no principle obstacles to the application of the method of quantum beats to these systems. Interpretation of results is expected to be more simple, in this case, because of the use of monochromatic sources of ionization and the absence of cross recombination effects typical of the ionization track. Another manifestation of spin coherence, observed experimentally but omitted in this review, is the beats induced by resonance microwave pumping [36-38]. The range of applications of this phenomenon for studying spin-correlated radical ion pairs has yet to be outlined. 

The intermediacy of a species analogous to (i) (Scheme 1.7) that some described as a resonance hybrid of a radical pair and an ion pair and others as an ultrafast equilibrium between radical and ion pair has been proposed in the photoionization reaction of benzyl derivatives [172, 174-176]. Also, interestingly, to explain the driving force behind the rotational motion that leads to an extremely rapid deactivation of S of TAM cations, Vogel and Rettig [177] proposed that SI promptly forms a biradicaloid charge-transfer state. 

Radical cations are produced photochemically by electron transfer transitions (p. 419) but only as ion pairs with the radical anion formed simultaneously. In most cases, the ion pair reverts to reactant by reverse electron transfer before any reaction can take place. Radical cations can, however, be prepared as stable entities by photolysis of compounds with low ionization potentials at low temperatures in glasses containing electron acceptors. Electrons expelled by photoionization wander through the glass until they are trapped by an acceptor. Since the acceptor is not in contact with the cation radical and since energy is required to extract the electron... 

Photoionization. - 2.1.1 Sulfite anion. The photoionization of the sulfite anion SOf was studied by Fessenden et a few years ago. The steady-state EPR spectra of the hydrated electron and the sulfite radical-anion SOs show no CIDEP effects and indicate the lifetime of the hydrated electron to be about 100 ps. The sulfite radical-anion is often used as g factor standard in photolysis EPR experiments [ [(SOs") = 2.00316]. Bussandri et al. studied the laser photolysis of sulfite ions in basic solution by FT EPR with very high time resolution. They observed the FT EPR spectra of hydrated electrons and sulfite radical-anions with absorption/emission (A/E) pattern caused by the radical pair mechanism (RPM CIDEP) with the electron line in emission [fif(eaq ) = 2.00044 at room temperature] and the sulfite radical-anion in absorption. In the time profiles of both lines, oscillations of the EPR intensities were observed in the first 300 ns. This coherent oscillation in both radicals is the first direct EPR observation of zero quantum coherence in freely diffusing radicals. Previously zero... 

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