Biphotonic photoionization

This view is been confirmed by an electrochemical product study (Hatta et al. 2001) that is discussed below. The pfCa value of the Thy radical cation has been determined at 3.2 (Geimer and Beckert 1998). When the position at N( ) is substituted by a methyl group and deprotonation of the radical cation can no longer occur at this position, deprotonation occurs at N(3) (Geimer and Beckert 1999 for spin density calculations using density functional theory (DFT) see Naumov et al. 2000). This N(3) type radical is also produced upon biphotonic photoionization of N(l)-substituted Thy anions [reaction (7)] in basic 8 molar NaC104 D20 glasses which allowed to measure their EPR spectra under such conditions (Sevilla 1976). 

Malone ME, Cullis PM, Symons MCR, Parker AW (1995) Biphotonic photoionization of cytosine and its derivatives with UV-radiation at 248 nm - an EPR study in low-temperature perchlorate glasses. J Phys Chem 99 9299-9308... 

Photoionization at 193 nm in oxygenated solution of poly(C) causes strand breakage with high efficiency, half of which occurs at times < 4 ms, the other half with a half-life of 7 ms (Melvin et al. 1996 Table 11.8). This kinetic behavior is very different from what is seen after -OH-attack and points to the direct involvement of the Cyt radical cation. In poly(U), the (biphotonic) photoionization shows similar results (Table 11.8). With poly(A), the formation of strand breaks is 20-times less efficient as compared to poly(C) (Table 11.8), and this is in agreement with the above conclusion that the A-+ or A- do not cause strand breakage to any major extent. 

Buettner, G.R., Hall, R.D., Chignell, C.F., and Motten, A.G. (1989) The stepwise biphotonic photoionization of chlorpromazine as seen by laser flash photolysis, Photochem. Photobiol., 49, 249-256. 

Primary photoprocesses and photochemical behaviour of proflavin have been investigated in aqueous and anionic micellar solutions. Micellar environments affect the pK of the singlet and triplet excited states owing to differences in surface and bulk pH. The photoredox behaviour is due to monophotonic photoionization and triplet-triplet annihilation gives rise to oxidized and reduced proflavin radicals in water, whereas biphotonic photoionization occurs in micelles. Photoredox processes have also been studied in Zn-tetraphenylporphyrin-methylviologen surfactant assemblies. 

The question whether or not similar effects play a role in the photoionization of phenols, could be studied only in aprotic surroundings (ionization energy around 8 eV). By using photon energies of about 5eV biphotonic ionization but also monophotonic fragmentation of the phenol molecule leading to phenoxyl radicals [6] takes place. To overcome this complication we used sterically hindered phenols where the first excited singlet relaxes by internal conversion only [7]. Results of biphotonic ionization with UV lasers of different pulse durations are reported and interpreted in this paper. 

When an electron acceptor is in the system, the ionization is greatly facilitated either by the contribution of charge transfer excitation or by the interaction of excited donor with acceptor to release an electron. These photoionizations occur by a uniphotonic process, whereas a biphotonic process obtains in the absence of an electron acceptor (9). Charge transfer complex and ionization are thus closely related. 

More recently, photoionization following the same mechanism as described for 132 has been reported for 1-naphthylmethyl radical (154) [147] and 4-biphenyl-methyl radical (140) [ 148] (Scheme 21). Thus, the lowest excited state of the radical forms a chloro-adduct in carbon tetrachloride, presumably as a result of charge transfer followed by trapping, while an upper excited state produced by biphotonic excitation photoionizes and the carbocation product is quenched by alcohol. 

In polar solvents the biphotonic nature of the photoionization was verified, but it was proposed that the intermediate is not the triplet state but rather a semi-... 

Photoemission from excited single states produced by photoionization of anthracene crystals occurs after two step laser excitation Biphotonic excitation of phenanthrene under 208 nm irradiation is a complex process involving both ionization andT-T annihilation. Change transfer exciton band structures have been characterized with samples of crystalline tetracene . Measurement of the photoionization efficiency in trans-stilbene crystals as a function of excitation energy shows that ionization occurs after rapid vibronic relaxation o. 

The photoionization of 9-anilino-l-naphthalenesulphonate (ANS) is shown to occur through a biphotonic process. An intermediate charge-transfer complex... 

Both silica and alumina have served as a host for oxidation of benzene by ultraviolet irradiation, leading to the benzene dimer cation radical (benzene) (Tanei, 1968). Photoionization here is thought to be biphotonic (p. 180). On the other hand, the formation of perylene and anthracene cation radicals on silica alumina is enhanced by ultraviolet irradiation, and the process is found to be monophotonic (Takimoto and Miura, 1972). The fate of the photo-ejected electron is, of course, not known, a state of ignorance which pertains to all of the cation-radical forming reactions on catalyst surfaces. 

The ionization energy of TMPD in solid and liquid solutions of hydrocarbons was studied by biphotonic ionization and detection of recombination fluorescence (Bullot and Gauthier, 1976). Generally, the photoionization threshold decreased on going from the solid to the liquid phase. The following sequence was observed ... 

Aromatic ketones. The photophysics and photochemistry of benzophenone have been widely studied as a model for aromatic ketones. Earlier results on the spectroscopic properties of the transient radicals and their reactivities have been reviewed by Kavarnos and Turro, and questions related to the photoionization of benzophenone have been summarized by Corner et TR EPR studies of benzophenone in micellar solutions have been published by Murai. The photoionization of benzophenone and some of its derivates (monosubstituted carboxylic acids, tetracarboxyhc add) were investigated in our laboratory using FT EPR. The spin polarization (TM CIDEP) of the hydrated electron and the successor radicals of the benzophenone radical-cation indicates that the biphotonic mechanism of the photoionization of all benzophenone derivatives occurs via the triplet state of the benzophenones. 

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