Biphotonic absorption

From pulse radiolysis lifetimes of phenol radical cations between 300 and 500 ns are known [4, 9]. Laser photolysis (3 ns, 266nm up to 15 mJ) ofN2-purged solutions of up to 10 3 mol dm 3 phenols yields phenoxyl radicals as dominating products (Figure 2). In the spectrum only a little hint for the phenol radical cations exists. The inset shows that the phenoxyl radical formation does not depend linearly from the energy but appears by biphotonic absorption contradictory to the fs-experiments described above. 

The ground-state depletion is more commonly observed in photochemical experiments such as flash photolysis than the simultaneous biphotonic absorption. Another common deviation from the Beer s law due to the sequential absorption of two photons will be discussed later in this section. 

Multiphotonic Absorption the simultaneous absorption of more than one photon by a molecule (see also Biphotonic Absorption). 

The rate of radical formation was found to be proportional to the square of the incident light intensity. These results were interpreted according to the biphotonic absorption by naphthalene molecules, followed by energy transfer to the polymer. The mechanism can be... 

Siegel and Judeikis [124] have shown that the photodegradation of polydimethylsiloxane is sensitized by naphthalene. A stepwise biphotonic absorption process, involving the first triplet state of naphthalene as an intermediate, followed by energy transfer, has been proposed. It seems that the decomposition of the polymer occurs via Si—CH3 bond rupture and abstraction of hydrogen atoms by the methyl radicals to produce —Si—CHj radicals. Breaking of the — Si—CH3 bond has also been proposed as the most probable reaction in the vacuum ultraviolet photolysis of polydimethylsiloxane at 123.6 and 147 nm [125]. 

On photoirradiation with high-intensity lasers, each molecule can absorb two or more photons at the same time. For biphotonic absorptions what is the absorption wavelength required to produce the dissociation of benzene ... 

Lasers can be coupled efficiently to fiber optic devices to deHver intense monochromatic light precisely to the desired region of the body, including internal organs (see Fiber optics). As in other cases of laser-induced photochemistry, biphotonic effects may be important (87). Lasers also offer the advantage of being able to concentrate the incident energy in a spectral bandpass matched to the absorption band of the sensitizer. 

Figure 1. Time-dependent absorption spectra obtained after laser excitation (248 nm, 20 ns) of benzophenone (frans-2,3-diphenylaziridin-l -yljimine (134d) in oxygen-saturated TFE.64 Inset influence of the laser dose on the yields of stilbene (monophotonic) and diphenylmethyl cation (biphotonic).

Cyclized polyisoprene has been used as a photoresist by being sensitized with bisazides(1-3). Recently, H.Harada et al. have reported that a partially cyclized 1,2-polybutadiene showed good properties as a practical photoresist material in reproducing submicron patterns (U ). S.Shimazu et al, have studied the photochemical cleavage of 2,6-di(h -azidobenzal)cyclohexanone in a cyclized polyisoprene rubber matrix, and have reported that the principal photoreaction is the simultaneous cleavage of the both azido groups by absorption of a single photon with a U3% quantum yield(5 ). Their result does not support the biphotonic process in the photolysis of bisazide proposed by A.Reiser et al.(6 ). 

The study of short lived excited states is limited by the low concentra- lions in which they are created on excitation with normal light sources. The use of high intensity sources such as flash lamps with suitable flashing rates and laser sources have been helpful in this respect. Triplet-triplet absorption, absorption by excited singlet state to higher singlet state and Absorption by exciplexes (Section 6.6.1) can be effectively observed by sequential biphotonic processes. 

Since j> is independent of I , the ratio of the intensity of P-type delayed emission to that of prompt fluorescence should show linear dependence on the intensity of absorption. The square law dependence indicates the necessity of two photons for the act of delayed emission and is hence known as biphotonic process. It has been observed in fluid solutions of many compounds and also in the vapour state. 

In the nanosecond biphotonic photolysis the vibronically excited level reached by absorption of the first photon relaxes by IC to the first excited state (Figure 3), which is stronger acid than the ground state by up to six orders in magnitude [11]. Quantum chemical calculations showed that the O - H bond becomes a bit longer and the C - OH bond becomes shorter and more rigid. The lifetimes of the first excited singlet state of the sterically hindered phenols... 

Aromatic solutes can photosensitize the homolysis of C—H bonds in hydrocarbon solvents at low temperatures in rigid media.222,241,242 The reaction is biphotonic in rigid media where triplet lifetimes are sufficiently long, intense irradiation can populate second excited triplets by T-T absorption. The second excited triplets of several aromatic compounds apparently can transfer their energy to solvent molecules and cause bond cleavage. 

Recently, Meisel et al.177 have shown that hydrated electrons are produced with low efficiency upon light absorption by the lowest excited state of Ru(bpy) +, i.e., in a biphotonic process. 

This had been proposed, namely for a Rose Bengal derivative because the yield and the rate of formation of the radicals are dependent on the light intensity. A more recent analysis of the process performed in a micellar environment seems to agree with an alternative path (steps 25 and 26) still explaining a biphotonic behaviour [174] a more reactive but unidentified intermediate would be formed by absorption of a photon by the triplet state. 

Biphotonic excitation Also called two-photon excitation. The simultaneous (coherent) absorption of two photons (either same or different wavelength) the energy of excitation being the sum of the energies of the two photons. 

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