Biphotonic process

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. 

The excimer is formed as an intermediate which may dissociate into an excited singlet and a ground state singlet. The delayed excimer emission is a biphotonic process in contrast to the prompt excimer emission which is monophotonic. 

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. 

The only other limitation of Beer s law concerns biphotonic processes in which the apparent extinction coefficient depends on the intensity of light this can become important in conditions of laser photolysis. 

There are two distinct ways in which a biphotonic process can take place. 

The emission of delayed fluorescence through triplet-triplet annihilation (see section 3.4.2) can be taken as an example of this type of biphotonic process. 

Biphotonic processes depend greatly on light intensity, and on the lifetimes of the excited states M. Their importance increases markedly in conditions of very intense, pulsed light excitation, e.g. in laser beams or in flash light. 

Figure 4.62 shows the Jablonski diagram of the oxygen molecule, restricted to the first few states relevant to photo-oxidation processes. The phosphorescence Sj-Tq is very weak and is difficult to detect because it comes in the NIR at 1270 nm. There are however two other emissions at 634 nm and 703 nm which are due to a biphotonic process... 

We have seen that the limitations of the time characteristics of electronic devices requires the use of optical delays between the pump and probe pulses in ps flash photolysis. There are also indirect ways of using optical properties to measure the kinetics of laser pulses and of fluorescence, known as autocorrelation and up-conversion . These rely on the non-linear properties of certain materials or chemical systems, i.e. they are based on fast biphotonic processes. 

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. 

Biphotonic process A process resulting from biphotonic excitation. 

Ramanujam, P. S., Hvilsted, S., Zebgei I., and Siesler, H. W. (1995). On the explanation of the biphotonic processes in polyesters containing azobenzene moieties in the side chain. Macromol. Rapid Commun. 16, 455-461. 

Dithiolyl radicals are produced by photolysis of appropriate 1,2-di-thiolylium ions in ethanolic solutions.The reaction is a biphotonic process and involves ethanol as reductant. 

A flash-photolytic study of phenoxazine has been reported.Three transient species were observed the first triplet state, 253, and 248. The results are consistent with the formation of 248 in a biphotonic process involving the triplet. In the presence of oxygen, neither the triplet nor 248 was observed continuous-irradiation experiments indicated that probably 253 reacts with oxygen, via unstable hydroperoxide intermediates, to give, as stable photooxidation product, 3i/-phenoxazin-3-one. 

Thus a sample containing 0.0070 M TCPA acceptor was irradiated with narrower slits in the monochromator to reduce light intensity by about two-thirds, and the irradiation time increased proportionately. The observed quantum yield of photocross I inking was in fact reduced by a factor of ten (Table II), supporting the supposition that a biphotonic process is involved in the reaction catalyzed by TCPA. 

A recent LFP, electron spin resonance, and product study on the photodissociation of 7/-(triphenylmethyl)anilines concludes that the primary photochemical event is homolytic cleavage [72]. However, the chromophore in these molecules is the aromatic amine. Triphenylmethyl cations were observed when the excitation wavelength was 248 nm but these were apparently formed by a biphotonic process of electron photoejection from the triphenylmethyl radical. 

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