Monophotonic processes

Two-photon processes caused by absorption of photons by reaction intermediates and excited states are common under condition of high-power laser excitation. The consequence of two-photon excitation can include the formation of new reaction intermediates (electron photoejection is common) and the partial depletion of intermediates formed in monophotonic processes. To minimize this problem, do not use higher laser power then required to obtain a good signal/noise ratio, and do not focus the laser too tightly. There are in fact techniques used to obtain a more diffuse and homogenous laser beam (see below). 

The quantum yield of photochemical processes can vary from a low fractional value to over a million (Section 1.2). High quantum yields are due to secondary processes. An initially excited molecule may start a chain reaction and give rise to a great number of product molecules before the chain is finally terminated. For nonchain reactions, the quantum yields for various competitive photophysical and photochemical processes must add up to unity for a monophotonic process if the reaction occurs from the singlet state only ... 

Fig. 3 Energy level scheme of phenol indicating the mechanistic differences for fs- and ns-biphotonic ionization as well as the assumed path for the monophotonic process according to [10].

Solvated electrons do not inevitably interfere in photoinduced electron transfer. Their observations are often made under laser irradiations in order to detect these transients efficiently. Under these conditions processes may occur in a multistep and biphotonic way [68], the triplet state being one of the possible intermedites [69], The two photon process of electron ejection may dominate under pulsed laser conditions of high excitation energy while a monophotonic process prevails under continuous laser intensity conditions. These differences may explain the quantum yields observed for instance for the electron photoejection from excited phenolate in water under different irradiation conditions (0.23 [70], 0.17 [71], 0.37 [72]). When using conventional light sources, a relatively low yield of solvated electron is to be expected [69, 72]. 

A reason for the limitation of the sensitivity to a level situated below the theoretical value for a monophotonic process, despite the chain character of the reaction, can be sought In an important loss of chain carriers or by an Inadequate supply of oxygen. 

The photolonlzatlon process was exhibited both by steady state ultraviolet Irradiations and flash photolysis studies (24-31). Feltelson (24) studied the electron ejection process by steady state ultraviolet irradiations at 254 nm In aqueous solutions as a function of pH and temperature. His work indicated that the hydrated electrons originated from the fluorescent state. Mlalocq et al. (31) monitored the formation of both e at 660 nm and the Indole cation radical IH at 600 nm In picosecond laser photolysis of Indole and tryptophan. They report a quantum yield of 0.21 for the formation of e and conclude that It arises primarily from a monophotonic process Involving an excited singlet state. 

Zechner et al. (30) also report that photolonlzatlon Is the major primary photoprocess for Indole In aqueous solutions. However, they conclude that electrons are ejected from a relaxed singlet state. In earlier work. Bent and Hayon (26 also observed spectrophotometrlcally the production of e q and IH In nanosecond laser pulses. They conclude that the photolonlzatlon of Indole occurs primarily by way of a monophotonic process Involving either a higher excited single state or a vlbratlonally excited lowest singlet state, or both. They report a photolonlzatlon quantum yield of 0.26 for Indole at 25°C. 

With photochromic systems, as with other areas of photochemistry, we are normally using monophotonic processes in which a molecule absorbs one photon. However, it is possible to have two-photon or multi-photon photochromic systems. These have certain attractive properties. Two possibilities exist [51]. In the first (sequential) case, a molecule absorbs one photon to form its excited state. This (or a subsequent species) may then absorb a second photon to give the product ... 

Figure 3.18 Schematic representation of transition moment integral for monophotonic and biphotonic transitions in naphthalene. (A) Transition forbidden by one photon process (B) Allowed by two photon process.

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. 

Monophotonic photochemical reactions are those where each absorbed quantum excites one molecule which then reacts. Rates are usually directly proportional to the light intensity. Where secondary reactions are set up, however, the proportionality changes, depending on the chain termination processes. If chain intermediates are terminated by unimolecular reactions,... 

Vialaton et al. have reported that irradiation of naphthalene in air saturated aqueous solution produces mainly 7-hydroxy-1,4-naphthoquinone (major), 2-formylcinnamaldehyde, and 2-carboxycinnamaldehyde with a quantum yield of 0.0025 shown in Scheme 1 [7]. They proposed that phototransformation of naphthalene may proceed by a monophotonic ionization process to produce the naphthalene radical cation which can depro-tonate and/or react with water to produce radicals that are oxidized by molecular oxygen to yield the observed 2-formylcinnamaldehyde and 2-carboxycinnamaldehyde. It was further proposed that the formation of 7-hydroxy-1,4-naphthoquinone (major) is likely to result from the photolysis of 1,4-naphthoquinone that can form as an unstable intermediate [8]. 

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. 

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 formation of the solvated electron and radical cation may result from the loss of an electron from an excited EDA complex to yield a radical cation and ground state acceptor molecule (2 0). This is a suitably low energy process to be monophotonic ... 

The dual exponential behavior was apparent at the lowest laser intensity ( 0.5 mJ) and well below the onset threshold at which similar behavior became apparent for 16. Although the probability of depositing 2 photons onto the tetrakisporphyrin ensemble 17 is twice that for the corresponding bisporphyrin 16, the high concentration and low photon density employed should ensure monophotonic excitation of 17. Therefore, the dual exponential decay kinetics observed at low laser intensity are not due to triplet-triplet annihilation but must arise from an additional process. 

The quantum efficiency of secondary photochemical processes varies from fractional value to several millions. This is due to the fact that initially excited molecule starts a chain of excitation (like photo degradation of ozone by chloro-floro hydrocarbons). Those reactions which are monophotonic occur from singlet excited state... 

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