Photophysical/photochemical process

The absorjDtion of a photon initiating photophysical and photochemical processes can itself be an extremely rapid... 

A number of electronic and photochemical processes occur following band gap excitation of a semiconductor. Figure 5 illustrates a sequence of photochemical and photophysical events and the possible redox reactions which might occur at the surface of the SC particle in contact with a solution. Absorption of light energy greater than or equal to the band gap of the semiconductor results in a shift of electrons from the valence band (VB) to... 

Fig. 5. Photophysical and photochemical processes in a semiconductor cluster whereand represent chemical species, adsorbed on the surface of the semiconductor particle, which are capable of undergoing reduction and oxidation at rates and respectively. The subscript...

Functionalized polyelectrolytes are promising candidates for photoinduced ET reaction systems. In recent years, much attention has been focused on modifying the photophysical and photochemical processes by use of polyelectrolyte systems, because dramatic effects are often brought about by the interfacial electrostatic potential and/or the existence of microphase structures in such systems [10, 11], A characteristic feature of polymers as reaction media, in general, lies in the potential that they make a wider variety of molecular designs possible than the conventional organized molecular assemblies such as surfactant micelles and vesicles. From a practical point of view, polymer systems have a potential advantage in that polymers per se can form film and may be assembled into a variety of devices and systems with ease. 

In Chapter 2 we discussed a number of techniques used to study the various photophysical and photochemical processes occurring in anthracene and similar molecules. In that discussion we were primarily interested in the singlet state. In this chapter we will discuss some of the techniques available for studying the photophysical and photochemical properties of the triplet state. Most of our discussion will be directed to the photochemistry of simple ketones. 

Zhen Z, Tug C-H (1991) Hydrophobic effects on photophysical and photochemical processes excimer fluorescence and aggregate formation of long-chain alkyl 4-(N, N-dimethy-lamino ) benzoate in water-organic binary mixtures. Chem Phys Lett 180(3) 211—215... 

The progress achieved in the detailed understanding of photophysical and photochemical processes that may be induced by light-irradiation in particular spin crossover systems has driven research efforts towards the development of materials that may be used for various technological applications. Only relatively recently, reports have appeared exploring this field for Fe(III) spin crossover materials. 

Only excimer emissions were observed during laser pulse, a broad plasma-like emission was detected later, and fragmented radicals became distinct. Ablation behavior can be interpreted in terms of photophysical and photochemical processes, including Si - Si annihilation. 

The possible fate of excitation energy residing in molecules is also shown in Figure 2. The relaxation of the electron to the initial ground state and accompanying emission of radiation results in the fluorescence spectrum - S0) or phosphorescence spectrum (Tx - S0). In addition to the radiative processes, non-radiative photophysical and photochemical processes can also occur. Internal conversion and intersystem crossing are the non-radiative photophysical processes between electronic states of the same spin multiplicity and different spin multiplicities respectively. 

Picosecond time-resolved spectroscopy has defined the relevant photophysical and photochemical processes associated with the charge-transfer excitation of an arene complex such as anthracene with tetranitromethane... 

The term hv is used in equations for photophysical and photochemical processes to represent a photon. 

Figure 5.4 Competing photophysical and photochemical processes occurring from Si and Tj...

Knowledge of the dynamics of excited states is of major importance in understanding photophysical, photochemical and photobiological processes. Two time-resolved techniques, pulse fluorometry and phase-modulation fluorometry, are commonly used to recover the lifetimes, or more generally the parameters characterizing the S-pulse response of a fluorescent sample (i.e. the response to an infinitely short pulse of light expressed as the Dirac function S). 

A unified understanding of the viscosity behavior is lacking at present and subject of detailed discussions [17, 18]. The same statement holds for the diffusion that is important in our context, since the diffusion of oxygen into the molecular films is harmful for many photophysical and photochemical processes. However, it has been shown that in the viscous regime, the typical Stokes-Einstein relation between diffusion constant and viscosity is not valid and has to be replaced by an expression like... 

FIGURE 3.3 Scheme of photophysical and photochemical processes involved in the production of cytotoxic species during photoexcitation of photosensitizer (PS adapted from Castano et ai, 2004). 

Once a molecule is excited into an electronically excited state by absorption of a photon, it can undergo a number of different primary processes. Photochemical processes are those in which the excited species dissociates, isomerizes, rearranges, or reacts with another molecule. Photophysical processes include radiative transitions in which the excited molecule emits light in the form of fluorescence or phosphorescence and returns to the ground state and nonradiative transitions in which some or all of the energy of the absorbed photon is ultimately converted to heat. 

As mentioned in the introduction, the ability to shape femtosecond laser pulses with unprecedented precision is the key to efficient control of photophysical and photochemical processes at the quantum level. In this section, we present the fundamentals of femtosecond pulse shaping and introduce specific pulse shapes that are used in the experiments and simulations presented in the following sections. We start with the electric field of a bandwidth-limited (BWL) femtosecond laser pulse written in terms of its positive frequency analytic signal... 

The ketone group is a useful model for other types of chromophores because it can be selectively excited in the presence of other groups in polymer chains such as the phenyl rings in polystyrene and so the locus of excitation is well defined. Furthermore there is a great deal known about the photochemistry of aromatic and aliphatic ketones and one can draw on this information in interpreting the results. A further advantage of the ketone chromophore is that it exhibits at least three photochemical processes from the same excited state and thus one has a probe of the effects of the polymer matrix on these different processes by determination of the quantum yields for the following photophysical or photochemical steps l) fluorescence,... 

Wigner s spin conservation rules govern energy transfer between two molecules, photodissociation energetics, T—T annihilation energetics and other photophysical and photochemical processes. 

A very important bimolecular deactivation process is the electronic energy transfer (ET). In this process, a molecule initially excited by absorption of radiation, transfers its excitation energy by nonradiative mechanism to another molecule which is transparent to this particular wavelength. The second molecule, thus excited, can undergo various photophysical and photochemical processes according to its own characteristics. 

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 ... 

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