Time-resolved chemically induced dynamic

Time-Resolved Chemically Induced Dynamic Electron Polarization and Optical Emission Studies... 

It was initially assumed that the degradation of the phenacyl aryl ether bond occurred through the u-tt triplet excited state [14-18]. However, quenching experiments and time-resolved chemically induced dynamic nuclear polarization (CIDNP) spectra have indicated that there is significant photodegradation through the singlet excited state [23-27]. 

Both CIDNP and ESR techniques were used to study the mechanism for the photoreduction of 4-cyano-l-nitrobenzene in 2-propanol5. Evidence was obtained for hydrogen abstractions by triplet excited nitrobenzene moieties and for the existence of ArNHO, Ai N( )211 and hydroxyl amines. Time-resolved ESR experiments have also been carried out to elucidate the initial process in the photochemical reduction of aromatic nitro compounds6. CIDEP (chemically induced dynamic electron polarization) effects were observed for nitrobenzene anion radicals in the presence of triethylamine and the triplet mechanism was confirmed. 

Time-resolved laser flash ESR spectroscopy generates radicals with nonequilibrium spin populations and causes spectra with unusual signal directions and intensities. The signals may show absorption, emission, or both and be enhanced as much as 100-fold. Deviations from Boltzmann intensities, first noted in 1963, are known as chemically induced dynamic electron polarization (CIDEP). Because the splitting pattern of the intermediate remains unaffected, the CIDEP enhancement facilitates the detection of short-lived radicals. A related technique, fluorescence detected magnetic resonance (FDMR) offers improved time resolution and its sensitivity exceeds that of ESR. The FDMR experiment probes short-lived radical ion pairs, which form reaction products in electronically excited states that decay radiatively. ... 

Appropriate modifications of the ESR spectrometer and generation of free radicals by flash photolysis allow time-resolved (TR) ESR spectroscopy [71]. Spectra observed under these conditions are remarkable for their signal directions and intensities. They may be enhanced as much as one hundredfold and may appear in absorption, emission, or in a combination of both modes. These spectra indicate the intermediacy of radicals with substantial deviations from equilibrium populations. Significantly, the splitting pattern characteristic for the spin density distribution of the intermediate remains unaffected thus, the CIDEP (chemically induced dynamic electron polarization) enhancement facilitates the detection of short-lived radicals at low concentrations. 

In order to study the viscosity effect on the quenching of triplet excited state of (53) by TEMPO, chemically induced dynamic electron polarization and transient absorption spectra have been measured in ethylene glycol, 1,2-propanol and their mixtures. The results indicate that the quenching rate constant is viscosity-dependent and decreases linearly with the increase in solvent viscosity. The spectroscopy and dynamics of near-threshold excited states of the isolated chloranil radical anion have been studied using photoelectron imaging taken at 480 nm, which clearly indicates resonance-enhanced photodetachment via a bound electronic excited state. Time-resolved photoelectron imaging reveals that the excited state rapidly decays on a timescale of 130 fs via internal conversion. ... 

Many of the fiindamental physical and chemical processes at surfaces and interfaces occur on extremely fast time scales. For example, atomic and molecular motions take place on time scales as short as 100 fs, while surface electronic states may have lifetimes as short as 10 fs. With the dramatic recent advances in laser tecluiology, however, such time scales have become increasingly accessible. Surface nonlinear optics provides an attractive approach to capture such events directly in the time domain. Some examples of application of the method include probing the dynamics of melting on the time scale of phonon vibrations [82], photoisomerization of molecules [88], molecular dynamics of adsorbates [89, 90], interfacial solvent dynamics [91], transient band-flattening in semiconductors [92] and laser-induced desorption [93]. A review article discussing such time-resolved studies in metals can be found in... 

Detailed studies on the decomposition of organic peroxides are of fundamental interest and of high importance in polymerization reactions. The time-scales of intermediate radical formation and of their subsequent decomposition determine process parameters such as the initiator efficiency in radical polymerizations. An improved understanding of the mechanism and dynamics of photo-induced fragmentation is achieved by systematic investigations in which quantum-chemical calculations are carried out in conjunction with highly time-resolved experiments. 

The seminal work of Marcus and Hush has had a significant impact on the development of PET. Pioneering efforts by Sutin, Hopfield, Jortner, and others established the connection between thermal electron transfer and photoelectron transfer [6]. This work set the stage for a notable series of experiments where laser flash spectroscopy [7], chemically induced nuclear polarization [8], resonance Raman spectroscopy [9], time-resolved microwave conductivity [10], and time-resolved photoacoustic calorimetry [11], to site only a few examples, have been successfully employed to chart the dynamics of PET in homogeneous solution, the solid-state, and organized assemblies. 

The chapters of this book are all theoretical in character. This reflects the fact that the conical intersection is a theoretical concept, and as such is not directly accessible to experimental observation. Nevertheless, the concepts, techniques and results discussed in this book are crucial for the interpretation of the observations in time-resolved spectroscopy and chemical kinetics on femtosecond time scales. It is hoped, therefore, that this book is of value not only for the theoretician, but also for the practitioneer in molecular spectroscopy, photochemistry, and collision-induced reaction dynamics. 

Laser-induced fluorescence data provide a wide variety of detailed information about physical and chemical reactions. Laser-based time-resolved (picosecond) fluorescence spectroscopic techniques have been used to investigate the mechanism of photo-stabilisation by UVAs such as benzophenones, ben-zotriazoles and polymer-bound UV stabilisers [117]. Such ultrafast spectroscopic measurements can provide insight into the dynamics of the primary energy dissipation processes in polymers and polymer additives following light absorption. Excimer LIF spectra of plasticised PVC showed two distinct regions... 

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