Picosecond Light-induced Processes

The time-scale of molecular vibrations is of the order of 10 13 s, just outside the ps range. Internal conversions and in particular vibrational cascades therefore fall into the femtosecond (10-15s) time-scale. However, the spin-forbidden processes of intersystem crossing take place in times of a few ps to several ns. The case of benzophenone is a good example of the compensation between spin and orbital angular momentum. The rise of the triplet state absorption shows that intersystem crossing is completed within some 20 ps. 

Many fluorescence kinetics fall into the ps time regime when non-radiative deactivation competes efficiently with the radiative transition. The natural 

The rotational relaxation of low-viscosity solvents takes place in times of ps. This can be observed through time-resolved spectroscopy when the dipole moment of the excited molecule differs substantially from that of the ground state species. Table 8.2 gives a few values of these relaxation times the alcohols show rather slow relaxation because of the hydrogen bonds which associate the solvent molecules. 

Some fluorescence lifetimes are observed in ps times, although these are unusual cases. In organic molecules the Sj—S0 fluorescence has natural lifetimes of the order of ns but the observed lifetimes can be much shorter if there is some competitive non-radiative deactivation (as seen above for the case of cyanine dyes). A few organic molecules show fluorescence from an upper singlet state (e.g. azulene) and here the emission lifetimes come within the ps time-scale because internal conversion to S and intersystem crossing compete with the radiative process. To take one example, the S2-S0 fluorescence lifetime of xanthione is 18 ps in benzene, 43 ps in iso-octane. 

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The aim of this book is to provide a concise introductory overview of the various light-induced processes in physics, chemistry, biology, as well as, in medicine and industry. It is largely based on the course of photophysics and photochemistry given at the University of Fribourg, and every effort has been made to keep it up to date with the latest developments in this field, in particular the probing of the fastest light-induced reactions on picosecond and femtosecond time scales. 

Within the conjugated polymers polythiophene is an important material for future technical applications, e. g. for LEDs, electronic devices, or optical switches [1]. In this context studies of light induced processes are of interest, especially details of the degradation of electronic energy within the ultrashort time scale of picoseconds and sub-picoseconds. 

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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