Electronic Excitation and Photophysical Processes

If the initial excitation was not into the lowest excited state of given multiplicity, a fast crossing to that state will occur via internal conversion, that is, typically to the S, or T, state (Kasha s rule, cf. Section 5.2.1). In some cases a funnel in S is accessible and internal conversion from S, to Sg can be so fast that the first thermal equilibration of the vibrational motion in these molecules is achieved in a minimum in the Sg state. Such a process is referred to as a direct reaction. Here as well, the excess kinetic energy of the nuclei may take the molecule over barriers in the S state into valleys other than the one originally reached analogous to the above-mentioned reactions, such processes are referred to as hot ground-state reactions. 

Finally, in the presence of heavy atoms or in other special situations (cf. Section 4.3.4) intersystem crossing may proceed so fast that it is able to 

In one way or another, picoseconds after the initial excitation, the molecule will typically find itself thermally equilibrated with the surrounding medium in a local minimum on the S, T, or So surfaces in S if the initial excitation was by light absorption, in T, if it was by sensitization or if special structural features such as heavy atoms were present, and in Sq if the reaction was direct. Frequently, the initially reached minimum in S (or T,) is a spectroscopic minimum located at a geometry that is close to the equilibrium geometry of the original ground-state species, so no net chemical reaction can be said to have taken place so far. 

The description given so far is best suited for unimolecular photochemical reactions. As mentioned above, if the process is bimolecular, both components together must be considered as a supermolecule. The description given here remains valid except that some motions on the surface of the supermolecule may be unusually slow since they are diffusion-limited. 

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All photochemical and photophysical processes are initiated by the absorption of a photon of visible or ultraviolet radiation leading to the formation of an electronically-excited state. 

The above-mentioned theoretical background shows that, irrespective of the chemical nature of the photosensitizer and its binding mode to the semiconductor surface, one should consider two main ways of the semiconductor CB populating direct and indirect. Direct processes include VB -> CB excitations, photosensitization via bulk doping (TTRS-driven processes) and photophysical processes involving the TTRMs term. Indirect processes, in turn, involve excitation of the surface and a subsequent electron transfer reaction (WRV1 + TTet). 

Following photo excitation a solution sample returns to thermal equilibrium by a variety of photochemical and photophysical processes. The faster processes, e.g. vibrational relaxation and solvent relaxation, have only recently begun to be studied by direct kinetic methods (1-5). Picosecond emission spectroscopy has been especially useful in elucidating these ultrafast processes (1,/3, 5). As electronically excited molecules relax, their fluorescence spectrum shows time dependence that reflects the relaxation processes. 

The photochemical and photophysical processes that occur in an electronically excited molecule are well described in many books [11]. Radicals in photoinitiators are produced through several following typical processes [li] ... 

If the adiabatic Bom-Oppenheimer approximation were exact, photochemistry and photophysical processes would be rather straightforward to describe. Molecules would be excited by the incident radiation to some upper electronic state. Once in this electronic state, the molecules could radiate to a lower electronic state, or they could decompose or isomerize on the upper electronic potential energy surface. No transitions to other electronic states would be possible. The spectroscopy of the systems would also be greatly simplified, as there would no longer be any phenomena such as lambda doubling, etc., which lifts degeneracy of some energy levels of the clamped-nucleus electronic Hamiltonian, //,. 

Much use has been made of micellar systems in the study of photophysical processes, such as in excited-state quenching by energy transfer or electron transfer (see Refs. 214-218 for examples). In the latter case, ions are involved, and their selective exclusion from the Stem and electrical double layer of charged micelles (see Ref. 219) can have dramatic effects, and ones of potential imfKntance in solar energy conversion systems. 

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... 

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. 

A term in photochemistry and photophysics describing an isoenergetic radiationless transition between two electronic states having different multiphcities. Such a process often results in the formation of a vibrationally excited molecular entity, at the lower electronic state, which then usually deactivates to its lowest vibrational energy level. See also Internal Conversion Fluorescence... 

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