Indirect excitation processes

Sensitized fluorescence. This is used for enhancing the weak fluorescence of a fluorophore. In this process, the excitation energy absorbed by the donor species is transferred to an acceptor species (fluorophores). A dramatic increase in the fluorescence of the acceptor molecule is observed through this indirect excitation process. 

Finally, in activated chemiluminescence, an added compound also leads to an enhancement of the emission intensity however, in contrast with the indirect CL, this compound, now called activator (ACT), is directly involved in the excitation process and not just excited by an energy transfer process from a formerly generated excited product (Scheme 5). Activated CL should be considered in two distinct cases. In the first case, it involves the reaction of an isolated HEI, such as 1,2-dioxetanone (2), and the occurrence of a direct interaction of the ACT with this peroxide can be deduced from the kinetics of the transformation. The observed rate constant (kobs) in peroxide decomposition is expected to increase in the presence of the ACT and a hnear dependence of kobs on the ACT concentration is observed experimentally. The rate constant for the interaction of ACT with peroxide ( 2) is obtained from the inclination of the linear correlation between obs and the ACT concentration and the intercept gives the rate constant for the unimolec-ular decomposition ( 1) of this peroxide (Scheme 5). The emission observed in every case is the fluorescence of the singlet excited ACT" ° . ... 

Distilled rather than natural water is often used as the solvent for determination of quantum yields for two major reasons. First, the total absorbance of the solution at the wavelength of irradiation should not exceed 0.02. Second, and more important, the presence of natural water constituents (e.g., humic material, nitrate) could enhance the total photolytic transformation rate by indirect photolytic processes as described in Chapter 16. Zepp and Baughman (1978) have argued that for many chemicals d>,r obtained in distilled water is nearly the same as that observed in natural waters (at least in uncontaminated freshwaters), because concentrations of natural water constituents that could undergo reactions with or quench photolysis of excited pollutants are generally very low. Furthermore, the effects of molecular oxygen, which may act as a quencher, can also be studied in distilled water. 

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). 

Photo-induced reaction on a metal surface usually consists of several elementary reactions and it is difficult to model the whole reaction process. However, any reactions need to be triggered by electronic excitation. As stated in Section 20.1.4, the major mechanism is indirect excitation thus we focus on modeling the indirect excitation reaction. Since desorption from the surface is one of the simplest processes and can be a prototype for other complex surface reactions, DIET or DIME are clearly the best to study [10, 48, 53, 57, 96]. In photochemistry, continuous wave or nanosecond lasers lead to DIET, where desorption increase linearly with fluence. In contrast, the DIMET process is caused by intense and short laser pulses on the picosecond or femtosecond time scale, with nonlinear dependence on fluence. Since the fluence is proportional to the number of created hot electrons in the bulk, linear... 

An important result from aerosol chamber studies was the discovery of the indirect photochemical process. Thus, Bricard et al. (1968) found that intense aerosol particle production can be observed in the chamber in the dark if ambient filtered air is sampled from a sunlit atmosphere. It is speculated that in the atmosphere some gaseous substance is excited by sunlight and is not collected by the filter used to obtain air which is free of aerosol particles. In the chamber these photochemically excited molecules initiate secondary thermal reactions leading to the formation of some supersaturated vapour (e.g. H,S04) which subsequently condenses (see also Subsection 3.6.3). 

In indirect photodissociation processes, the initial absorption occurs into a bound discrete excited state which subsequently interacts with the continuum of a final dissociating state. The process of predissociation, in which the bound potential curve is crossed by a repulsive state of a different symmetry, is illustrated in Figure 16. The cross section consists in this case of a series of discrete peaks, broadened by the predissociation process. 

The band gap energy has been discussed from the photo-current action spectrum. The band gap energy estimated from the photo-cmrent spectra is abont 2 eV for the assumption for the indirect photo excitation process. We can illustrate a model of the band diagram of n-type semiconductive passive oxide for the photo-induced process in Fig. 38. The indirect transition may pos-... 

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