Diffuse reflectance laser flash-photolysis surface studies

Gopidas, K. R. Kamat, P. V. George, M. V. Photochemical processes on oxide surfaces. A diffuse reflectance laser flash photolysis study, Mol. Cryst. Liq. Cryst. 1990, 183, 403. 

The mechanism of the quenching of triplet benzophenenone by both electron and hydrogen donors involves change transfer effects". Diffuse reflectance laser flash photolysis studies of the reactions of triplet benzophenone with hydrogen atom donors on surfaces have also been reported". A dual phosphorescence of benzophenone at 77K in HjO/EtOH in glasses indicates that in this environment a... 

Wilkinson et al. developed, at the end of the 1980s, the diffuse reflectance laser flash-photolysis technique [1,10], which proved to be crucial for transient absorption and emission studies on surfaces, providing both spectroscopic and kinetical information. This technique for studying solid and opaque media became so important for surface studies as the conventional flash-photolysis was and still is for transparent media, after its discovery by G. Porter in the 1950s. 

The use of triplet excited states as probes to study the surface properties of many solids is particulary interesting, due to the fact that they usually exhibit long lifetimes which in most cases come closer to those obtained for rigid matrices. These long lifetimes increase in many cases the efficiency of several photochemical processes. Therefore spectroscopic and kinetic studies can be performed in a wide and interesting variety of situations. As we said before, the development of the diffuse reflectance laser flash photolysis technique [1,10] by Wilkinson et al. was crucial for the development of these studies on surfaces. 

An alternative to laser flash photolysis which is useful for studying opaque (but reflecting) samples, is diffuse reflectance spectroscopy [27]. This spectroscopic technique measures the ratio of the intensity of light reflected from the sample, I, to that reflected from a background or reference reflective surface, Iq. In time-resolved... 

In a recent article Thomas [24] emphasises the prime importance of surfaces in chemical reactions, particularly in the fields of catalysis and corrosion chemistry. Also eluded to in the article is the concept that photochemical and photophysical techniques can yield Information relating to molecules adsorbed on surfaces and the fact that such information "reports back" on the environment of the excited state (i.e. the polarity and nature of surface sites). The technique of laser flash photolysis in diffuse reflectance should be as valuable to the understanding of heterogeneous photoreactions as the transmission mode has been for homogeneous photoreactions. It has already shown great potential in the study of materials adsorbed on surfaces. 

The above provides a broad overview of some of the areas of study to which the technique of laser flash photolysis, in diffuse reflectance mode, has been applied with respect to opaque samples. It is hoped that this chapter in conjunction with other published review material [13,14,43,44] provide a complete picture of "the state of the art" of laser flash photolysis of solid surfaces, and reiterates the great potential of this new mode for flash photolysis studies at interfaces. 

Also, Fourier transform infrared absorption spectroscopy provides relevant information regarding the specific interactions of different probes within substrates [17], especially in the diffuse-reflectance mode when applied to the study of powdered opaque surfaces that disperse the incident radiation. The extension of this technique to obtain time resolved transient absorption spectra in the IR wavelength range (laser flash-photolysis with IR detection) will certainly play in the near future an important role in terms of clarifying different reaction mechanisms in the surface photochemistry field [17c, 18]. 

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