Fluorescence Spectroscopy of Adsorbed Atoms and Molecules

If fluorescence is excited by a laser pulse of a duration shorter than the lifetime of the excited state of an adsorbate, the subsequent decay of the emission intensity can be measured by time-resolved techniques. Then the slope of the logarithm of intensity versus time gives the total decay rate, 1/r. The part related to nonradiative relaxation is determined by the energy transfer to the substrate. Thus such measurements allow one to study different de-excitation channels at a surface. 

Atoms or molecules adsorbed on a metal surface do not fluoresce because of highly effective quenching via creation of electron-hole pairs in the substrate. However, their fluorescence can be observed if they are separated from a metal by a dielectric spacer layer. The Langmuir-Blodgett technique allows one to deposit a fatty acid layer of a well-controlled thickness and thus provides an opportunity to study fluorescence of an adsorbate versus its distance from the surface (see Fig. 5.9). 

3) Each fatty acid layer has a thickness of 26.4 A determined by the length of a single molecule. 

In semiconductors, PL originates from the radiative recombination of pho-toexcited electron-hole pairs. Their nonradiative lifetime is determined by both bulk and surface recombination. Therefore, the major problem in observing PL from semiconductor surfaces and interfaces is to minimize signals which arise from defects and impurities in the bulk. This can be achieved with an improvement in material quality by means of an epitaxial growth together with the fabrication of special structures, such as semiconductor heterostructures and quantum wells. The control of the epitaxial growth process allows one to vary the relative contribution of bulk and surface recombination independently of each other. 

PL is an extremely sensitive probe of surface recombination properties. Being combined with a careful in situ control of surface reconstruction and chemical termination, it allows one to assess how these factors influence the surface 

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Two-photon Fluorescence Spectroscopy of Adsorbed Atoms and Molecules 167... 

In the preceding chapter it had already been discussed that it is less the synthesis itself which may be the bottleneck in high-throughput zeolite science but rather the analysis of the solids formed in a high-throughput program. There are several standard characterization techniques which are typically employed to characterize zeolitic materials. These include powder XRD for phase identification, X-ray fluorescence analysis (XRF) or atomic absorption spectrometry to analyze elemental composition, sorption analysis to study the pore system, IR-speclroscopy, typically using adsorbed probe molecules to characterize the acid sites, NMR spectroscopy and many others. For some of these techniques parallelized solutions have been developed and described in the literature, other properties are more difficult to assess in a parallelized or even a fast sequential fashion. 

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