Space- and time-resolved laser spectroscopy and photochemistry of organic solids

Abstract

In inhomogeneous organic solid systems it is indispensable to measure photoprimary processes as a function of position and to elucidate photochemical processes. For this purpose some space- and time-resolved spectroscopic methods have been developed. Total internal reflection fluorescence spectroscopy makes it possible to obtain a concentration profile or fluorescent species from the surface/interface to the bulk and to discriminate the dynamics of the surface/interface layers from that of the bulk. Laser photolysis, which involves monitoring the transient species under multiple-reflection conditions, gives dynamic information on thin films of submicron thickness. As an example, the diffusion coefficient of holes in a photoconductive film has been estimated by analysing the absorption decay of photo-induced charge carriers. By a combination of time-correlated single-photon counting with a confocal laser-scanning microscope, three-dimensionally submicrometre space-resolved and picosecond time-resolved fluorescence measurements can be made. This has also been extended to transient absorption spectroscopy. To study the photochemistry of organic solid particles, a laser manipulation method utilizing laser-trapping phenomena has been introduced and combined with microscope spectroscopy. One can choose a single particle in the dispersed solution, characterize it, induce chemical reactions on it and fabricate it. These recent advances in space- and time-resolved spectroscopy and its high potential in organic solids photochemistry are summarized and new trends in photochemistry are discussed.