Laser nanochemistry

Abstract

Various time- and space-resolved spectroscopies have been developed and applied to thin films and nanoparticles. Dynamic reflection spectroscopies of total internal, diffuse, and regular reflection modes analyze photophysical and photochemical processes at the interface/surface layers with thicknesses of a few tens to a few hundreds of nm and of optically scattering materials. The excited singlet, triplet, and ionic states are identified, and intersystem crossing, isomerization, electron transfer and recombination, and photothermal conversion due to excited-state annihilation are analyzed, just as by transmittance mode spectroscopy. Fluorescence and Rayleigh light-scattering spectroscopies are developed for elucidating excited-state dynamics of single nanoparticles. The optical properties are related to their size, shape, internal structure, and environmental conditions. We prove that organic molecular materials show novel nanometer-size effects due to structural confinement. The high-intensity laser excitation induces ablation whose dynamics and mechanism are considered on the basis of time-resolved spectroscopy and imaging. For nanosecond and femtosecond ablation, we propose cyclic multiphotonic absorption and photomechanical mechanisms, respectively, while purely photochemical ablation was confirmed. Ablation studies have opened a new research approach toward expansion and contraction dynamics of polymer films, nanoparticle preparation, crystal growth control, crystallization in saturated solution, and others.