Picosecond Emission Spectroscopy

A picosecond emission apparatus utilizing a mode-locked Na+3/phosphate glass laser and a streak-camera-optical-multichannel-analyzer detection system is described, and some general classes of experiments in this research area are discussed. Special emphasis is placed on nonexponential decays. A new type of diffusion controlled chemical reaction based on rotational diffusion is proposed for study using picosecond fluorescence depolarization methods. These methods are also useful for the assessment of solvent dependent molecular shape changes of flexible molecules and the relationship these structural changes might have with solvent dependent chemical reactivity. The study of fluorescence probe molecules on surfaces or in biological membranes or macromolecules is described. The picosecond fluorescence depolarization technique extends the study of rotation of whole macromolecules or polymers to small molecules or to small molecular segments in a macromolecule or a polymer. Picosecond emission spectroscopy, combined with picosecond absorption spectroscopy of the solvated electron, allows assessment of one-photon photoionization as a primary process in photochemistry and photobiology. A tentative result indicates that tryptophan photionizes both by direct electron ejection and by an intramolecular scavenging effect. Because of slow interchange between conformations that can and cannot lead to intramolecular scavenging, a double exponential is observed. Picosecond emission and depolarization studies of the fluorescence probes ANS and TNS give direct evidence for strong solvent attachment effects in the excited electronic state. A sort of "crystallization" of water molecules around the solute occurs in the time range of about 50 psec and is D20 dependent. ANS and TNS in mixed water/ethanol solvents show nonexponential decays which are interpreted in terms of site inhomogeneities in the liquid state. This discovery suggests that studies of wavelength dependent emission decays and picosecond "hole burning" experiments are feasible in the liquid state.

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