Significant improvements in the performances of the Super-ACO storage ring free-electron laser (FEL) at 800 MeV have been obtained recently: enhancement of the output power in the ultraviolet, laser duration of 10 h for the same injection of positrons, long-term stability with a micropulse of 60 ps FWHM. A first series of experiments using this FEL has then been successfully performed. Taking advantage of the time structure, the polarization and the high power of the source at 350 nm, the polarized fluorescence decays of the reduced nicotinamide adenine dinucleotide coenzyme (NADH) were studied in aqueous solution, using the single-photon counting (SPC) technique. The experimental setup is described, including the Super-ACO FEL characteristics and diagnostics. The FEL working point has been first optimized by monitoring the SPC apparatus function. A complete fluorescence experiment required about 30 min of data acquisition, during which the best integrated instrumental response had a FWHM of 110 ps. Measurements performed in such a way lead to the unambiguous separation of two close lifetime components of 0.28 and 0.62 ns in the fluorescence decays of NADH at 20 °C, in good agreement with previous works. The thermodynamic parameters obtained from temperature studies show that the NADH fluorescence heterogeneity is consistent with the ground-state folding equilibrium of the coenzyme, as characterized by many other spectroscopic techniques. From the fluorescence anisotropy decays, an apparent hydrodynamic radius of about 6 Å is determined, while on the other hand, a large initial depolarization of the fluorescence indicates a fast independent motion of the nicotinamide ring. The quality of the collected data fully meets the requirements for the study of more complex systems such as fluorescent compounds bound to proteins or membranes. Thus, the feasibility of use of a storage ring UV FEL for this type of time-resolved experiments on the subnanosecond time scale has been demonstrated.
Combining the use of a UV storage ring free electron laser and of synchrotron radiation, a time resolved core level spectroscopy study has been performed on photoexcited Si(111)2×1 surfaces with subnanosecond resolution. This enabled us to measure band bending fluctuations, caused by surface carrier dynamics, during the first nanosecond after photoexcitation; differences in the Si2p core level lineshape, dependent on the pump-probe time delay, were also observed. The presence of defects was found to reduce the fluctuations and make the carrier recombination process faster.
Equilibrium and dynamical properties of a storage-ring free-electron laser are shown as resulting from the combined effects of two different saturation mechanisms. The first one is the laser-induced energy spread of the electron beam. The second mechanism is due to an imperfect longitudinal synchronism between the electron and laser pulses. Consequences are briefly described.
This paper deals with three problems of ;t different nature. We first discus the conditions required to obtain reliable and usable magneto-optical and absorption data at any temperature down to that of liquid helium; this discussion includes the description of our cryostat and absorption device, as well as a critical study of the optimization of the signal-to-noise ratio of the instruments. Secondly, a new fitting procedure is proposed for the analysis of absorption and magnetic circular dichroism curves and the extraction of useful parameters. Finally, the significance of these parameters is examined for anisotropic centres isolated in an isotropic matrix ; the results thus obtained are illustrated by means of magneto-optical experiments on CaF2 +Nd3+ crystals. * Equip de recherche no. 5 du C.N.R.S. * D is used here for absorbance (A) since this last parameter (A) is widely used in the description of MCD data.
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