Pulsed studies fluorescence mechanism

In the present work, we have examined poly(N-vinylcarbazole) (abbreviated hereafter as PVCz) and pyrene-doped poly(aethyl methacrylate) (PMMA) films by using a tine-resolved fluorescence spectroscopic aethod. Fluorescence spectra and their dynanic behavior of the forner fila were elucidated with a high intensity laser pulse and a streak camera, which nakes it possible to neasure dynaaics just upon laser ablation. This aethod reveals aolecular and electronic aspects of laser ablation phenomena (17). For the latter fila a laser pulse with weak intensity was used for characterizing the ablated and Basked areas. On the basis of these results, we demonstrate a high potential of fluorescence spectroscopy in aolecular studies on laser ablation and consider its mechanism. Experimental... 

Matsuno et al. (1991) reports a method to induce thrombosis in the rat femoral artery by means of a photochemical reaction after injection of a fluorescent dye (rose Bengal, 10 mg/kg i.v.) and transillumination with a filtered xenon lamp (wave length 540 nm). Blood flow is monitored by a pulsed Doppler flow meter. Occlusion is achieved after approximately 5-6 min. Pretreatment with heparin dose-dependently prolongs the time required to interrupt the blood flow. The model also enables the study of thrombolytic mechanisms, which had been evaluated with t-PA. A comparative data for hirudin in various models was carried out by Just et al. (1991). 

We have employed a pulsed laser photolysis - pulsed laser induced fluorescence technique to carry out direct, real time studies of OH reactions with DMS and DMS-dfc in N2, air, and O2 buffer gases. Both temperature and pressure dependencies have been investigated. We find that the observed rate constant (kQbs = d[0H]/[0H] [DMS]dt) depends on the O2 concentration. Our results are consistent with a mechanism which includes an abstraction route, a reversible addition route, and an adduct + O2 reaction which competes with adduct decomposition under atmospheric conditions. 

The short pulse duration combined with the high photon density of ps-and fs-lasers have provided the means to study the properties of the excited states by emission and transient absorption measurements. Fluorescence of the lowest and higher excited states of azobenzene can be detected, but most work is being directed toward the dynamics of isomerization. Because questions about the isomerization mechanism are prominent in this field, this work will be discussed in Section 1.6 The Isomerization Mechanism. 

Fig. 6. Time-resolved fluorescence spectra of Cf. aurantiacus excited at 715 nm. Numbers on right margin indicate time of observation relative to the arrival time of the peak of the excitation pulse in ps. Dashed-line spectrum in the bottom frame shows the absorption spectrum of intact cells. Figure source Mimuro (1990) Studies on excitation energy flow in the photosynthetic pigment system structure and energy transfer mechanism. Bot Mag, Tokyo. 103 248.

Fig. 12. (A) Time-resolved fluorescence spectra of A. nidulans phycobilisomes measured at 77 K. Excitation by 6-ps, 580-nm argon laser pulse. Three small ticks in the topmost spectrum (at 932 ps) indicate locations of maximum fluorescence at 0 ps. (B) Rise and decay of various fluorescent components derived from deconvolution of the fluorescence spectra. Assignment of individual fluorescent components are shown in the right margin. (C) Energy flow among individual chromophores in the phycobilisomes. The asterisk in (B) and (C) indicates a linker polypeptide is attached to the trimer. See text for discussion. Figure source Mimuro (1989) Studies on excitation energy How in the photosynthetic pigment system structure and energy transfer mechanisms. Bot Mag Tokyo 103 244.

The multiphoton absorption cycle was confirmed by a comparison of the temporal profile of the fluorescence of anthracene-doped polystyrene films with computational results based on the cyclic process [97]. In the computational studies, the ground state, first excited singlet state, and lowest triplet state have been included. The calculated temperature rise during the laser pulse depends nonlinearly on the laser intensity. Rapid internal conversion within the triplet manifold is the most effective mechanism for depositing heat at the irradiated surface. 

Three papers on photoprocesses in phenol have appeared. Grabner in steady-state photolysis studies reports quantum yields of fluorescence, hydrated electron, and H-atom formation from excited phenol in aqueous solution at excitation energies of 254 and 229 nm corresponding to the two lowest excited singlet states between 10 and 65 °C. The mechanisms postulated are indicated in Scheme 1. Zechner et o/. have studied solvent effects on the primary photoprocess of phenol in several solvents. Data on product yields are exemplified in Table 9. A study of electron ejection in aqueous phenol and phenolate solution used 27 ps pulses at 265nm. The phenolate undergoes extremely rapid electron ejection. 

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