Picosecond emission spectroscopy

The laser-excited fluorescence from dibenzofuran in a biphenyl host at 4.2 K and three different sites in the lattice have been identified. The two-photon excitation spectrum of single crystals of carbazole at 4.2 K has been analysed in detail. Very extensive vibrationally unrelaxed fluorescence is observed following dye-laser excitation of single vibronic levels of naphthazanine and the rates and pathways of relaxation are examined by picosecond emission spectroscopy. The photophysics of rubrene peroxide have been measured for the first time by Bayrakceken. Picosecond fluorescence has been used to study the spectra and kinetics for Sj— So and Si transitions for... 

Robinson, G. W., Caughey, T. A., Auerbach, R. A. Picosecond emission spectroscopy with an ultraviolet sensitive streak camera. In Advances in Laser Chemistry/Springer Series in Chemical Physics. Tlewafl, A. H. (Ed.) p. 108, Berlin, Heidelberg, New York Springer 1978 Robinson, G. W. et al. J. MoL Struct 47, 221 (1978)... 

Picosecond Emission Spectroscopy with Intensified Photodiode Arrays... 

Following photo excitation a solution sample returns to thermal equilibrium by a variety of photochemical and photophysical processes. The faster processes, e.g. vibrational relaxation and solvent relaxation, have only recently begun to be studied by direct kinetic methods (1-5). Picosecond emission spectroscopy has been especially useful in elucidating these ultrafast processes (1,/3, 5). As electronically excited molecules relax, their fluorescence spectrum shows time dependence that reflects the relaxation processes. 

Photodissociation of Haloaromatic Compounds. Picosecond emission spectroscopy was recently employed (27,28) to study the photodissociation of haloaromatics and the subsequent fluorescence of the formed radicals. An earlier paper by Huppert et. al. (29) describes the predissociative pathways of the haloaromatics following excitation at 266 and 355 nm. What makes the more recent work different Is that, after the initial excitation by a 266-nm 25-ps FWHM pulse, a second 355-nm 25-ps pulse irradiates the same area of the sample. With this two-color fluorescence technique they were able to observe clearly the reaction dynamics of the radicals which were formed by the initial light pulse. 

Using picosecond flash spectroscopy Gupta et al. 2k) reported for 2-hydroxyphenylbenzotriazole in ethanol a short-lived transient (6 ps) followed by a transient absorption whose lifetime is estimated to be 600 ps. The authors assigned the short-lived transient to the "vertical singlet" while the long-lived transient is presumably the "proton transferred species". These measurements of transient absorptions with the picosecond flash method confirm our results derived from the fluorescence emission using the phase fluorimetric method. 

The combination of the picosecond single electron bunch with streak cameras, independently developed in 1979 at Argonne National Laboratory [55] and at University of Tokyo by us [56], enabled the very high time resolution for emission spectroscopy. The research fields have been extended to organic materials such as liquid scintillators [55-57], polymer systems [58], and pure organic solvents [59]. The kinetics of the geminate ion recombination were studied [55,57,59]. 

Details of the picosecond pulse radiolysis system for emission (7) and absorption (8) spectroscopies with response time of 20 and 60 ps, respectively, including a specially designed linear accelerator (9) and very fast response optical detection system have been reported previously. The typical pulse radiolysis systems are shown in Figures 1 and 2. The detection system for emission spectroscopy is composed of a streak camera (C979, HTV), a SIT... 

Figure 1. The schematic diagram of the picosecond pulse radiolysis system for emission spectroscopy.

More recently, powerful time-resolving techniques began to evolve. Nanosecond [13] and picosecond [14] flash absorption and emission spectroscopy made it possible to obtain UV spectra of transient species with very short lifetimes. 

An important issue associated with molecular machines is the detection of actuations on the nanoscale level. When a chemical stimulus induces movement in a machine, several spectroscopic techniques, such as nuclear magnetic resonance (NMR) spectroscopy, UV-Vis spectroscopy, emission spectroscopy and X-ray photoelectron spectroscopy (XPS) can be used to detect their outputs. More intri-guingly, electrochemical and photochemical inputs often provide [6, 8g] a two-fold advantage by inducing the mechanical movements and detecting them. Additionally, the dual actions of the these two types of stimuli can be exploited when the time-scale of the molecular actuations, which ranges from picoseconds to seconds, falls within the detection time-scale of the apparatus. 

Serpone et al. have examined colloidal titanium dioxide sols (prepared by hydrolysis of TiCl4) with mean particle diameters of 2.1, 13.3, and 26.7 nm by picosecond transient absorption and emission spectroscopy [5]. Absorption decay for the 2.1 nm sols was found to be a simple first-order process, and electron/hole recombination was 100% complete by 10 ns. For the 13.3 and 26.7 nm sols absorption decay follows distinct second-order biphasic kinetics the decay times of the fast components decrease with increase in particle size. 10 ns after the excitation pulse, about 90% or more of the photogenerated electron/hole pairs have recombined such that the quantum yield of photooxidations must be 10% or less. The faster components are due to the recombination of shallow-trapped charge carriers, whereas the slower components (x > 20 ns) reflect recombination of deep-trapped electrons and holes. 

The relaxation time for this new dynamic equilibrium varies from femtoseconds to picoseconds. The fast reorientation of solvent molecules causes a fast solva-tochromic shift in the fluorescence band of the organic chromophores. Solvation dynamics is measured in terms of (8v (0) 8v (/)), where the fluctuating frequency v(t) is the difference in solvation energies between the two electronic states involved, i.e., v(t)= sE(t)/h [110]. In time-resolved emission spectroscopy the time dependence of the excited-state distribution is monitored via the frequency shift of the emission... 

S. Schneider, A. Mindl, G. Elfinger, and M. Melzig, Photochromism of spirooxazines. I. Investigation of the primary processes in the ring-opening reaction by picosecond time-resolved absorption and emission spectroscopy, Ber. Bunzenges Phys. Chem., 91, 1222-1224 (1987). 

Ono and Ware"" have measured the absorption, emission, and excitation spectra, the fluorescence decay times, and the quantum yields of a series of substituted diphenylmethylenes in rigid matrices at low temperatures. Acean-thrylene shows S2- So emission in hexane with a yield of 0.017 and lifetime of 4.3 ns. The low-temperature fluorescence spectra of bis-2-naphthyl-alkanes and their derivatives have been studied. Excimer formation is an activated process. The fluorescence and absorption spectra of 1,1-diphenyl-ethylenes have been analysed in some detail by Gustav and Bolke. " The S — Si transitions in trans isomers of phenylnaphthylethylenes have been assigned by picosecond absorption spectroscopy. Effects of solvent viscosity and the role of conformers in the mechanism of isomerization are elucidated. The production of non-equilibrium conformer concentrations in glassy solutions of diarylethylenes at 77 K due to restrictions imposed by the solid matrix has also been reported. Free jet excitation and emission spectra of diphenyl-butadiene show clearly the lowest excited Ag state and give a lifetime of 52.8 ns for 0-0 excitation.Electric field-induced charges in the optical... 

The mechanisms of a number of photochemical reactions and photophysical processes have been elucidated by means of picosecond emission, absorption, and Raman spectroscopy. 

Picosecond spectroscopy enables one to observe ultrafast events in great detail as a reaction evolves. Most picosecond laser systems currently rely on optical multichannel detectors (OMCDs) as a means by which spectra of transient species and states are recorded and their formation and decay kinetics measured. In this paper, we describe some early optical detection methods used to obtain picosecond spectroscopic data. Also we present examples of the application of picosecond absorption and emission spectroscopy to such mechanistic problems as the photodissociation of haloaromatic compounds, the visual transduction process, and inter-molecular photoinitiated electron transfer. 

In the preceding discussion, we have presented several optical detection methods that can be used when one employs picosecond emission or absorption spectroscopy as a means of... 

Time-resolved emission spectroscopy has provided valuable information on the nature of excited states in polymers. Two distinct types of excimers have been observed in poly(Y-vinylcarbazole) using picosecond time-resolved fluorescence. The sandwich-type excimer emitting at 420 nm was formed in several nanoseconds, whereas a second excimer emitting at 375 nm was formed immediately after a lOps electron pulse. (Scheme 13). Similar observations were also... 

Studies of Molecular Dissociation by Means of Ultrafast Absorption and Emission Spectroscopy and Picosecond X-Ray Diffraction P, M. Rentzepis and B. Van Wonterghem... 

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