Time-resolved laser spectroscopy, study

J. Carlsson, P. Jdiisson, L. Sturesson, C. Proese Fischer Multi-configuration Hartree-Fock calculations and time-resolved laser spectroscopy studies of hyperfine structure constants in sodium. Phys. Scr. 46, 394 (1992)... 

Recently, the electron-transfer kinetics in the DSSC, shown as a schematic diagram in Fig. 10, have been under intensive investigation. Time-resolved laser spectroscopy measurements are used to study one of the most important primary processes—electron injection from dye photosensitizers into the conduction band of semiconductors [30-47]. The electron-transfer rate from the dye photosensitizer into the semiconductor depends on the configuration of the adsorbed dye photosensitizers on the semiconductor surface and the energy gap between the LUMO level of the dye photosensitizers and the conduction-band level of the semiconductor. For example, the rate constant for electron injection, kini, is given by Fermi s golden rule expression ... 

The first valence isomer to be investigated was the bicyclic (hexamethyl-, HM-) Dewar benzene. However, y irradiation of this substrate produced the electronic spectrum of HM-128 [360]. Similarly, nsec time-resolved laser spectroscopy failed to reveal evidence for the bicyclic radical cation [362]. The first indication for the existence of such a species as a discrete entity was provided by a CIDNP study [361]. These results are best discussed in connection with ab initio calculations carried out for the parent C6H6 system. At the 6-31 G level these calculations support the existence of two cationic states with the unpaired spin density either... 

The wide use of photoinduced radical or cationic polymerization of unsaturated monomers in UV curing technologies [1-3] have stimulated many detailed investigations of the processes involved [4-10]. Time-resolved laser-spectroscopy has been extensively employed [7 and references therein][8][11-20] to study in real time, the dynamics of the excited states of molecules used as photoinitiators. Reaction models accounting for the photoinHiation steps of the polymerization have been proposed which allow relationships between the excited state reactivity and the practical efficiency as photopolymerization initiators to be discussed thoroughiy. 

The enhancement of the curing rate by additions of thioxanthone was studied with time-resolved laser spectroscopy and H-NMR CIDNP. It led to the conclusion that two competitive processes occur from the excited triplet-state sensitizers. One is electron transfer and the other is enei transfer. Whether it is the former or the latter depends upon the structure of the initiator used. ... 

The transfer of electronic energy into rovibronic energy has much larger cross sections than the electronic to translational energy ( E - T) transfer [1009]. These processes are of crucial importance in photochemical reactions of larger molecules and detailed studies with time-resolved laser spectroscopy have brought much more insight into many biochemical processes [1010-1012],... 

These examples reveal that these extremely fast biochemical processes could not have been studied without time-resolved laser spectroscopy. It not only provides the necessary spectral resolution but also the sensitivity essential for those investigations. More examples and details on the spectroscopy of ultrafast biological processes can be found in [1518-1522]. 

In this section we will study time-resolved laser spectroscopy and generally discuss radiative properties of atoms and molecules, and methods of studying these properties. Since very short laser pulses with a power density sufficient to well saturate optical transitions can be obtained, a large fraction of the irradiated ground-state atoms can be transferred to the excited state. Using step-wise excitations with synchronized lasers a large number of atoms can be excited into very highly excited states. When the laser pulse ceases the exponential decay of the excited state can be monitored. Note, that primarily the population number N(t) decays exponentially, i.e.. 

The high time resolution, achievable with pulsed or mode-locked lasers (Chap. 11) opens the possibility for studying the dynamics of collision processes and relaxation phenomena. The interesting questions of how and how fast the excitation energy, selectively pumped into a polyatomic molecule by absorption of laser photons, is redistributed among the various degrees of freedom by intermolecular or intramolecular energy transfer can be attacked by time-resolved laser spectroscopy. 

Table 3.1. Types of studied elementary processes using time-resolved laser spectroscopy...

The hydrolysis of the uranyl(VI) ion, UO " 2> has been studied extensively and begins at about pH 3. In solutions containing less than lO " M uranium, the first hydrolysis product is the monomeric U02(OH)", as confirmed using time-resolved laser induced fluorescence spectroscopy. At higher uranium concentrations, it is accepted that polymeric U(VI) species are predominant in solution, and the first hydrolysis product is then the dimer, (U02)2(0H) " 2 (154,170). Further hydrolysis products include the trimeric uranyl hydroxide complexes (U02)3(0H) " 4 and (1102)3(OH)(154). At higher pH, hydrous uranyl hydroxide precipitate is the stable species (171). In studying the sol-gel U02-ceramic fuel process, O nmr was used to observe the formation of a trimeric hydrolysis product, ((U02)3( -l3-0)(p.2-0H)3) which then condenses into polymeric layers of a gel based on the... 

Molecular Studies on Laser Ablation Processes of Polymeric Materials by Time-Resolved Luminescence Spectroscopy... 

After a delay of several ps, the luminescence of Eu " is already very weak, and narrow long-lived lines of trivalent RE dominate in the spectrum. The lines at 589, 617, 651, and 695 nm (Fig. 4.1c) have never been detected in natural apatite by steady-state spectroscopy. According to their spectral position they may be ascribed to Eu ", but they are different from known lines in synthetic apatites activated by Eu (Jagannathan and Kottaosamy 1995 Morozov et al. 1970 Piriou et al. 1987 Piriou et al. 2001 Voronko et al. 1991). In order to clarify this problem we studied artificially activated samples by laser-induced time-resolved luminescence spectroscopy. 

In principle, absorption spectroscopy techniques can be used to characterize radicals. The key issues are the sensitivity of the method, the concentrations of radicals that are produced, and the molar absorptivities of the radicals. High-energy electron beams in pulse radiolysis and ultraviolet-visible (UV-vis) light from lasers can produce relatively high radical concentrations in the 1-10 x 10 M range, and UV-vis spectroscopy is possible with sensitive photomultipliers. A compilation of absorption spectra for radicals contains many examples. Infrared (IR) spectroscopy can be used for select cases, such as carbonyl-containing radicals, but it is less useful than UV-vis spectroscopy. Time-resolved absorption spectroscopy is used for direct kinetic smdies. Dynamic ESR spectroscopy also can be employed for kinetic studies, and this was the most important kinetic method available for reactions... 

The putative benzyloxynitrene can be intercepted with tetramethylethylene to form the expected aziridine. Time-resolved IR spectroscopy was unable to detect the 0-nitrene, but detected the presence of PhCH2N=0 formed with a time constant of 250 ns after the laser pulse. Thus, the lifetime of benzyloxynitrene is also 250 ns. The TRIR spectroscopic studies indicated that benzyloxynitrene reacts with oxygen a rate constant of 10 M s. This value strongly suggests that the 6>-nitrene, in contrast to the Al-nitrenes has a triplet ground state. 

A promising recent development in the study of nitrenium ions has been the introduction of time-resolved vibrational spectroscopy for their characterization. These methods are based on pulsed laser photolysis. However, they employ either time resolved IR (TRIR) or time-resolved resonance Raman (TRRR) spectroscopy as the mode of detection. While these detection techniques are inherently less sensitive than UV-vis absorption, they provide more detailed and readily interpretable spectral information. In fact, it is possible to directly calculate these spectra using relatively fast and inexpensive DFT and MP2 methods. Thus, spectra derived from experiment can be used to validate (or falsify) various computational treatments of nitrenium ion stmctures and reactivity. In contrast, UV-vis spectra do not lend themselves to detailed structural analysis and, moreover, calculating these spectra from first principles is still expensive and highly approximate. 

Chung, K. H., Klenze, R., Park, K. K., Paviet-Hartmann, P. Kim, J. I. 1998. A study of the surface sorption process of Cm(III) on silica by time-resolved laser fluorescence spectroscopy (I). Radiochimica Acta, 82, 215-219. 

Stumpf, Th., Bauer, A., Coppin, F. Kim, J. I. 200]a. Time-resolved laser fluorescence spectroscopy study of the sorption of Cm(III) onto smectite and kaolinite. Environmental Science and Technology, 35, 3691 -3694. 

Stumpf, Th. Fanghanel, Th. 2002. A time-resolved laser fluorescence spectroscopy (TRLFS) study of the interaction of trivalent actinides (Cm(III)) with calcite. Journal of Colloid and Interface Science, 249, 119-122. 

Gabriel, U., Charlet, L., Schlapfer, C. W., Vial, J. C., Brachmann, A. Geipel, G. 2001. Uranyl surface speciation on silica particles studied by time-resolved laser-induced fluorescence spectroscopy. Journal of Colloid and Interface Science, 239, 358-368. 

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