Laser spectroscopic

According to Kramers model, for flat barrier tops associated with predominantly small barriers, the transition from the low- to the high-damping regime is expected to occur in low-density fluids. This expectation is home out by an extensively studied model reaction, the photoisomerization of tran.s-stilbene and similar compounds [70, 71] involving a small energy barrier in the first excited singlet state whose decay after photoexcitation is directly related to the rate coefficient of tran.s-c/.s-photoisomerization and can be conveniently measured by ultrafast laser spectroscopic teclmiques. 

Optical metiiods, in both bulb and beam expermrents, have been employed to detemiine tlie relative populations of individual internal quantum states of products of chemical reactions. Most connnonly, such methods employ a transition to an excited electronic, rather than vibrational, level of tlie molecule. Molecular electronic transitions occur in the visible and ultraviolet, and detection of emission in these spectral regions can be accomplished much more sensitively than in the infrared, where vibrational transitions occur. In addition to their use in the study of collisional reaction dynamics, laser spectroscopic methods have been widely applied for the measurement of temperature and species concentrations in many different kinds of reaction media, including combustion media [31] and atmospheric chemistry [32]. 

As a scientific tool, ab initio quantum chemistry is not yet as accurate as modem laser spectroscopic measurements, for example. Moreover, it is difficult to estimate the accuracies with which various methods predict bond energies and lengths, excitation energies and the like. In the opinion of tlie author, chemists who... 

The determination of the laser-generated populations rij t) is infinitely more delicate. Computer simulations can certainly be applied to study population relaxation times of different electronic states. However, such simulations are no longer completely classical. Semiclassical simulations have been invented for that purpose, and the methods such as surface hopping were proposed. Unfortunately, they have not yet been employed in the present context. Laser spectroscopic data are used instead the decay of the excited state populations is written n (t) = exp(—t/r ), where Xj is the experimentally determined population relaxation time. The laws of chemical kinetics may also be used when necessary. Proceeding in this way, the rapidly varying component of AS q, t) can be determined. 

Apart from LIF, other laser-based HPLC detectors are LS and Raman. Laser spectroscopic methods for detection in LC have been reviewed [567]. 

The density here refers to the spatial coordinate, i.e. the concentration of the reaction product, and is not to be confused with the D(vx,vy,vz) in previous sections which refers to the center-of-mass velocity space. Laser spectroscopic detection methods in general measure the number of product particles within the detection volume rather than a flux, which is proportional to the reaction rate, emerging from it. Thus, products recoiling at low laboratory velocities will be detected more efficiently than those with higher velocities. The correction for this laboratory velocity-dependent detection efficiency is called a density-to-flux transformation.40 It is a 3D space- and time-resolved problem and is usually treated by a Monte Carlo simulation.41,42... 

The events taking place in the RCs within the timescale of ps and sub-ps ranges usually involve vibrational relaxation, internal conversion, and photo-induced electron and energy transfers. It is important to note that in order to observe such ultrafast processes, ultrashort pulse laser spectroscopic techniques are often employed. In such cases, from the uncertainty principle AEAt Ti/2, one can see that a number of states can be coherently (or simultaneously) excited. In this case, the observed time-resolved spectra contain the information of the dynamics of both populations and coherences (or phases) of the system. Due to the dynamical contribution of coherences, the quantum beat is often observed in the fs time-resolved experiments. 

A number of other laser spectroscopic techniques are of interest but space does not permit their discussion. A few specialized methods of detecting laser absorption worthy of mention include multiphoton ionization/mass spectrometry (28), which is extremely sensitive as well as mass selective for gas-phase systems optically detected magnetic resonance (29) laser intracavity absorption, which can be extremely sensitive and is applicable to gases or solutions (30) thermal blooming, which is also applicable to very weak absorbances in gases or liquids (31) and... 

Closs and Trifunac, 1970 Baldwin and Andrist, 1971 Lepley and Closs, 1972 Bethell and McDonald, 1977). The formation of free radicals from aromatic carbenes is often easily detected by the fast laser spectroscopic techniques discussed earlier. The radicals generally have characteristic absorption spectra and reactivity patterns that make their identification certain. The direct insertion reaction of singlet carbenes is not expected to generate free radicals. 

The detected transient reacts with alcohols to give the expected ethers in very high yield (23). The time-dependence of this reaction can be accurately monitored using laser spectroscopic techniques. In pentane solution it reacts with t-butyl alcohol with a bimolecular rate constant of 3.4 x 109M-1s-1. When the solvent is changed to cyclohexane, the rate constant decreases by a... 

Laser spectroscopic study of the reaction of FL with alcohols in either acetonitrile or hydrocarbon solution appears to show that 3FL is consumed rapidly by the alcohol the absorptions associated with 3FL decay faster in the presence of alcohol than in its absence. However, the product of this reaction is the ether expected from the singlet carbene (Table 7). Moreover,... 

For more information about laser spectroscopic investigations of vibrational or rotational energy transfer see ref. 102a-d). 

Scully, M. O., Kattawar, G. W., Lucht, R. P., Opatmy, T., PUloff, H., Rebane, A., Sokolov, A. V., and Zubairy, M. S. 2002. FAST CARS Engineering a laser spectroscopic technique for rapid identification of bacterial spores. Proc. Natl. Acad. Sci. USA 99(17) 10994-11001. 

Tunable laser spectroscopic techniques such as laser-induced fluorescence (LIF) or resonantly enhanced multi-photon ionization (REMPI) are well-established mature fields in gas-phase spectroscopy and dynamics, and their application to gas-surface dynamics parallels their use elsewhere. The advantage of these techniques is that they can provide exceedingly sensitive detection, perhaps more so than mass spectrometers. In addition, they are detectors of individual quantum states and hence can measure nascent internal state population distributions produced via the gas-surface dynamics. The disadvantage of these techniques is that they are not completely general. Only some interesting molecules have spectroscopy amenable to be detected sensitively in this fashion, e.g., H2, N2, NO, CO, etc. Other interesting molecules, e.g. 02, CH4, etc., do not have suitable spectroscopy. However, when applicable, the laser spectroscopic techniques are very powerful. 

For molecular desorption, laser spectroscopic studies of the desorbing molecule can give full internal state distributions, Df Ef, 6f, v, J, f M ), Ts), where f M ) is some distribution function describing the rotational orientation/alignment relative to the surface normal. For thermal desorption in non-activated systems, most atoms/molecules have only modest (but important) deviations from a thermal distribution at Ts. However, in associative desorption of systems with a barrier, the internal state distributions reveal intimate details of the dynamics. Associative desorption results from the slow thermal creation of a transition state, with a final thermal fluctuation causing desorption. Partitioning of the energy stored in V into... 

Van der Waals complexes of C2v symmetry between 1,2,4,5-tetrazine and a number of light gases (He, Ar, H2) have been observed and characterized by laser spectroscopic studies of free supersonic jet expansion of the tetrazine in the carrier gas (84CHEC-(3)53l). In these complexes, one equivalent of noble gas sits on top of the aromatic TT-system of the heterocycle. 1,2,4,5-Tetrazine, its 3-methyl, and 3,6-dimethyl derivative as well as aminotetrazine have all been used as heterocycles with noble gases, water, HC1, benzene, and acetylene, playing the role of the second partner. 

Laser light is at the root of a great variety of laser spectroscopic techniques, allowing many chemical problems to be solved. Let us list these briefly. 

Van der Waals complexes between 1,2,4,5-tetrazine (38) and a number of light gases (He, Ar, H2) were observed and characterized by laser spectroscopic studies of free supersonic expansion of (38) in the carrier gas. The observed complexes are of the form X (38) or X2- (38), where X is He, Ar or H2. The spectra are consistent with the gas in both types of complexes being bound on or near the out of plane C2 axis on top of and/or below the 1,2,4,5-tetrazine ring. For the He and H2 complexes, analysis of the rotational structure indicates that the van der Waals bond length is 3.3 A (78JCP(68)2487,79JCP(7l)4757). 

Molecular and laser spectroscopic approaches arc making possible a deeper resolution of the dynamics of atomic and molecular motions and the potential energy surfaces governing energy transference within a molecule or group of molecules as chemical bonds are made and broken. 

Eli Y2O3 nanolayers coated on different dielectric nanoparticles Recently, the structural and optical properties of Eu3+ Y2C>3 films coated on a variety of dielectric nanoparticles have been investigated using transmission electron microscope (TEM), X-rays diffraction (XRD) and site-selective laser spectroscopic methods (Chen, X.Y. et al., 2005 Chen et al., 2003a). Eu3+ ions are employed as probes for the study of crystallization and multi-site structure as well as the luminescent centers in nanolayers. It was found that the luminescent nanolayers exhibit distinct thermodynamics and luminescence properties. 

Emission spectroscopy and, to a lesser degree, absorption spectroscopy have provided considerable information on and insight into the chemistry occurring during the process of combustion. In particular, many of the transient free-radical molecules important in the chain reactions were identified and characterized through their emission spectra in flames. Now, new laser spectroscopic techniques offer the promise of obtaining more detailed and precise information, especially for the ground electronic states of many of the molecules involved in combustion. 

Figure 2. Schematics of experimental letups for three laser spectroscopic probe...

Malaga et al. [114] also proposed a diradical intermediate based on picosecond laser spectroscopic data. Excitation of l,3-hexadienes/9-cyanoanthracene showed an unidentified absorption band in addition to the respective ion radicals, which might be attributed to 1,6-diradical intermediates. 

By means of laser spectroscopic studies it has been established that chlorobenzene does not quench the triplet state of A,A-dimethylaniline and the reaction is exclusively singlet-mediated402. The radical anion of chlorobenzene could not be observed and its cleavage rate constant is considered to be very high403. Kinetic analysis has shown that the rate constants for the in-cage fragmentation of the radical anion and the back recombina-... 

The first picosecond laser spectroscopic study to examine charge carrier trapping and recombination dynamics was reported by Gratzel, Serpone and co-workers [15]. The mean lifetime of a single electron/hole pair was determined to be 30 15 ns at low occupancy of electron/hole pairs in the titanium dioxide particles. At high occupancies, where recombination followed second-order kinetics, the bulk rate coefficient for recombination was (3.2 1.4) x 10-11 cm3 s 1. 

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