Clusters electronic relaxation

Niv M Y, Krylov A I and Gerber R B 1997 Photodissociation, electronic relaxation and recombination of HCI in Ar-n(HCI) clusters—non-adiabatic molecular dynamics simulations Faraday Discuss. Chem. Soc. 108 243-54... 

For all known cases of iron-sulfur proteins, J > 0, meaning that the system is antiferromagnetically coupled through the Fe-S-Fe moiety. Equation (4) produces a series of levels, each characterized by a total spin S, with an associated energy, which are populated according to the Boltzmann distribution. Note that for each S level there is in principle an electron relaxation time. For most purposes it is convenient to refer to an effective relaxation time for the whole cluster. 

The [Fe4S4p clusters contain four equivalent irons and give relatively narrow signals (3, 7, 62, 63) (Fig. 2E). The electron relaxation time is evaluated around 5 X 10 s (Table I), which is somewhat smaller than that of the Fe(II) monomer. Also, the signals of [Fe4S4l (15, 64-68) (Fig. 2D) and [Fe4S4fi+ (8, 13, 69-71) (Fig. 2F) systems are sharp. 

The time scales of excitation characterizing the violent cluster perturbations which we want to describe are so short that, to a very good approximation, ions can be considered as fixed during the excitation process and for an early stage of the electronic relaxation (typically up to 100 fs). 

The method can partly take account of the electronic relaxation effects in the excitations. However, for the 1-D clusters of Si2He and Si32He8, the... 

Kiv, M., A. Krylov, and R. Gerber 1997a, Photodissociation, electronic relaxation and recombination of HCl in Ar (HCl) clusters Nonadiabatic molecular dynamics simulations . Faraday Discussions 108, 243 254. 

A short laser pulse ( 200 fs) generates hot electrons in the metal clusters. A second laser pulse monitors the hot electron population. This technique enables the direct observation of electron relaxation as a result of electron-phonon coupling. Electron-phonon coupling is a decisive property enabling differentiation between bulk and molecular properties. Although this possibility has been exploited by different authors, the particles used have not been single-sized, which is necessary if valid results are to be obtained. The use of clusters of a distinct size such as Aun and Auss" and almost monodisperse 15-nm gold colloids and 3 nm and 35 nm Pt particles recently enabled more precise measurements to be made and better results to be obtained than in the past. It could be shown that the hot electron relaxation depends on the particle size in two different ways ... 

As already mentioned, these two electronic investigations of hot electron relaxation and the optical extinction of various clusters, are only part of many attempts to visualize quantum size behavior and, as a consequence, the transition from metal to molecule. Other chapters will provide more information about the results obtained. Catalysis using metal clusters, whether homogenous or heterogeneous, must be influenced by this fundamental difference between molecular clusters and clusters with a metallic inner life . It is surprising that these aspects have not been discussed previously in relation to catalytic effects. The purpose of this contribution is, there-... 

Core excitation of clusters is reviewed as a unique approach to probe element- and site-speciflcally size-dependent properties of free clusters in the gas phase. The fundamental characteristics of core level spectroscopies that rely on resonant excitation are briefly reviewed. Specifically, this includes primary photoabsorption as well as subsequent processes, such as photoionization, electronic relaxation, radiative relaxation, and finally fragmentation of the singly or multiply charged clusters. Elxper-imental techniques as well as selected results on simple model systems, such as variable size rare gas clusters, are presented. 

The experiments needed to assign the resonances, determine the molecular stmcture, and characterize the cluster electronic stmcture are the same as those employed for paramagnetic metalloproteins in general, " which, in turn, are largely the same as those employed for comparable diamagnetic proteins,with the exception that faster repetition rates and much shorter mixing times are necessary to characterize the cluster environment. Since nuclear relaxation times can vary from 1 ms to several seconds in a cubane a range of experimental... 

The reader should be warned that this seeming simplicity of the C2s band system of hydrocarbon PE spectra is spurious, and largely a consequence of the low (inherent and instrumental) resolution of the experiment. Theoretical treatments including the effects of electronic relaxation and electron correlationshow that the observed shapes of the individual C2s bands are the result of the superposition of a whole series of Franck-Condon envelopes, each corresponding to one of many cation states involving multiply excited configurations. However, the positions of these individual bands tend to cluster around the one band (dominantly) associated with the Koopmans state due to simple... 

When the number of atoms increases, the calculations become more complex but the general philosophy still holds. Once the energy levels are calculated with their S values and wavefunctions, the Cjj coefficients, which represent the ratio between i and i, can be evaluated and used to compute the shifts and their temperature dependence with equation 16. Analogously, the Qj coeflScients squared can be used for the analysis of nuclear relaxation. Of eourse, also the analysis of the effects of magnetic coupling on the electronic relaxation rates of the single ions become more difficult. An analysis of the NMR spectra of a C04S11 cluster is available [37],... 

For the description of free metal clusters, as observed and investigated in supersonic beams, the electrons relax (and tunnel) into the neighboring vacuum. In order to model... 

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