Clusters Coulomb excitation

In a next step, we compare nuclear and cluster response in the generic case of Coulomb excitation, as modeled by an initial shift of the electrons (respectively neutrons) with respect to ions (respectively protons). We first consider the nuclear giant dipole resonance. The lower panel of Figure 7.9 shows the power spectrum of the dipole along the z axis (symmetry axis) of after Coulomb excitation for several amplitudes with average excitation energies as indicated. The small-amplitude case represents the nuclear excitation spectrum in the linear regime as it is known from nuclear RPA calculations. We... 

For comparison we now take a similar cluster case. Figure 7.10 shows results for comparable Coulomb excitations (modeled by initial shift) of the cluster NaQ" " in the 144 Cylindrically Averaged Pseudopotential Scheme (CAPS) configuration, which consists of two subsequent rings each covering four ions and topped by one single ion [105]. 

Figure 7.10 Spectra of cluster resonances at small and large amplitudes (lower panel), as produced by Coulomb excitation. The upper panel shows the number of emitted electrons as a function of time...

Altogether, we thus see a striking similarity, up to details, between the nuclear and the cluster responses for this clean Coulomb excitation mechanism with the prominent feature that the resonance modes turn out to represent robust, harmonic oscillations. It remains to be seen to what extent this behavior persists in other experimental situations. In the following we focus on heavy-ion collisions and laser-cluster interactions as complementary and widely studied excitation mechanisms. 

The in-beam Mossbauer technique combining Coulomb-excitation and recoil-implantation, which was described in the Sect. 6.3.1, provides a unique feature for studying the anomalously fast diffusion, i.e., one-by-one measurement Every y-ray emission from Fe follows the implantation process. As a consequence, in the lattice the Fe probe always remains fully isolated from other Fe atoms implanted before, and therefore, the spectrum obtained with this method is completely free from overlapping cascades as well as from clustering of Fe atoms. Both of them would change completely the diffusion properties of Fe atoms. This method, therefore, guarantees an experimental condition under which we can follow a few jumps of Fe atoms immediately after the implantation into anomalously fast diffusion systems, such as a-Zr, Sc, and Pb. 

In fact, with small particles or clusters, a range of excited state lifetimes could be observed by spectroscopic methods . The observed non-Arrhenius dependence indicated the importance of multiphonon electron tunnelling, probably to preexistent traps. The shorter lifetimes observed at shorter emission wavelenths indicated significant coulombic interaction between traps. 

The dynamics of proton binding to the extra cellular and the cytoplasmic surfaces of the purple membranes were measured by the pH jump methods [125], The purple membranes selectively labeled by fluorescein Lys-129 of bacteri-orhodopsin were pulsed by protons released in the aqueous bulk from excited pyranine and the reaction of the protons with the indicators was measured. Kinetic analysis of the data implied that the two faces of the membrane differ in then-buffer capacities and in their rates of interaction with bulk protons. The extracellular surfaces of the purple membrane contains one anionic proton binding site per protein molecule with pA" 5.1. This site is within a Coulomb cage radius from Lys-129. The cytoplasmic surface of the purple membrane bears four to five pro-tonable moieties that, due to close proximity, function as a common proton binding site. The reaction of the proton with this cluster is at a very fast rate (3 X 1010 M-1 sec ). The proximity between the elements is sufficiently high that even in 100 mM NaCl, they still function as a cluster. Extraction of the chromophore retinal from the protein has a marked effect on the carboxylates of the cytoplasmic surface, and two to three of them assume positions that almost bar their reaction with bulk protons. Quantitative evaluation of the dynamics of proton transfer from photoactivated bacteriorhodopsin to the bulk has been done by using numerical... 

Fig. 31. The excited state efficiency 4es as a function of the reaction exothermicity AGe (data from [198, 199]) in the annihilation reactions of MoeClJj with the tungsten halide cluster acceptor WXgYj and Mo6Cl[ 4 in dichloromethane solutions. AGe values are calculated from the differences in the standard redox potentials corrected for the Coulombic interactions between reactions and product. The shape of the parabolic curve corresponds to the outer reorganization energy An = 1.2eV.

The photophysical processes of semiconductor nanoclusters are discussed in this section. The absorption of a photon by a semiconductor cluster creates an electron-hole pair bounded by Coulomb interaction, generally referred to as an exciton. The peak of the exciton emission band should overlap with the peak of the absorption band, that is, the Franck-Condon shift should be small or absent. The exciton can decay either nonradiatively or radiative-ly. The excitation can also be trapped by various impurities states (Figure 10). If the impurity atom replaces one of the constituent atoms of the crystal and provides the crystal with additional electrons, then the impurity is a donor. If the impurity atom provides less electrons than the atom it replaces, it is an acceptor. When the impurity is lodged in an interstitial position, it acts as a donor. A missing atom in the crystal results in a vacancy which deprives the crystal of electrons and makes the vacancy an acceptor. In a nanocluster, there may be intrinsic surface states which can act as either donors or acceptors. Radiative transitions can occur from these impurity states, as shown in Figure 10. The spectral position of the defect-related emission band usually shows significant red-shift from the exciton absorption band. 

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