Size-dependent electronic relaxation

The theory of energy transfer considered in this subsection was used to interpret the experiments with PbSe quantum dots (58) on the size-dependent energy relaxation in a quantum dot. In this paper it was shown that smaller dots have faster relaxation. In the theoretical paper by Hong et al. (59) it was assumed that the above energy transfer from a quantum dot exciton to surface states of the dot is a dominant channel of the electronic energy relaxation. Hong et al. considered in their calculations a spherical quantum dot of radius R and the transfer rate was obtained from the calculation of the power dissipation W on the surface of the quantum dot by the relation... 

Relaxation of CT samples also depends strongly on Sn content. For the most Sn-abundant sample x-ray analysis showed the presence of almost completely oriented Sn crystallites of 100-500 nm size, and electronic microscopy revealed that metal particles were separated by distances less than their size. It means that in this case we have practically metal film. The time response of such a sample appeared to be the same as that for pure Sn film. We attribute pulsewidth-limited rise of negative transmission and reflection (curves 10,11) to excitation of electrons in metal. Subsequent 5 ps rise reflects electron-phonon relaxation and further long decay is due to lattice cooling. Contribution of this dynamics is observed for the... 

Because of the fundamental importance of solvent-solute interactions in chemical reactions, the dynamics of solvation have been widely studied. However, most studies have focused on systems where charge redistribution within the solute is the dominant effect of changing the electronic stale.[I,2] Recently, Fourkas, Benigno and Berg studied the solvation dynamics of a nonpolar solute in a nonpolar solvent, where charge redistribution plays a minor role.[3,4] These studies showed two distinct dynamic components a subpicosecond, viscosity independent relaxation driven by phonon-like processes, and a slower, viscosity dependent structural relaxation. These results have been explained quantitatively by a theory of solvation based on mechanical relaxation of the solvent in response to changes in the molecular size of the solute on excitation.[6] Here, we present results on the solvation of a nonpolar solute, s-tetrazine, by a polar solvent, propylene carbonate over the temperature range 300-160 K. In this system, comparisons to several theoretical approaches to solvation are possible. 

The relatively large number of experiments on vibrational relaxation compared to other means of access to the molecular dynamics (such as translational, rotational, or electronic relaxation) stems from the richness of vibrational structure. A molecule composed of N atoms presents 3N — 6 vibrational modes and still only three translational degrees of freedom. In addition the vibrational overtone manifold cannot always be ignored due to accidental (strongly size-dependent) degeneracy and Fermi resonance with fundamental modes. 

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 ... 

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 Si nanocrystals exhibit photoluminescence upon irradiation with UV light at 230 nm. The MPL spectrum is shown in Figure 10. The spectrum is similar to that reported for 4 nm Si nanocrystals upon excitation with 350 nm at 20 K and also to that PL spectrum of Porous Silicon (49). In these systems the red luminescence is interpreted as a consequence of quantum crystallites which exhibit size-dependent, discrete excited electronic states due to a quantum effect (6,50,51). This quantum confinement shifts the luminescence to higher energy than the bulk crystalline Si (1.1 eV) band gap. This indirect gap transition is dipole forbidden in the infinite preferred crystal due to translational symmetry. By relaxing this symmetry in finite crystallite, the transition can become dipole allowed. As pointed out by Brus (49), the quantum size effect in Si nanocrystals is primarily kinetic mainly due to the isolation of electron-hole pairs from each other. 

One of the favorite materials studied so far is PbSe. Its advantage is that the effective mass of electrons and holes in a nanocrystal is nearly the same, that is, it can be expected that the energy levels are split symmetrically within the conduction and valence band with respect to the middle of the gap. The size dependence of the absorption spectra looks similarly as that determined for InP (compare with Figure 9.5). PbSe is also of great interest because the energy gap of the bulk material is 0.28 eV which makes it possible to study relaxation effects over a large range... 

---