Cluster-trapped electron

The formation of hydrated electrons (in the water-saturated zeolites X and Y) has been identified through their absorption spectra, their short lifetimes distinct from the long-lived cation cluster-trapped electrons, and their reactivity towards typical hydrated electron quenchers such as methylviologen. Based on these spectra (Fig. 5), yields between 4 x 10 mol and 6 x 10 mol were measured for electrons in fully hydrated NaY. These high radiolytic yields were also explained by electron transfer from the ionized zeolitic skeleton to the water clusters [Eqs. (1) and (7)]. 

Since solids do not exist as truly infinite systems, there are issues related to their temiination (i.e. surfaces). However, in most cases, the existence of a surface does not strongly affect the properties of the crystal as a whole. The number of atoms in the interior of a cluster scale as the cube of the size of the specimen while the number of surface atoms scale as the square of the size of the specimen. For a sample of macroscopic size, the number of interior atoms vastly exceeds the number of atoms at the surface. On the other hand, there are interesting properties of the surface of condensed matter systems that have no analogue in atomic or molecular systems. For example, electronic states can exist that trap electrons at the interface between a solid and the vacuum [1]. 

One can see that with decreasing average thickness of the silica grain boundaries below about 1 run the intensity of the PL strongly decreases. The investigations being currently done in our laboratory should decide if this decrease is due to a delocalization of the electrons and holes. Such an effect has been described for CdS clusters trapped in zeolites [41]. To optimize the electroluminescence from nc-Si films will require to find the optimum compromise. 

Neumark and co-workers [56] pointed out the similarity of the cluster results to the transient behavior in aqueous 1 solution, which has been studied via ultrafast pump-probe measurements [50]. Bradforth and co-workers [50] observed IR (800 nm) transient absorption after UV (255 nm) excitation with 50 fs time resolution. In 1 solution, a promptly arising transient disappears within 50 fs, and absorption due to solvated electron rises with a 200 fs time constant. For longer time-scales, the trapped electron shows a biexponential decay with time constants of 8 and 60 ps, which is due to recombination with the nearby iodine atom. The close resemblance of time-scales for the rise of the solvated electron and isomerization in I (water) ( = 5 and 6) implies that the electron trapping pathway in solution can be modeled as a rearrangement of the solvent hydrogen-bond network in gas-phase clusters. 

Four of the most powerful methods presently applied to elucidate metal cluster geometric structure will be presented in the following. These are mass-selected negative ion photoelectron spectroscopy, infrared vibrational spectroscopy made possible by very recent advances in free electron laser (FEL) technology, gas-phase ion chromatography (ion mobility measurements), and rf-ion trap electron diffraction of stored mass-selected cluster ions. All methods include mass-selection techniques as discussed in the previous section and efficient ion detection schemes which are customary in current gas-phase ion chemistry and physics [71]. 

CdS clusters of narrow size distribution were studied by Eychmuller et al. [63], In this case a rather narrow luminescence band can be observed near the absorption band. The decay kinetics of this excitonic luminescence is multiexponential with a typical lifetime on the order of nanoseconds, much longer than the expected exciton lifetime. The temperature dependence of the excitonic luminescence shows complex behavior. Again, the authors use the three-level thermal equilibrium model to explain the data. The excitonic luminescence is identified as delayed luminescence occurring by detrapping of trapped electrons. Furthermore, they invoke the concept... 

The immediate result after the excitation of a semiconductor cluster by a short laser pulse is the generation of an exciton bounded by the cluster surface. The exciton has a very short lifetime and in general does not contribute to the optical nonlinearity except in the very early time domain. Instead, the exciton is rapidly trapped by surface defects within subpicoseconds, forming a trapped electron-hole pair [50, 60, 62, 63, 93], as discussed in Section III. It is the presence of these trapped electron-hole pairs that affects the cluster absorption spectrum and gives rise to the optical nonlinearity [50, 93-97]. 

Figure 17. (a) A series of difference absorption spectra recorded at selected times after excitation of 40-A CdS clusters in Nafion with a 355-nm, 30-ps, 87-pJ laser pulse. (b) A plot of the maximum negative absorbance change at 450 nm induced in the 40-A CdS clusters in a Nafion sample as a function of 355-nm, 30-ps pump pulse energy. The bleaching saturates when there is about one trapped electron-hole pair per cluster. (Both figures taken from reference 50.)... 

This model predicts that a trapped electron is more efficient than a trapped hole in bleaching the exciton absorption in the case of CdS. This is because a trapped hole is not capable of localizing an electron with small effective mass, and therefore, is inefficient in reducing the electron-hole overlap. This interesting prediction was proven by a pulse radiolysis experiment [90] where the electron and hole can be separately injected into the cluster and their effects probed separately. It was found that the electron is much more efficient than the hole in bleaching the exciton absorption of CdS clusters. One can also use sensitized photoinduced electron transfer to inject either an electron or a hole into the clusters and study their effects separately. Such experiments should be very informative. 

With the basic mechanism understood, the resonant nonlinearity of semiconductor clusters can now be quantitatively analyzed. Since one trapped electron-hole pair can bleach the exciton absorption of the whole cluster, the bleaching efficiency per absorbed photon of a nanocluster is the same as that of a molecule, as described by Eq. (20). For a given rp and r, the resonant third-order optical nonlinearity of a nanocluster is simply determined by the (a - ax) term. 

Another example of such equilibria is the case of methanol in hydrocarbons. Since there exists an attractive force between a charge and a dipole moment of a polar molecule, it was speculated that electron attachment might be possible. Studies of the electron behavior in solutions of methanol and isooctane and tetramethylsi-lane, respectively, showed that excess electrons do not react with isolated methane molecules. At higher concentrations, the methanol molecules form clusters and an attachment/detachment equilibrium was found to exist with pentamers. Larger clusters (n > 10) seem to trap electrons irreversibly (Gangwer et al., 1977). 

The kinetics of the two electron transfers kei and kei) determine the trapping probability for the electrons. The overall rate of the electron transfer from the NR to the cluster (kei) is defined by the LUMO states of the clusters with respect to the semiconductor band edge. Clusters with a low LUMO can trap electrons efficiently and reduce the back reaction. However, with a too low LUMO the electrons are bound so strong on the clusters that the electron transfer rate from the cluster to the hydrogen atoms kei) is reduced, resulting in a lower Hi production. 

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