Simultaneous pair electronic excitations

The resonance Raman enhancement profiles In Figures 7 and 8 show that the maximum Intensity of the Fe-O-Fe symmetric stretch falls to correspond to a distinct absorption maximum In the electronic spectrum. This Implies that the 0x0 Fe CT transitions responsible for resonance enhancement are obscured underneath other, more Intense bands. Although strong absorption bands In the 300-400 nm region (e > 6,000 M" cm"l) are a ubiquitous feature of Fe-O-Fe clusters, the Raman results make It unlikely that they are due to 0x0 -> Fe CT. An alternative possibility Is that they represent simultaneous pair excitations of LF transitions In both of the... 

It has been seen in Section III,B,2 that in general more than 80% of the whole dissipated energy appears at a given moment as electronic excitation. The excited states have generally short lifetime they are therefore considered here as transient, compared to the lattice defects, which are indeed quasi-permanent. The influence of these various electronic imperfections upon the properties of solids, and more specially upon their catalytic properties is still little known. However, a simplified view of the problem results when considering only the pairs of excess free carriers produced in the course of irradiation. These tend to recombine and then stationary concentration depends simultaneously on both the recombination time and the intensity of the incident radiation. As soon as the irradiation ceases, this stationary concentration rapidly recovers the thermal equilibrium value. The problem hence reduces to the determination of the conditions under which the stationary concentration of the free carriers under irradiation will differ notably from the thermal equilibrium value. 

Excitations within the pair clusters of Section 13.1.1 provide the dominant contributions to the description of the complicated correlated motion of interacting electrons. The pair clusters dominate since the correlated motion is especially important for electrons that are close to each other and since at most two electrons (with opposite spins) may coincide in space. However, for an accurate treatment of the correlated motion of the electrons (within the orbital model), we must consider clusters of all sizes. For the three-electron clusters, we thus introduce amplitudes that represent the simultaneous interaction of three electrons, resulting in the excitation of three electrons from three occupied spin orbitals to three unoccupied ones. Furthermore, we must allow excitations to occur also within clusters containing a single electron. Such one-electron processes represent a relaxation of the spin orbitals and occur since the Hartree-Fock mean field experienced by each electron before the excitations were introduced is modified by the many-electron excitation processes occurring within the remaining clusters. 

Perhaps the most important general conclusion to be drawn from the one-electron Hj analysis is that chemical bonding intrinsically has nothing to do with electron pairs or electron spin. One remarkable feature that never excites any comment is the bond length of precisely two Bohr radii. Because of that the molecule resembles a system in which an electron simultaneously completes the ground states of two H atoms at the most probable distance of a0 from each nucleus. This construct is strictly non-classical. In the most precise work [86] the internuclear distance is found to be fractionally less than... 

There exists an extensive literature on theoretical calculations of the vibrational damping of an excited molecule on a metal surface. The two fundamental excitations that can be made in the metal are creation of phonons and electron-hole pairs. The damping of a high frequency mode via the creation of phonons is a process with small probability, because from pure energy conservation, it requires about 6-8 phonons to be created almost simultaneously. 

The time-resolved spectroscopic studies thus show that the charge-transfer excitation of CIP results in the simultaneous production of the reactive 17-electron carbonylmetal radicals [Mn(CO)5-, Co(CO)4 , etc.] together with the reduced acceptor radical (Cp2Co, Q-, NCP-, etc.). Furthermore, the time scale of the CT photoexcitation indicates that these radicals are initially formed as geminate pairs within the solvent cage. (50). The absence of productive photochemistry on steady-state irradiation of contact ion pairs alone in solution (i.e., without added phosphine) is consistent with the spontaneous return of the transient radicals to the ground state intact, for example,... 

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