Dimer charge oscillations

Comparison of the spectra before and after iodination (Fig. 3) shows that the broad absorption around 9000 cm (l.l eV), associated with the CT transition in the dimer with two electrons, shifts to 5000 cm (0.6 eV) in the dimer with one single electron. The spectra in the mid-infrared region exhibit the typical absorptions due to the dimer charge oscillations (related to the totally symmetric modes a and... 

The prominent new bands observed in the low-temperature infrared spectrum are due to the activation of dimer charge oscillations induced by the Au crystal components of totally symmetric (a ) intramolecular modes at frequencies somewhat lower than those of the corresponding unperturbed Ag components. The latter can be measured directly in the Raman spectrum and the set of vibronic frequency shifts (u i — n,) so derived allows one to evaluate the relative strength of the e-mv coupling constants gi of the various a intramolecular modes. This is made possible by solving... 

The mvestigation of molecular vibrations is a powerful too which can increase our knowledge on structures and on electron molecular vibrations, which are due to charge oscillation between dimerized molecules, coupled with totally symmetric intramolecular modes Raman scattering studies account for totally symmetric vibrations. In addition, Raman spectroscopy can take advantage of resonant effects. In fact, when resonant conditions are fulfilled, selected molecular vibrations are obtained as well as infor> mation on the electronic manifold involved in the resonance process. 

Thus there are two factors responsible for the maximum in the solvent entropy and its deviation from the pzc. Those parameters are the different configurations the monomers and dimers are able to assume on the surface of the electrode, and that, as we discussed above, depend on the free energy associated with each configuration [Fig. 6.85(a)]. The second parameter is related to the entropy of libration, i.e., how the water molecules oscillate and how these oscillations arc affected by the electrode charge [Fig. 6.85(b)]. The vibrational movements of the molecule do notgivatly affect the position of the maximum in the entropy-charge curve. 

An additional complication affecting silicon surface chemistry is the well-established fact that dimers tilt away from the symmetric position (c.f. Fig. 1(b)). Associated with dimer tilting is a charge transfer from the down atom to the up atom. Hence, the dimers exhibit somewhat zwit-terionic character, with one electron-poor atom and one electron-rich atom. Such a property of the Si(100)-(2 x 1) surface makes it possible to use nucleophilic and electrophilic attachment reactions. At temperatures less than 120 K, dimer tilting on Si(100)-(2 x 1) can be observed in STM experiments [3,9], while at higher temperatures the direction of the tilt oscillates on a time scale faster than the order milliseconds sampling times of the STM. 

Induction forces, on the other hand, may be responsible for the enchanced V y intensity. The squared transition moment expected for the dimer with two independent ethylene partners is twice that of the monomer. Inspection of Table 1 shows that the observed probability is somewhat higher than this. Vibration in each molecule induces oscillations in the polarizable charge distribution of the partner, leading to an enhanced dipole moment derivative. Numerical results for bound to a polarizable atom show that this enhancement is of the same magnitude as that observed (17). 

We can summarize this brief discussion of the electronic spectroscopy of dimers by noting that,in the dimer with two electrons, which is the simplest molecular cluster analog of an half-filled band system, the effect of the effective on-site correlation U is twofold (i) it shifts the charge transfer band to higher energies (ii) it reduces the oscillator strength of the CT transition. This behavior is shown in Fig. 2 for dimensionless quantities. 

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