Electron molecular-vibration interactions

Two further extensions of the CND0/S3 model also have proven valuable. First, it has been utilized to evaluate electron-molecular-vibration interactions in benzene (, 24), TCNQ (25, 26), and TTF ( ). A review of these calculations and their applications to interpret the transport properties of molecular crystals has been given elsewhere QJ). In addition, the CND0/S3 model also has been extended to encompass chalcogenide molecules, most notably S (28, 29), Se- (29), S.N, (30), As.S, (31), As.S, (3), and As Sef (31). 44— 46... 

Transition metal electrodes prepared by sputtering and electrochemical deposition often show derivative-hke (i.e., bipolar) [17, 34—38] or negative absorption (anti-absorption) bands [46-50]. Spectral features change from normal absorption to anti-absorption through bipolar shapes with increasing amount of the metal deposited [17]. The bipolar band shape was ascribed to a Fano-type resonance of electronic interactions between molecular vibrations and the metal [34, 35]. The anti-... 

Interaction between molecular vibrations and the orbital motion of electrons allows excitation of non-totally symmetric molecular vibrations during an electronic transition (breakdown of the Borti-Oppenheimer approximation). As an extreme manifestation of such an interaction, it happens that degenerate electronic states in non-linear molecules become unstable toward distortion (Jahn-Teller theorem). In fact, distortion due to non-totally symmetric vibrations will result in a lower molecular symmetry, with the consequence of lifting the orbital degeneracy [3]. 

Lewis proposed that the energy of high-mobility electrons can be transferred to molecules of the liquid by interaction with molecular vibrational modes of energy of tenths of electronvolts, usually. For n-hexane it can be as high as 0.37 eV in the infrared. The transfer could take place in either of two ways. In the first, called the weak interaction mode, a quasicontinuous loss along the electron trajectory is envisaged, resulting from the dielectric response of the liquid to the electric field of the electron. It was estimated that the time required for excitation of the liquid is 10 s, and the interaction distance would be approximately 0.6 nm. 

N. O. Lipari and C.B. Duke, J. Chem. Phys. 63, 1748 (1975). C.B. Duke, Electron Interactions with Molecular Vibrations ... 

The system s full partition function includes terms related to the i different types of energies nuclear, electronic, molecular vibrations, molecule rotations, as well as translation and interactions between the different types of molecules. 

Identifying electronic and vibrational properties of xanthophylls should provide not only structural information. Gaining information about excited state energy levels would help to design and interpret kinetic experiments, which probe molecular interactions and the energetic relationship between the xanthophylls and chlorophylls. 

In general, all observed intemuclear distances are vibrationally averaged parameters. Due to anharmonicity, the average values will change from one vibrational state to the next and, in a molecular ensemble distributed over several states, they are temperature dependent. All these aspects dictate the need to make statistical definitions of various conceivable, different averages, or structure types. In addition, since the two main tools for quantitative structure determination in the vapor phase—gas electron diffraction and microwave spectroscopy—interact with molecular ensembles in different ways, certain operational definitions are also needed for a precise understanding of experimental structures. 

Apart from molecular vibrations, also rotational states bear a significant influence on the appearance of vibrational spectra. Similar to electronic transitions that are influenced by the vibrational states of the molecules (e.g. fluorescence, Figure 3-f), vibrational transitions involve the rotational state of a molecule. In the gas phase the rotational states may superimpose a rotational fine structure on the (mid-)IR bands, like the multitude of narrow water vapour absorption bands. In condensed phases, intermolecular interactions blur the rotational states, resulting in band broadening and band shifting effects rather than isolated bands. 

Theoretical estimations and experimental investigations tirmly established (J ) that large electron delocalization is a perequisite for large values of the nonlinear optical coefficients and this can be met with the ir-electrons in conjugated molecules and polymers where also charge asymmetry can be adequately introduced in order to obtain non-centrosymmetric structures. Since the electronic density distribution of these systems seems to be easily modified by their interaction with the molecular vibrations we anticipate that these materials may possess large piezoelectric, pyroelectric and photoacoustic coefficients. 

In an effort to understand the mechanisms involved in formation of complex orientational structures of adsorbed molecules and to describe orientational, vibrational, and electronic excitations in systems of this kind, a new approach to solid surface theory has been developed which treats the properties of two-dimensional dipole systems.61,109,121 In adsorbed layers, dipole forces are the main contributors to lateral interactions both of dynamic dipole moments of vibrational or electronic molecular excitations and of static dipole moments (for polar molecules). In the previous chapter, we demonstrated that all the information on lateral interactions within a system is carried by the Fourier components of the dipole-dipole interaction tensors. In this chapter, we consider basic spectral parameters for two-dimensional lattice systems in which the unit cells contain several inequivalent molecules. As seen from Sec. 2.1, such structures are intrinsic in many systems of adsorbed molecules. For the Fourier components in question, the lattice-sublattice relations will be derived which enable, in particular, various parameters of orientational structures on a complex lattice to be expressed in terms of known characteristics of its Bravais sublattices. In the framework of such a treatment, the ground state of the system concerned as well as the infrared-active spectral frequencies of valence dipole vibrations will be elucidated. 

The other mechanism involves atomic-size roughness (i.e., single adatoms or small adatom clusters), and is caused by electronic transitions between the metal and the adsorbate. One of the possible mechanisms, photoassisted metal to adsorbate charge transfer, is illustrated in Fig. 15.4. It depends on the presence of a vacant, broadened adsorbate orbital above the Fermi level of the metal (cf. Chapter 3). In this process the incident photon of frequency cjq excites an electron in the metal, which subsequently undergoes a virtual transition to the adsorbate orbital, where it excites a molecular vibration of frequency lj. When the electron returns to the Fermi level of the metal, a photon of frequency (u>o — us) is emitted. The presence of the metal adatoms enhances the metal-adsorbate interaction, and hence increases the cross... 

As noted in the introduction, vibrations in molecules can be excited by interaction with waves and with particles. In electron energy loss spectroscopy (EELS, sometimes HREELS for high resolution EELS) a beam of monochromatic, low energy electrons falls on the surface, where it excites lattice vibrations of the substrate, molecular vibrations of adsorbed species and even electronic transitions. An energy spectrum of the scattered electrons reveals how much energy the electrons have lost to vibrations, according to the formula ... 

Interaction between electronic and vibrational motions in a molecular entity. 

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