Vibronic effects, molecules

Quantum Systems in Chemistry and Physics is a broad area of science in which scientists of different extractions and aims jointly place special emphasis on quantum theory. Several topics were presented in the sessions of the symposia, namely 1 Density matrices and density functionals 2 Electron correlation effects (many-body methods and configuration interactions) 3 Relativistic formulations 4 Valence theory (chemical bonds and bond breaking) 5 Nuclear motion (vibronic effects and flexible molecules) 6 Response theory (properties and spectra atoms and molecules in strong electric and magnetic fields) 7 Condensed matter (crystals, clusters, surfaces and interfaces) 8 Reactive collisions and chemical reactions, and 9 Computational chemistry and physics. 

Nuclear Motion, Vibronic Effects and Flexible Molecules... 

Nuclear motion, vibronic effects, and flexible molecules (Chair J. Maruani) Response theory—Properties and spectra Atoms and molecules in strong electric and magnetic fields (Chair J. Gerratt)... 

Nuclear motion vibronic effects, flexible molecules ,... 

In the present review article 1985 s results obtained in applications of the concept of vibronic interactions to the investigation of electric properties of molecules (dipole and multipole moments and polarizabilities) are presented. Molecular aspects of these topics are almost untouched in the publications listed in the preceding paragraph. The idea of dipole instability was used first as a basis of the so-called vibronic theory of ferroelectricity (Bersuker, 1966 Bersuker and Vekhter, 1978). Meanwhile, the manifestation of the electronic or vibronic degeneracy in the electric responses of molecules, being no less essential than other vibronic effects, has some special features. 

This effective Hamiltonian will improve the taking into account of deformation vibrations [15,26,29,30], all of degrees of freedom of the system, namely vibronic effects [38,45-47]. However, the action of these factors (including stretch-bend resonance interactions, that perturb overtone part of vibrational spectra most of YCX3 molecules [15,26,29,30,48]) on vibrational... 

In a rigid band model, the excited-state relaxations (and thus the vibronic effects) are expected to decrease linearly with the number of atoms in the chain hence, lattice relaxation would be insignificant for long conjugated molecules. The existence of a vibronic progression in the absorption spectra of conjugated macromolecules indicates, however, that self-localization phenomena (with associated lattice relaxation) occur in the excited states. 

Scheme 15.4 Some of the molecules, described in the text, where the vibronic effect was found...

The interpretation of MCD spectra requires a close interaction with theory. So far, this has been nearly exclusively semiempirical theory, but the time appears ripe for ab initio calculations, at least for the smaller among r-electron systems, including not only purely electronic but also vibronic effects. Concurrently, high-resolution spectra will need to be measured for many of the molecules for which only low-resolution spectra are available today. A qualitatively higher level of understanding of electronically excited states of n systems can be expected to emerge from this combination. 

Non-adiabatic coupling is also termed vibronic coupling as the resulting breakdown of the adiabatic picture is due to coupling between the nuclear and electi onic motion. A well-known special case of vibronic coupling is the Jahn-Teller effect [14,164-168], in which a symmetrical molecule in a doubly degenerate electronic state will spontaneously distort so as to break the symmetry and remove the degeneracy. 

The full quantum mechanical study of nuclear dynamics in molecules has received considerable attention in recent years. An important example of such developments is the work carried out on the prototypical systems H3 [1-5] and its isotopic variant HD2 [5-8], Li3 [9-12], Na3 [13,14], and HO2 [15-18], In particular, for the alkali metal trimers, the possibility of a conical intersection between the two lowest doublet potential energy surfaces introduces a complication that makes their theoretical study fairly challenging. Thus, alkali metal trimers have recently emerged as ideal systems to study molecular vibronic dynamics, especially the so-called geometric phase (GP) effect [13,19,20] (often referred to as the molecular Aharonov-Bohm effect [19] or Berry s phase effect [21]) for further discussion on this topic see [22-25], and references cited therein. The same features also turn out to be present in the case of HO2, and their exact treatment assumes even further complexity [18],... 

Now, we examine the effect of vibronic interactions on the two adiabatic potential energy surfaces of nonlinear molecules that belong to a degenerate electronic state, so-called static Jahn-Teller effect. 

R, Englman, The Jahn-Teller Effect in Molecules and Crystals, John Wiley. Sons, Inc., Interscience, New York, 1972 I, B. Bersucker and V. Z. Polinger, Vibronic Interactions in Molecules and Crystab, Springer Verlag, 1989. 

---