Theoretical Chemistry Group Heidelberg

1. Theory of Jahn-Teller effect and vibronic coupling

   The treatment of vibronic coupling phenomena has been pioneered (together with W. Domcke and L.S. Cederbaum) by a model type approach, where the pertinent coupling matrix elements are taken to be a low order Taylor series expansion in the nuclear coordinates. This was partly adapted from Jahn-Teller theory, but has also been employed to generalize the latter, by taking several Jahn-Teller active modes and electronic states into account (M. Döscher and H. Köppel, Chem. Phys. 225, 93 (1997)). Many electronic bands of small polyatomics could be explained in this way; see, for example: H. Köppel, W. Domcke and L. S. Cederbaum, Adv. Chem. Phys. 57, 59 (1984); H. Köppel and W. Domcke, Encyclopedia of Computational Chemistry, Ed. P. von Rague Schleyer, Wiley, New York, 1998, p. 3166. For a current monograph on the field (covering also the next topic below), see: W. Domcke, D. R. Yarkony and H. Köppel (Eds.), Conical Intersections: Electronic Structure, Dynamics and Spectroscopy (World Scientific, Singapore, 2004).
   

2. Construction of diabatic electronic wavefunctions

   A simplified construction scheme for diabatic electronic states has been proposed and been investigated numerically for several important test cases. In this scheme the aforementioned linear coupling approach (see item 1) is applied to the adiabatic-to-diabatic mixing angle only. In this way the crucial (singular) nonadiabatic coupling matrix elements appearing in the adiabatic electronic basis are removed. Various test cases (A. Thiel and H. Köppel, J. Chem. Phys. 110, 9371 (1999); H. Köppel, J. Gronki and S. Mahapatra, J. Chem. Phys. 115, 2377 (2001)) and applications to small, triatomic systems (S. Mahapatra et al., Chem. Phys. 259, 211 (2000)) show very encouraging results

3. Femtosecond quantum molecular dynamics

   Femtosecond molecular processes are treated computationally by means of wavepacket propagation techniques. This includes the ultrafast internal conversion dynamics triggered by conical intersections of potential energy surfaces (H. Köppel, M. Döscher and S. Mahapatra, Int. J. Quantum Chem. 80, 942 (2000)) and associated photophysical phenomena, such as the geometric phase effect (J. Schön and H. Köppel, J. Chem. Phys. 108, 1503 (1998)). More recently, also chemical processes have been studied such as H+H2 reactive scattering (S. Mahapatra, H. Köppel and L.S. Cederbaum, J. Phys. Chem. A 105, 2321 (2001)) and the 1,2-hydrogen shift occurring in the system vinylidene-acetylene (R. Schork and H. Köppel, J. Chem. Phys. 115, 7907 (2001)).

4. Electronic structure and dynamics of elementary catalytic reaction steps

   The electronic structure and quantum dynamics of hydrogen elimination and related elementary reaction steps in homogeneous catalysis is studied computationally. The project is part of the Sonderforschungsbereich 623 'Molekulare Katalysatoren: Struktur und Funktionsdesign'. Please consult their homepage for further information.

Previous Research Areas Prof. Köppel

1. Dynamic core-hole localization effects

   In systems with several nominally equivalent core-hole sites such as C2H4 or CO2 the instantaneous creation of an asymmetric distortion through excitation of non-totally symmetric modes leads to the phenomenon of dynamic core-hole localization. The vibrational structure of the core-hole spectra cannot be interpreted using the Condon principle and the adiabatic approximation; a diabatic picture is more appropriate (H. Köppel et al., J. Chem. Phys. 106, 4415 (1997); N. Dobrodey, H. Köppel and L.S. Cederbaum, Phys. Rev. A 60, 1988 (1999)).

2. Quantum chaos / Statistics of molecular energy levels

   In complex molecular spectra with heavy mixings between the different degrees of freedom a statistical analysis is more appropriate than a more conventional one-by-one assignment of the spectral lines. Through a semiclassical analysis its results can also be linked to the question of regular vs. chaotic classical motion (Th. Zimmermann et al., J. Phys. Chem. 91, 4446 (1987)). Complex spectra of C2H4+ and NO2 have thus been analysed and classified which look similar from a simple by-eye inspection (D. Leitner, H. Köppel and L.S. Cederbaum, J. Chem. Phys. 104, 434 (1996)).

3. Peierls distortion in quasi-one dimensional solids

   The Peierls-distortion operative in quasi-one dimensional solids with a half-filled valence band can be viewed as the analogue of the Jahn-Teller distortion in condensed systems such as polydecker sandwich compounds (M. Lavrentiev, H. Köppel and M.C. Böhm, Chem. Phys. 169, 85 (1993)). Its consequences on phonon spectra and low-energy excitations as well as modification by electron correlation and finite-size effects have been explored by extensive model calculations (I. Baldea, H. Köppel and L.S. Cederbaum, Phys. Rev. B 60, 6646 (1999); Eur. Phys. J. B 20, 289 (2001)).