(A) Calculation of electronic states and transitions using many-particle methods
This work aims at the precise ab initio calculation of electronic excitation energies and intensities in the field of photoabsorption and photoemission, photoionization and the Auger decay of molecules . 1-3) Besides the application of conventional quantum chemical methods, the main interest focuses on the application and development of field-theoretical many-particle methods. 4,5)
(B) Spectroscopy and radiationless relaxation of polyatomic molecules
Primary interest in this work is the investigation of strong non-Born-Oppenheimer effects. Research focuses especially on a microscopic understanding of ultrafast radiationless decay processes of electronically excited states of molecules and molecular ions . 6,7) Recently an innovative method was proposed which allows the efficient ab initio calculation of the elements of the coupling matrix required in these studies . 8)
(C) Electron-molecule scattering, photoionization and Auger decay
The first part of this work concentrates on the ab initio calculation of the electron-molecule scattering process . 9) Many-particle effects are taken into account using an optical potential . 10) Applications 11) were performed that include, e.g., studies of the vibrational dynamic that emerges during the scattering process as well as the calculation of rotation-excitation cross sections (rotational rainbows). Closely related to electron scattering are processes such as photoionization 12) and the Auger decay decay 13) where only one outgoing but no incoming electron are present (so-called half collisions).
(D) Wave packet dynamics
The motion of nuclei that move on Born-Oppenheimer surfaces and experience the influence of coupled potential energy surfaces may be solved either conventionally, i.e. within the time-independent picture or by the propagation of wave packets. For a number of problems, e.g., the study of decaying molecules (photodissociation) the time-dependent approach supercedes the classical one. Besides the study of models 14) this research has focused on the development of an efficient algorithm used for the wave packet propagation . 15)
(E) Chaos and statistics
This research field comprises classical chaos theory and its influence on corresponding quantum chemical systems. Particularly nice examples are atoms and molecules in strong magnetic fields where a wealth of interesting and new phenomena already has emerged . 16,17) A classification of the complex spectra and their interpretation by semi-classical considerations is possible from the statistical analysis of the energy levels . 18)
(F) Structure and dynamics of quasi one-dimensional systems
In this project the multi-mode Peierls distortion of quasi one-dimensional systems is studied with special consideration of the internal degrees of freedom of the monomers . 19) Examplary applications studied in detail are the equilibrium geometry of polydecker sandwich compounds , 20) phonons and non-linear excitations.
(G) Stable multiply-charged anions of isolated small molecules and clusters
This work has focused particular attention on the theoretical evidence for the existence of small multiply-charged negative ions . 21) Precise many-particle methods are used in the large-scale calculations required, e.g., for the assessment of the electronic stability of this interesting class of species. Current research focuses on medium-sized to large highly-charged anions of molecules and clusters 22) as well as on the development of simplified theoretical models that allow for a deeper understanding of the interesting phenomena observed.
(H) Fundamental Aspects, Electronic Structure and Dynamics of Atoms and Molecules in Strong Magnetic Fields
This project includes the investigation of fundamental properties of molecules in strong magnetic fields . 17) We hereby focus on the separation of the different types of motion, the Born-Oppenheimer approximation in external fields, symmetry properties of molecules in fields etc... The ab initio computation of the electronic structure of many electron systems in strong magnetic fields is also a central aspect of these research activities 23). Highly excited Rydberg atoms in strong magnetic fields belong to the simplest physical systems which exhibit regular, chaotic as well as intermittent dynamics and serve therefore as a laboratory for the appearance of chaos in simple microscopic quantum systems. Both classical as well as quantum mechanical studies are performed here 24) Due to the nonseparability of the different degrees of freedom the transition from regularity to chaos in the electronic motion shows up also in the center of mass motion.
(K) Charge migration driven by electron correlation
In many cases, the ionization of a many-electron system, like a molecule, does not bring the system to a stationary electronic state, but rather to a superposition of several or even many cationic states, creating in that way an electronic wave packet which starts to evolve in time. The process of ionization represents the removal of an electron from the system, or the creation of a "hole" in the electronic cloud and, therefore, the dynamics triggered by the ionization manifest themselves as a time-evolution of the created hole. If the initial hole is localized, these dynamics can even represent a migration of the hole throughout the system. In the late 90s, we predicted . 25) that such hole migration can be solely driven by the electron correlation and electron relaxation, and can take place on an ultrashort time scale, typically only a few femtoseconds . 26-28). This phenomenon has been termed /charge migration/ and contrasts strongly the standard electron (or hole) transfer which is driven by the nuclear motion and is hence much slower. There are by now ample /ab initio/ computed examples which show that charge migration is a rich phenomenon with many facets . 26,29,30) that are rather characteristic of the molecule studied. For a review, see . 31).
The rapidly growing interest in charge migration phenomenon is driven by the fact that although in many cases the process represents an ultrafast charge oscillation from one site of the molecule to another, our studies. 29,32) have already indicated that the coupling to the slower nuclear motion can lead to a trapping of the charge. In other words, this purely electronic process appears as the first step of an effective transfer of the charge from one moiety of the system to another. The presence of the charge in different parts of the system naturally triggers different nuclear rearrangements, that is, induces different chemistry. The ability to control the pure electronic charge migration step, therefore, offers the extremely interesting possibility to influence the chemical reactivity of the system at a very early stage of its quantum evolution.
(M) LICI - Light -induced conical intersections
Conical Intersections (CIs), a manifestation of the breakdown of the Born-Oppenheimer approximation, are ubiquitous in polyatomic molecules and are well known to play a crucial role in the quantum dynamics of these systems, acting as "photochemical funnels" inducing extremely fast radiationless transitions between electronic states. While to form a "natural" CI a molecule must have at least two independent nuclear degrees of freedom, so that CIs cannot occur in diatomics, by exposing a diatomic to a strong resonant laser field so-called Light-Induced CIs (LICIs) can be formed, with the angle between the polarization direction of the field and the molecular dipole moment providing the additional degree of freedom 33).
Of course LICIs can be formed also in polyatomic molecules, either through internal degrees of freedom (e.g. in fixed-in-space molecules) or for freely rotating molecules through the external (rotational) nuclear coordinates 34). LICIs can be formed both by standing and running laser waves and can have a strong impact e.g. on photodissociation processes and in molecular alignment 35,36). In this research area work is devoted to further investigate the properties and implications of LICIs, in particular their potentialities in the control of quantum dynamics and photoinduced processes.
Next to LICIs, an analogous phenomenon can be induced by light in electronic resonances, i.e. in electronic states undergoing electronic decay e.g. through electron emission, for example Auger Decay or Interatomic Coulombic Decay. In this case, complex potential energy surfaces are associated to these states, so that strong, resonant laser light can induce so-called DICEs, i.e. the analogous of LICIs for the case of electronic resonances 37,38). The investigation of light-induced DICEs constitutes another major topic in this research area.