- Theoretical surface science
We are interested in both stationary and, even more so, in dynamical aspects of adsorbates at solid surfaces. These play a role in such diverse fields as heterogeneous catalysis, surface lithography, photocatalysis, and electrochemistry. For dynamics, adiabatic and non-adiabatic processes are studied. In the first case, the adsorbate remains in the electronic ground state while in the latter electronic excitations are involved. The most prominent examples of non-adiabatic events are photo- and STM- (Scanning Tunneling Microscope) induced reactions. Both are interesting for applications (microstructuring of materials, surface photo- and femtochemistry, photocontrol, single molecule manipulation), and for fundamental reasons (treatment of ‘open’, driven quantum systems in contact with a ‘bath’ of surrounding modes). Using quantum dynamical (open-system density matrix theory, wave packet propagation) and quantum chemical methods, the following topics / reactions are investigated (for reviews, see [42,73,76] – numbering refers to publication list).
- Photodesorption of atoms and small molecules from metal surfaces
- Photoswitching of molecules at surfaces [101,107,111,120,124,126,141,142]
- STM-induced desorption and reactions (e.g., switching) of atoms and molecules at semiconductor and metal surfaces [36,40,47,53,60,75,80,89,93].
- Electron-stimulated desorption of small molecules from metal surfaces [55,56].
- (Femtosecond) Laser control of surface chemical reactions (UV/vis, IR)
- Quantum chemistry of photo- /STM-active adsorbates [34,40,75,80,88,90,93,100,101,106,127].
Examples of adiabatic processes are elastic, inelastic, and reactive scattering of atoms and molecules from rigid and nonrigid surfaces and the energy transfer of vibrationally excited adsorbates to the surface. We develop, test and apply, various time- independent and time-dependent quantum methods to treat scattering/energy transfer of atoms and molecules at/to metal and semiconductor surfaces. Specific examples are:
- Time-independent reactive scattering and rate theory for the dissociative sticking of diatomic molecules at rigid metal surfaces [10,11].
- Development and application of wavepacket methods for the scattering of atoms and molecules at rigid and nonrigid metal surfaces [23,31,33,41,44,45].
- Development and application of open-system density matrix based methods for gas-surface scattering [26,35,48,52,96].
- Vibrational relaxation of adsorbates by vibration-phonon or vibration-electron hole pair coupling [53,60,72,73,76,77,78,81,83,93,103,110,115,125,131,132]
- IR-laser pulse induced vibrational excitation and reactions of adsorbates
- Photodesorption of atoms and small molecules from metal surfaces
- System-bath dynamics
Many of the examples of above are of the type: A subsystem (or chromophore) interacts with an environment, which leads to energy and phase relaxation in the system. These system-bath problems can be treated in various ways. In quantum dynamics, we often use reduced density matrix theory, by solving an open-system Liouville-von Neumann (LvN) equation either with or without the Markov approximation and by direct matrix propagation. LvN equations can also be solved by stochastic wave- packet methods instead. An alternative approach is the solution of the full system-bath, time-dependent Schrödinger equation, by approximate quantum wavepacket methods. Examples are the MCTDH (Multi-Configurational Time-Dependent Hartree), the LCSA (Local Coherent State Approximation), and TDSCF (Time-Dependent Self-Consistent-Field) methods. Further, quantum-classical dynamics or molecular dynamics with friction (Langevin dynamics), can be employed. Method-oriented examples of our work are:
- Direct, reduced density matrix theory [12,14,18,26,32,39,48,50,54,81,88,96,97,117,138].
- Stochastic, dissipative, or coupled wavepacket methods [21,29,35,43,59,71,100,134].
- Approximate wavepacket methods: MCTDH, LCSA, TDSCF [44,77,78,83,112,138].
- Quantum-classical dynamics [31,44] and Langevin dynamics [31,100,118,134].
- Electron dynamics
The quantum mechanical description of (laser-driven) electron dynamics in molecular and solid-state systems is of relevance for molecular and nanoelectronics, photoche- mistry, and spectroscopy. Here, explicitly time-dependent methods in one-electron approximation (wavepacket propagation or open-system density matrix theory with relaxation and dephasing terms), and explicitly time-dependent correlated many- electron methods (TD-CI, Time-Dependent Configuration Interaction, and MCTD- HF, Multi-Configurational Time-Dependent Hartree-Fock) are developed and used. For transport problems, also the Landauer formalism is adopted. Specific examples are:
- The pump-probe and two-photon photoemission (2PPE) spectroscopy of interface and adsorbate states [51,65,68].
- The laser-driven electron transport through metal-insulator-metal contacts and metal films, using jellium models [61,65,66,67].
- Electron transport through molecular junctions .
- Calculation of electronic ground and excited states of real molecules, and their response to external electric fields using explicitly time-dependent methods [69,86,87].
- Laser-driven dynamics and control of electrons in real molecules [69,86,87,97,98].
- Extension of time-dependent many-electron methods (TD-CI) to dissipative systems [97,98,114].
- Theoretical photophysics and spectroscopy
Theoretical spectroscopy is a useful alternative to experiment: Not only is the assignment of spectroscopical signals to specific state-to-state transitions possible, but also environmental and temperature effects can be modelled in detail. We use quantum chemical ab initio methods or quantum dynamical techniques to evaluate spectra of isolated and embedded molecular species. Examples are:
- Time-independent and time-dependent (wavepacket) calculation of electronic and / or vibrational spectra of isolated molecular systems [19,28].
- Time-dependent open-system density matrix theory of electronic and / or vibrational spectra of large molecules or molecules embedded in an environment [18,39,50,51,84,95].
- Quantum chemistry of electronically excited states of biomolecules and dyes using CI, DFT-CI, and TD-DFT and R-MPn methods, possibly combined with molecular dynamics [57,58,64,75,82,84,91,95,97,136].
- Vibronically resolved electronic spectra using correlation functions [113,122,130,136,139]
- Time-dependent electronic spectroscopy by “dynamics on the fly” 
- Resonance Raman spectra using correlation functions 
- X-ray photoemission spectra 
Spectroscopy is often photophysics, i.e., no bonds are broken or made. The same is true for pure, laser-induced electron dynamics. In our group we are also interested in photochemistry, as stated above.
- Electronic structure of molecules, solids, and biomolecules
The electronic structure of molecules and solids in their ground and excited states is investigated with the help of ab initio or DFT methods. Standard programs such as GAUSSIAN03, CRYSTAL, and CASTEP are used for this purpose. Examples are:
- The electronic structure of ceramic high-Tc superconductors using ab initio cluster and band structure calculations [3,4,6,9].
- Electronic structure of low-dimensional solids:
‘Gap engineering’ and other aspects of layered materials [7,133,140].
‘Quantum size effects’ in metal films [8,49].
- Adsorption of molecules on surfaces [16,70,88,106,121,143].
- Electronic structure of switchable molecules
… and of transition metal compounds
- Electronic structure of flavoproteins as blue-light receptors
- Electronic structure of other biomolecules
Tools of molecular and solid state quantum mechanics are developed / improved for modelling and understanding the structure and dynamics of molecular systems, isolated or in contact with an environment. Examples, some of which have already been described are:
- Stationary electronic structure theory:
- Self-consistent treatment of long-range Coulomb interactions in solids .
- Implementation of scalar-relativistic effective core potentials in periodic Hartree-Fock schemes [1,2].
- Time-dependent electronic structure theory:
- Multi-Configurational Time-Dependent Hartree-Fock (MCTDHF) method [66,86].
- Time-Dependent Configuration Interaction (TD-CI) method [69,87,97,98,99].
- System-bath nuclear dynamics:
- See “system-bath dynamics” of above; in addition:
- Methods / concepts of calculating energy relaxation of excited atoms or molecules in an environment
- Optimal and local control theories [85,92,94,98].
- Time-dependent approaches to excited state spectroscopy [128,137]