Dynamics and Spectroscopy of Molecular Systems: From the Infrared to the X-ray Regime
Abstract:
In this set of lectures I will present fundamental concepts of Theoretical Molecular Physics in the context of spectroscopy in the condensed phase [1]. Starting from the Born-Oppenheimer ansatz for the molecular wave function, potential energy surfaces for nuclear motion will be discussed, accounting for the effects of non-adiabatic coupling. This also includes a brief digression into the field of electronic structure theory. Next, dynamics methods will be addressed, ranging from the classical to the quantum domain and in the latter case from coherent to incoherent dynamics. The connection to linear and nonlinear spectroscopy will be established on the basis of the (non-)linear response function formalism. Non-linear experimental setups such as multi-dimensional spectroscopies will be introduced as a means to unravel the nature of interactions as well as correlations in the dynamics of different chromophores.
Applications will be presented according to different spectral ranges and associated dynamics. Each example will include a brief introduction into the relevant concepts and models. (i) Nuclear dynamics can be followed by infrared spectroscopy. Here, the focus will be on hydrogen-bonded systems, which feature rich dynamics due to the often pronounced anharmonicity of the potential energy surface. Particular examples include ultrafast cascaded energy redistribution, correlated motion of nucleic acid base pairs, and vibrational dephasing in ionic liquids [2-4]. (ii) UV/Vis spectroscopy of coupled electron-vibrational dynamics will be discussed for the example of excitation energy transfer in natural and biological photosynthesis [5]. A central aspects concerns the general role of nuclear vibrations. Recent experiments established evidence that vibrations are not only serving the purpose of forming a heat bath, but are actively promoting electronic excitation energy transfer. This will be illustrated for the so-called Fenna-Matthews-Olson complex of green sulfur bacteria [6]. (iii) X-ray spectroscopy of core levels provide a highly localized probe of the electronic structure in the vicinity of the excited atom. I will present results for transition metal compounds in solution to demonstrate that fundamental concepts of chemical bonding between the metal atom and its ligands can be scrutinized [7,8].
- [1] V. May, O. Kühn, Charge and Energy Transfer Dynamics in Molecular Systems, Wiley-VCH, Weinheim, 2011.
- [2] K. Heyne et al., Revealing Anharmonic Couplings and Energy Relaxation in DNA Oligomers by Ultrafast Infrared Spectroscopy, J. Phys. Chem. B 112, 7909 (2008).
- [3] Y. Y. Yan, O. Kühn, Unraveling the Correlated Dynamics of the Double Hydrogen Bonds of Nucleic Acid Base Pairs in Solution, J. Phys. Chem. B 115, 5254 (2011).
- [4] S. Chatzipapadopoulos et al., Vibrational Dephasing in Ionic Liquids as a Signature of Hydrogen Bonding, ChemPhysChem 16, 2519 (2015).
- [5] M. Schröter et al., Exciton-Vibrational Coupling in the Dynamics and Spectroscopy of Frenkel Excitons in Molecular Aggregates, Physics Reports 567, 1 (2015).
- [6] J. Schulze, O. Kühn, Explicit Correlated Exciton-Vibrational Dynamics of the FMO Complex, J. Phys. Chem. B 119, 6211 (2015).
- [7] E. Suljoti et al., Direct Observation of Molecular Orbital Mixing in a Solvated Organometallic Complex, Angew. Chem. Int. Ed. 52, 9841 (2013).
- [8] S. I. Bokarev et al.,State-Dependent Electron Delocalization Dynamics at the Solute-Solvent Interface: Soft X-ray Absorption Spectroscopy and Ab Initio Calculations, Phys. Rev. Lett. 111, 083002 (2013).