Modeling Fluorescence Observables, Particularly for FRET Experiments, using Markov Chain Analysis of Molecular Dynamics and Quantum Mechanics Simulations

We present a new method for simulating fluorescence observables, particularly those related to bulk and single-molecule fluorescence-detected resonance energy transfer (FRET) experiments. In this method, a molecular dynamics (MD) simulation is used to sample configuration space and quantum mechanics (QM) calculations are used to estimate the electronic coupling between the donor and acceptor probes for snapshots along the MD trajectory. A Markov chain method is used to sample the resulting electronic coupling trajectory allowing accurate simulation of any desired fluorescence observables, such as FRET efficiency histograms or time-resolved donor fluorescence decays. The Markov chain results will be compared with the results of simple histogram and averaging schemes showing that the Markov chain is the only one that yields realistic results in well known examples such as the rapid diffusion limit. This combination of computational methods also avoids some pitfalls of traditional FRET analysis such as the kappa-squared and the ideal dipole approximations. Because the simulation results can be compared directly with experimental observables, this method may allow more detail to be derived from experiment than is traditionally possible. 3037-PosB80

Probing Aqueous Ions with Non-local Auger Relaxation

The decay of core holes is often regarded as a local process, but in some systems, it involves the autoionization of neighbouring atoms or molecules. Here, we explore such non-local autoionization (Intermolecular Coulombic Decay, ICD) of surrounding molecules upon 1s ionization of aqueous-phase Na+, Mg2+ and Al3+ ions. The three ions are isoelectronic but differ in the strength of the ion-water interactions which is manifested in experimental Auger electron spectra by varying intensities. While for strongly interacting Mg2+ and Al3+ the non-local decay is observed, for weakly bound Na+ no signal was measured. Combined with theoretical simulations we provide a microscopic understanding of the non-local decay processes. We assigned the ICD to decay processes ending with two-hole states delocalized between the central ion and neighbouring water. The ICD process is also shown to be highly selective with respect to water molecular orbitals. The ICD lifetime was estimated to be around 40 fs for Mg and 20 fs for Al. Auger spectroscopy thus represents a novel tool for exploring molecules in the liquid phase, providing simultaneously structural and electronic information.