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SURFSPEC Report Summary

Project ID: 279619
Funded under: FP7-IDEAS-ERC
Country: Norway

Final Report Summary - SURFSPEC (Theoretical multiphoton spectroscopy for understanding surfaces and interfaces)

When molecules are exposed to light, the will respond to the applied perturbations, either by absorbing the light or by scattering it. These processes allow us to obtain a lot of information about the molecules, their molecular and electronic structure, the electronic and nuclear dynamics of the molecule, as well as the interaction of the molecule with an environment, be this a solvent, a larger (bio)molecular complex, or a surface or an interface.

Experimentally, these responses can be studied using a variety of spectroscopic techniques. With experimental advances, more intense fields have become accessible, often with shorter pulse lengths, and several light sources are often used to obtain insight into time evolution of processes or to probe specific interactions, such as the interactions that arise from a surface or an interface.

With the increased complexity of experimental set-ups, the analysis of the observed data in terms of their molecular responses gets increasingly complicated, also as a consequence of more complex systems being studied. Computational studies can assist in these analyses, but the increased complexity of spectroscopic processes and the increased complexity of the systems being studied requires the development of novel computational methodology. This has been the focus of the SURFSPEC project.

In this project we have developed a novel approach to the calculation of interactions between an electronic density and an arbitrary number of external electromagnetic fields. This has been achieved by using techniques from computer science, such as recursive programming and automatic differentiation, to develop a program that by itself designs the computational pathway and executes this evaluation to obtain these properties.

We have extended this technique to also include the effects of a complex surrounding. Several approaches have here been developed, the most central one being a polarisable embedding approach, in which the mutual polarisation of the reactive centre of the system and the surrounding environment, be this a solvent or a biomolecule, is included, as well as local field effects. We have also extended the polarisable continuum model to calculate these responses, as well as a three-level model combining the quantum description of the chromophore with both the polarisable force field and the polarisable continuum model. We have also developed a deep understanding of the computational requirements for the accurate modelling of multiphoton processes in biomolecular systems using the polarisable embedding approach.

We have also developed methods to study the interactions between an electromagnetic field and the electronic density in the time domain for electron dynamics. This allows both to efficiently calculate molecular responses in a wide frequency range, as well to study processes at the atto to femtosecond timescale. With the new-generation light sources, this will become increasingly important. The electron dynamics developments have been implemented based on two- and four-component relativistic Hamiltonians, allowing both molecules with heavy elements to be studied, as well as to accurately describe processes dependent on spin-orbit effects, such as X-ray L-edge absorption spectra.

The project has opened new avenues for computational studies of spectroscopic processes, providing theoreticians and experimentalists with the necessary toolbox to study complex light-matter interactions in complex biological systems. A particularly interesting area for these methods are multiphoton absorption processes and multidimensional vibrational spectroscopies. More work remains in order to define models that can also reliably describe light-matter interactions at surfaces and interfaces, but we have initiated work on including also non-electrostatic effects into our computational models, often much more important for interfaces than for more homogenous samples, and this work is currently being pursued in our group.

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