Peter Hamm

Charge migration and charge transfer in molecular systems

The transfer of charge at the molecular level plays a fundamental role in many areas of chemistry, physics, biology and materials science. Today, more than 60u2009years after the seminal work of R. A. Marcus, charge transfer is still a very active field of research. An important recent impetus comes from the ability to resolve ever faster temporal events, down to the attosecond time scale. Such a high temporal resolution now offers the possibility to unravel the most elementary quantum dynamics of both electrons and nuclei that participate in the complex process of charge transfer. This review covers recent research that addresses the following questions. Can we reconstruct the migration of charge across a molecule on the atomic length and electronic time scales? Can we use strong laser fields to control charge migration? Can we temporally resolve and understand intramolecular charge transfer in dissociative ionization of small molecules, in transition-metal complexes and in conjugated polymers? Can we tailor molecular systems towards specific charge-transfer processes? What are the time scales of the elementary steps of charge transfer in liquids and nanoparticles? Important new insights into each of these topics, obtained from state-of-the-art ultrafast spectroscopy and/or theoretical methods, are summarized in this review.

Implications of short time scale dynamics on long time processes

This review provides a comprehensive overview of the structural dynamics in topical gas- and condensed-phase systems on multiple length and time scales. Starting from vibrationally induced dissociation of small molecules in the gas phase, the question of vibrational and internal energy redistribution through conformational dynamics is further developed by considering coupled electron/proton transfer in a model peptide over many orders of magnitude. The influence of the surrounding solvent is probed for electron transfer to the solvent in hydrated I−. Next, the dynamics of a modified PDZ domain over many time scales is analyzed following activation of a photoswitch. The hydration dynamics around halogenated amino acid side chains and their structural dynamics in proteins are relevant for iodinated TyrB26 insulin. Binding of nitric oxide to myoglobin is a process for which experimental and computational analyses have converged to a common view which connects rebinding time scales and the underlying dynamics. Finally, rhodopsin is a paradigmatic system for multiple length- and time-scale processes for which experimental and computational methods provide valuable insights into the functional dynamics. The systems discussed here highlight that for a comprehensive understanding of how structure, flexibility, energetics, and dynamics contribute to functional dynamics, experimental studies in multiple wavelength regions and computational studies including quantum, classical, and more coarse grained levels are required.

Nonadiabatic effects in electronic and nuclear dynamics

Due to their very nature, ultrafast phenomena are often accompanied by the occurrence of nonadiabatic effects. From a theoretical perspective, the treatment of nonadiabatic processes makes it necessary to go beyond the (quasi) static picture provided by the time-independent Schrödinger equation within the Born-Oppenheimer approximation and to find ways to tackle instead the full time-dependent electronic and nuclear quantum problem. In this review, we give an overview of different nonadiabatic processes that manifest themselves in electronic and nuclear dynamics ranging from the nonadiabatic phenomena taking place during tunnel ionization of atoms in strong laser fields to the radiationless relaxation through conical intersections and the nonadiabatic coupling of vibrational modes and discuss the computational approaches that have been developed to describe such phenomena. These methods range from the full solution of the combined nuclear-electronic quantum problem to a hierarchy of semiclassical approaches and even purely classical frameworks. The power of these simulation tools is illustrated by representative applications and the direct confrontation with experimental measurements performed in the National Centre of Competence for Molecular Ultrafast Science and Technology.

Concepts and Methods of 2D Infrared Spectroscopy: Simple simulation strategies

One of the strengths of 2D IR spectroscopy is the ability to quantitatively link experimental results to computer simulations, be it molecular dynamics simulations, quantum chemistry calculations, or ideally a combination of both on the level of mixed quantum mechanics/molecular mechanics (QM/MM) calculations. In the present chapter, we outline how such simulations are performed and present some examples with computer code that can be reproduced on a personal computer. We also describe more sophisticated models that have been developed. The motivation of the chapter is not to get the most accurate agreement with experiment, but to outline the essential concepts with working examples. In this chapter we use the molecular dynamics simulation package Gromacs 3.3 [183] (which can be downloaded for free from http://www.gromacs. org), the quantum chemistry program Gaussian09 for electronic structure calculations [58], and simple Mathematica or C codes (together with Numerical Recipes routines [152]). All the relevant computer programs in this chapter can be downloaded from the book webpage (http://www.2d-ir-spectroscopy.com), so the reader has operational programs to start with which can then be modified at will. For each of the Mathematica programs, Matlab versions are available on the book webpage as well. 2D lineshapes: Spectral diffusion of water Perhaps the most accurate quantities that can currently be modeled are 2D IR lineshapes.

Concepts and Methods of 2D Infrared Spectroscopy: Basics of 2D IR spectroscopy

In this chapter we apply the mathematical methodology that we have developed in the preceding chapters to predict what the 1D and 2D IR spectra will look like for some generic systems. It turns out that 2D IR line shape and cross-peak patterns depend upon the experimental setup chosen to measure the 2D IR spectra, and some are better than others. Thus, this chapter is organized according to the common ways of collecting 2D IR spectra. Linear spectroscopy Before discussing 2D IR spectra, we illustrate the concepts of the preceding chapters by applying the methodology to linear infrared spectroscopy. For linear spectra measured using weak infrared light, and assuming that all the molecules are in their ground vibrational state before the laser pulse interacts with the sample, we only need to consider two vibrational levels and one Feynman diagram (Fig. 4.1a, b). Using this Feynman diagram, we develop the response function step by step: At negative times, the system is in the ground state, described by the density matrix ρ =|0〈 〉0|. At time t = 0, we generate a ρ10 off-diagonal matrix element of the density matrix (we also generate ρ01 element from the corresponding complex conjugate Feynman diagram, which is not necessary to consider because it is redundant). The probability that this happens is proportional to the transition dipole moment μ 10 . […]