Attosecond Electron Dynamics in Molecular Ions

Chemistry is the science that occupies itself with the composition, structure, properties, and re-arrangement of molecules. Consequently, many chemists perform experiments aimed at monitoring the relative positions of atoms in the course of a chemical reaction and/or dissociation process as a function of time. These are changes that occur on the femtosecond (or longer) timescale and are conveniently studied using readily available femtosecond lasers. However, faster timescales do exist in molecular systems of chemical interest. As was recently pointed out by Remacle and Levine (An Electronic Timescale in Chemistry, PNAS 103, 6793 (2005)), sudden ionization of molecules can lead to the formation of molecular ions with a non-equilibrium electronic structure that subsequently evolves in time, and that can lead, in extended molecules, to electron transfer on a timescale of just a few femtoseconds. Recently developed attosecond extreme-ultraviolet (XUV) lasers, and recently developed approaches for the implementation of attosecond lasers in time-resolved pump-probe experiments now make it possible to investigate this type of ultrafast electronic response experimentally. To do so is the goal of the present proposal. The current state of the art of attosecond science allows experiments to be configured where the attosecond pulses are used to initiate or probe the electron dynamics at hand. A laser in the visible or infrared (IR) can be used as the 2nd laser in a pump-probe sequence that either follows or preceeds the attosecond pulse. This proposal contains two types of experiments. (1) attosecond XUV pump-IR probe experiments will be performed to explore the idea of Remacle and Levine: the attosecond XUV laser will be used to perform a sudden ionization of the molecule under investigation, and multi-photon ionization by the IR probe accompanied by angle- and energy-resolved detection of photoelectrons and accompanying ions will be used to obtain information about the time-dependent electron motion induced by the ionization event. A number of promising molecules for this type of experiment (donor-acceptor complexes, small peptide chains, and hydrogen- or Van der Waals-bonded complexes, where the ionization can be localized on a sub-unit of the molecule) are identified in the proposal. (2) IR pump-XUV probe experiments will be performed to assess the degree to which IR multi-photon ionization can serve as a probe of single-active electron dynamics. The attosecond probe will determine to what extent singly-charged molecular ions left behind by the IR ionization have become coherently excited by this ionization process. The experiments in this proposal are extremely challenging and at the forefront of the rapidly developing field of attosecond science. Consequently - following what quickly seems to become a tradition in the attosecond field ? we will carry out this project in close collaboration with leading experimentalists and theoreticians outside the Netherlands. The experiments will be carried out in collaboration with Prof. Albert Stolow from the National Research Council in Ottawa, Canada, where the postdoctoral fellow that we seek to hire through this proposal will also perform part of the experimental work. And the experimental work will benefit from a close collaboration with Dr. Francoise Remacle from the University of Liege, Belgium and Prof. Raphy Levine from the Hebrew University of Jerusalem, Israel. All these scientists will be closely involved in the project and consequently the drafting of the present proposal has truly been a joint effort.