ATTOCHEM · Attosecond imaging and control of chemical dynamics

Attosecond imaging and control of chemical dynamics

The project ATTOCHEM is focused on ultrafast electron motion during chemical reactions. While the motion of the nuclei during a chemical reaction typically proceeds on the timescale of tens to thousands of femtoseconds, electron motion reaches well into the attosecond regime. Observing electron motion on the attosecond time scale has become possible through the development of laser sources that generate optical fields with high intensity and precisely controlled field evolution. Here, such attosecond technology is applied to imaging and controlling of electron and nuclear dynamics during light-induced chemical reactions. Chemical reactions triggered by light absorption are ubiquitous in nature. Examples include photosynthesis in green plants, DNA damage, and the process of vision in the human eye. A microscopic understanding of these processes provides the groundwork to their technological exploitation. These may involve breakthroughs such as new green energy sources or learn how to inhibit DNA damage that may lead to cancer. For a microscopic understanding of photochemical reactions it is of great importance to know precisely how electrons and nuclei move on their natural time scales. In addition to observation of ultrafast electronic and nuclear motion, laser light allows for controlling chemical reactions towards a desired outcome. Understanding how electrons and nuclei can be steered during a chemical reaction adds to the microscopic understanding gained by imaging their motion. Despite low yields, such laser-driven synthesis may allow the production of new chemicals by opening up reaction pathways that are not accessible by conventional means. In the project ATTOCHEM a novel experimental technique, called Sub-cycle Tracing of Ionization Enabled by infra-Red (STIER) has been developed and applied to imaging and controlling chemical dynamics. This technique employs intense and precisely controlled laser pulses in the visible and mid-infrared spectral ranges. In a proof-of-concept experiment, STIER was used to realize a streak camera for strong-field ionization [3], allowing attosecond measurements of electron motion in strong fields. STIER allows for manipulating the outcome of photochemical reactions. The mid-IR laser addresses vibrational or electronic degrees of freedom of a molecule, while the visible pulse initiates the reaction. We have achieved control over dissociation processes in polyatomic molecules, and observed signatures of light-induced molecular potentials. STIER has been successfully employed in imaging an oscillating charge density in an atomic ion [5]. The demonstrated all-optical momentum microscope will allow for visualizing the electron motion in molecules that occurs during the motion of the nuclei, e.g. during chemical reactions. In summary, realizing the STIER technique has opened up new research directions in strong-field physics. The following three directions immediately follow from the results obtained during the action: (i) Time-resolving strong-field ionization: STIER allows for measuring time delays in ionization, the momentum offset after tunneling and instantaneous ionization rates. (ii) Controlling chemical reactions: New routes to manipulating molecular dynamics by coupling rotational, vibrational and electronic degrees of freedom using the unique combination of laser pulses. (iii) Time-resolved photoelectron spectroscopy: In a pump-probe scheme with two visible pulses, the mid-IR allows for separating pump and probe steps. Isolating the probe signal allows for studying the dynamics induced by the pump pulse with sub-10-fs time resolution, e.g. imaging non-stationary valence electron densities during chemical processes. By carrying out this project I have received substantial training-through-research, including project management, complex experimental techniques including pump-probe experiments using visible and mid-IR pulses, as well as written and oral dissemination of scientific results. Moreover, I have had the opportunity to extend my network of international collaborators. These will be assets in obtaining an independent position in research.

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