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Attosecond imaging and control of chemical dynamics

Periodic Reporting for period 2 - ATTOCHEM (Attosecond imaging and control of chemical dynamics)

Reporting period: 2017-11-17 to 2018-11-16

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 scienti
The first six months of the project were dedicated to the development of the necessary optical setup for the STIER experiments, followed by first successful experiments in the prototype system H2. After further development, a first set of experiment towards coherent control of photochemical reactions in acetylene were carried out.

A carrier-envelope phase meter has been implemented into the experimental set-up and used to demonstrate sub-femtosecond time resolution of the STIER technique [3].

Further development of the experimental set-up allowed applications of STIER with different polarization geometries and on processes such as double ionization and dissociative ionization of H2.

STIER has been extended to pump-probe experiments. I have demonstrated the real time, three-dimensional imaging of the oscillating valence electron density in an atomic ion after photoionization. These results mark an important step towards imaging the electron motion within a molecule while it undergoes a chemical reaction.

In the return phase, first COLTRIMS experiments with the newly developed 100 kHz, 2 µm laser source at LMU were carried out. We have observed triple ionization of argon atoms at relatively low intensity, indicating the large recollision energy of electrons produced with 2 µm radiation. Photoelectron spectra from nitrogen molecules were recorded. However, realizing orbital imaging experiments will require further improvements of the laser system. A proposal to the Emmy-Noether programme of the Deutsche Forschungsgemeinschaft has been submitted, as planned.

Overview of the results and their exploitation:

(i) Streaking strong-field ionization
a. Single ionization: published [3]
b. Double ionization: manuscript in preparation

(ii) Imaging electron motion in space and time: published [5]

(iii) Manipulating dissociation of hydrogen: manuscript in preparation

(iv) Controlling molecular dissociation via vibrational wave packets: in progress


Further dissemination activities include invited talks at two international conferences (GRC on Multiphoton processes, and LPHYS) in 2018.
Paper [5] has been accompanied by a press release, and posts on twitter and a nature blog.  
The demonstrated STIER technique goes beyond the state of the art by adapting the principle of the Attosecond Streak Camera to strong-field ionization. This allows for conducting time-resolved measurements beyond the pulse duration of optical pulses. The experimental results also provide new insight into strong-field photodissociation of hydrogen and acetylene on light-induced potential energy surfaces. Furthermore, the combination of STIER with the laser-induced orbital imaging represents a versatile technique for the imaging of time-dependent valence electron densities. In addition, I plan to explore the applications of STIER for tracing electron dynamics in solids.

I expect that the STIER method and its applications will receive broad attention in the scientific community. As fundamental research results, I do not expect immediate socio-economic consequences of the project. The developed technique may be of interested in the applied sciences and support future developments.
(a) Overview of the STIER experiments, and (b) Results for STIER applied to ionization of Neon.
The electron distribution inside an argon ion changes from a donut shape into a peanut shape within