Ultrafast dynamic chemical imaging

The investigations in C.2.7 aim at reaching a central MAP goal: the control and observation of electron motion in complex molecules. Recent light source developments within MAP permit exploring chemical processes with unprecedented time resolution allowing to understand ultrafast chemical reactions on the natural timescale of the motion of the fastest involved particles, the electrons. In our project we focus on strongly coupled electron-nuclear dynamics unfolding on an attosecond to few tens of femtoseconds timescale after strong-field or XUV-induced ionization of molecules. The theoretical description of such dynamics requires modeling beyond the Born-Oppenheimer approximation and is performed in project C.2.9.

Our project has two objectives: A) the control of elementary chemical processes by steering the electron motion in (complex) molecules with waveform-controlled few to single cycle laser pulses and B) the time-resolved observation of this control with a resolution down to the attosecond regime.

Waveform controlled few-cycle laser fields have allowed to control electron localization in the dissociative ionization of molecular hydrogen and its isotopes ^{(1, 2)}. After initial ionization, these molecules contain only a single electron, which can be localized and steered by a light-induced coherent superposition of two electronic states of gerade and ungerade symmetry. The electron localization becomes visible upon the break-up of the molecule: the directional emission of charged and uncharged fragments depends on the electric field waveform of the laser pulse. We have also explored an alternative route to control electron localization in molecular hydrogen with a two-color laser field⁽³⁾.

An important milestone towards the control of electrons in complex molecular systems was achieved in recent studies on a first multi-electron molecular system ⁽⁴⁾. The directional emission of ionic fragments upon the break-up of carbon monoxide (CO), indicating the control of electron motion during dissociation, was monitored via velocity-map imaging (VMI). The figure shows a cut through the 3D momentum distribution of C+ ions that have been created in the dissociative ionization of CO with a 4 fs, phase-stable laser pulse. A clear asymmetry in the ion emission is observed along the (vertical) laser polarization axis. Quantum calculations that were performed within collaboration with C.2.9 suggested that two phenomena are responsible for the observed asymmetry in the ion emission and thus for the control of the dissociation reaction by the waveform of the laser field. First, the ionization of CO in a near-single cycle phase-stable laser field is strongly orientation dependent. The angular distribution of the ion emission indicates the electronic structure of the orbitals from which the ionization took place (the solid line in the figure corresponds to the calculated ion distribution and asymmetry for the experimental parameters)⁽⁵⁾. Second, excitation of CO⁺ by electron recollision and the coupling of several excited states of CO⁺ lead to dissociation and an observable phase-dependent asymmetry⁽⁴⁾.

In the first part of the extension of this project, we plan to study the steering of electrons in molecular systems such as D₂ and DCl utilizing a recently developed source of intense infrared few-cycle laser radiation at 2 mm wavelength. In addition, in further experiments on the control of molecular reactions by the laser waveform, the distinction of various reaction pathways will be enabled utilizing a recently commissioned reaction microscope (REMI), allowing the coincident detection of created ions and electrons. We aim for the observation of strongly coupled electron-nuclear dynamics occurring after single or double ionization of a molecular species with attosecond time resolution.