Ultrafast chemical imaging

Our project aims at the observation and the control of ultrafast chemical transformations. We will utilize imaging techniques to A) control elementary chemical processes (e.g. a dissociation reaction) by steering the electron motion in molecules with waveform-controlled few-cycle laser pulses and B) monitor ultrafast chemical transformations by means of laser-induced electron diffraction. As an important improvement over studies that will be performed with 750 nm radiation, we plan to utilize intense waveform controlled few-cycle radiation at 2 μm (the source is currently under development).

A) In 2006, it was demonstrated for the first time that waveform-controlled few-cycle laser pulses can be applied to steer electrons in molecular bonds and control a chemical reaction.^[1] In the first demonstration experiment, performed on the dissociative ionization of D₂ molecules, imaging techniques played a vital role in the visualization of the control. The direction of the emission of D⁺ ions was monitored as a function of the carrier-envelope phase (CEP) and thus the waveform of the few-cycle laser field as shown in the scheme. After creation of a coherent superposition of electronic states in D₂⁺, the binding electron in D₂⁺ localizes and is driven back and forth between the two nuclei. When the molecular bond reaches a critical distance at which tunneling of the electron from one nucleus to the other is suppressed, the electron “freezes” out on one nucleus leaving a positively charged ion (D⁺) flying in one and a neutral atom (D) in the opposite direction. Changing the CEP by π mirrors the electric field evolution of the laser pulse and reverses the direction of the fragments. We want to investigate how the control of electronic motion in molecules can be exploited to heteronuclear, heavier and more complex systems. We will closely collaborate with C.2.6, investigating these questions on a theoretical level.

B) When a molecule is exposed to and ionized by an intense, few-cycle light pulse, electrons are driven strongly by the field and re-scatter from their parent ion. The recombination of electrons with their parent ion leads to the generation of attosecond XUV bursts in the process known as high-harmonic generation (HHG). The XUV radiation and the scattered electrons carry information about the molecular structure.^[2,3] Very recent theoretical work by Morishita et al.,^[4] gives promises that it will be possible to obtain the first structural images of molecules with attosecond precision. Their results on the isomerization reaction of acetylene to vinylidene show that the electron scattering patterns obtained at around 10 Up contain clear structural information.^[4] In collaboration with the Kansas State University (Kansas, USA) we will investigate structural imaging by laser induced electron diffraction. We aim at advancing the technique, such that chemical transformations can be studied.