Attosecond electron wave packet interferometry

The typical time scale of electronic motion in atoms and molecules is a few hundred attoseconds. Tremendous progress has been made over the last decade to experimentally access this time-scale thanks to dramatic breakthroughs in laser technology that made it possible to measure and partly also to control the dynamics of electrons on sub-laser-cycle time-scales, thereby defining the vibrant research field of Attosecond Physics. Sufficient temporal resolution, however, is not the only demand for exploring the motion of valence electrons in atoms and molecules. Equally important is the ability to obtain insight into the bound electronic structure. While approaches that allow the reconstruction of changes of Angstrom-scale structures on sub-femtosecond time-scales have been developed, a complete picture of electronic rearrangement will only emerge, when methods are developed that can measure also the phase of the complex quantum mechanical valence electronic distribution rather than only intensity distributions. A well-known method that can measure the phase of complex quantities is interferometry, routinely applied in optics. The applicant, together with co-workers, has recently introduced in a proof of principle experiment a method based on interferometry with pairs of electron wave packets coherently created on sub-laser-cycle time- scales by emission from atoms or molecules in an intense laser field, which is capable of measuring quantum phase-sensitive structural information with a sub-10-attosecond precision. The aim of this project is to establish this type of electron wave packet interferometry as a metrology for phase- sensitive restructuring dynamics of electrons in the attosecond time domain and to extend its applicability from atoms to diatomic and even polyatomic molecules. The project will apply sub-cycle sculpted near- and mid- infrared laser pulses to create and steer the interfering electron wave packets, and coincidence momentum spectroscopy to measure the emerging three-dimensional interferograms. The extension of the metrology to molecules will benefit enormously from the application of a unique laser source in the mid-infrared (6m) wavelength regime. The long cycle-duration of the laser electric field will allow for tracing valence electron motion within an extended time window from about 100 attoseconds up to several femtoseconds with sub-100-attosecond temporal resolution. By extending attosecond electron wave packet interferometry to molecules, the project will allow the investigation of several important topics in the interaction of intense laser pulses with atoms and molecules, such as the sub- femtosecond dynamics of (transiently) populating excited electronic states during an ionization event, the multi- electron character of the valence electron dynamics triggered by field ionization and/or excitation, or the attosecond restructuring dynamics of valence electron distributions in polyatomic molecules as they undergo a bond-breaking process. This project, thus, has the potential to significantly enhance our understanding of the electronic origin of intra-molecular chemical bond-breaking and restructuring processes on a pre-chemical time-scale.

In this project, we carried out theoretical and experimental studies on the demonstration and the application of attosecond electron wave packet interferometry with waveform-controlled ultrashort laser fields. Using a reaction microscope, we recorded interference patterns of photoelectrons released by strong laser fields through tunneling ionization. These interference patterns contain the temporal and structure information of the released electron and its parent ion. In the frame of the project, we achieved the following results: We demonstrated a two-dimensional electron wave packet interferometer by using a two- color laser field. The most important advantage of this interferometer is its capability of disentangle electron wave packet interferences on different time scales of from attosecond to a few femtosecond in measured three-dimensional photoelectron momentum distributions. The method, in general, can be applied to more complex quantum systems and it is compatible with pump-probe techniques for time-resolved studies. We applied the attosecond electron wave packet interferometry to hydrogen molecules and achieved channel-resolved measurements of electron wave packet interferences which connect to different types of laser-induced molecular dissociation. We introduced molecular alignment as a tool to control the laser-induced molecular ionization and dissociation of hydrocarbon molecules. The method exploits the removing of electrons from inner and outer valence orbitals for controlling a desired molecular dissociation. We further performed dissociation channel-resolved measurements of electron wave packet interferences, which contain the information of electron dynamics during the laser-molecular interaction. We implemented an electron wave packet interferometer in a pump-probe scheme to investigate the electron and nuclear dynamics during molecular ionization and dissociation. Our results indicate that not only the nuclear dynamics on the femtosecond time scale but also the electron dynamics on the attosecond time scale can be resolved in the measured interference signals.