Structural and multi-dimensional spectroscopy of atoms and molecules on ultrafast time scales using CEP-controlled few-cycle light fields and (continuum-continuum) high-harmonic generation

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Small quantum systems such as atoms and molecules are characterized by both their spatial and energy-level structure. It is an old and ongoing endeavor in physics, chemistry, and molecular biology to measure the shape of atoms and molecules and to extract the characteristic energies of all quantum states. In particular, the temporal/dynamical evolution of spatial structure (motion) and the associated energy-level modifications (transitions) is at the heart of chemical reactions. As electrons are the core ingredient of chemical bonding, these dynamics occur on very fast time scales of few femtoseconds down into the sub-femtosecond regime.Here, we propose an experimental approach to access both structural and energy-resolved information on shortest time scales by employing the nonlinear-optical process of high-harmonic generation (HHG) of laser light, including a novel HHG mechanism, as a probe. According to own prior theory work, the traditional high-harmonic generation process can be modified to reveal time-resolved information about the spatial electrostatic-potential structure created by an atomic or molecular ion that was field ionized by a few-cycle laser pulse. We here propose to experimentally demonstrate and further develop this method by observing these so-called continuum-continuum harmonics.On the other hand, performing (even conventional continuum-bound) high-harmonic generation with carrier-envelope phase (CEP)-stable few-cycle pulses, and by making use of the macroscopic spatial degree of freedom for analysis and control, a novel type of multi-dimensional spectroscopy can be created. In traditional two-dimensional spectroscopy techniques, properties such as the population and, more importantly, the coherence of electronic quantum-state evolution can be temporally resolved, leading, for instance, to the recent discovery of (quantum-coherent) wavelike energy transport in photosynthesis. Here, we propose to promote these techniques to the few and sub-femtosecond regime in order to target the primary few- and multi-electron dynamics during light-matter interaction in atoms and molecules.The experimental combination of these approaches to dynamical structural as well as quantum-state observation, technically achieved by a four-quadrant time-delay stage in tandem with a broad-bandwidth high-harmonic soft-x-ray spectrometer, will create a transformative technology to extract physical mechanisms of electron dynamics in atoms and molecules and to test and push forward our current understanding of multi-electron processes and strong-field laser science.