Session G04: Attosecond Transient Absorption and Four-Wave Mixing Spectroscopies

Live

Isolated attosecond pulses in the extreme ultraviolet combined with near infrared pulses from a Ti:sapphire laser are used to initiate and measure electronic and vibrational superpositions and decaying states in atoms and small molecules. Especially, the background-free method of attosecond four-wave mixing allows a new level of time-dynamic analysis, and multidimensional methods with near infrared pulse shaping can be used to isolate individual states. Attosecond pulses create massive superpositions of electronic states in atoms as well as vibrationally selectable states in molecules, and few-femtosecond decay dynamics are obtained due to autoionization or competing predissociation. Alignment-dependent core-hole decays in molecules reveal the role of nonlocal interactions on decay processes. Detailed outcomes are compared to theory.   

Live

Novel free electron laser X-Ray sources open powerful and unique measurement windows into ultrafast molecular dynamics. Using a realistic and fully ab-initio molecular Hamiltonian, we demonstrate the direct probing of coherences at conical intersections using X-Ray stimulated Raman signals. The signal is visible only at times where the wavepacket is in vicinity of the conical intersection. The wavepacket coherences survive for several hundred femtoseconds. Fundamental features such as wavepacket locality, conical intersection energy splitting and coherence duration in uracil can be directly accessed from the recorded signal. Our data set allows us to explore a vast parameter space affecting the signal: X-Ray probe wavelengths spanning several hundred electron volts, optimal probe bandwidths, and different molecular orientations and isotropic contributions. This gives a solid background for potential experimental realization. Resonant signals that probe the dynamics in the vicinity of a selected atom are presented and optimized using coherent control schemes.   

Live

Light-induced couplings between electronic states can be used to control the optical properties of media. In this context, strong-field transient absorption spectroscopy forms an excellent tool to characterize these couplings, and their effect on electron dynamics, leading to new insights into the light-induced states, autoionization dynamics, light-induced transparency, nonlinear emissions, population transfer etc. We perform such measurements by employing controlled, tunable wavelength infrared pulses to tailor the electronic couplings and study their effect on transient absorption lineshapes. We demonstrate control over Autler-Townes splitting of a bound state, and its evolution to a light-induced structure. We show that non-commensurate extreme-ultraviolet and infrared fields allow background free study of nonlinear four-wave-mixing emissions, which exhibit non-trivial spectral and temporal features. Four-wave-mixing with tunable pulses allows us to quantify the couplings between autoionizing states, study dark state dynamics, and probe the light-induced counterparts of Fano resonances. Systematic studies of light-induced effects help to refine the theoretical models and open up new applications of attosecond transient absorption spectroscopy. This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Award DE-SC0018251.   

Live

Attosecond spectroscopy has the potential to address fundamental questions in molecular sciences. A promising approach is offered by the element- and site-sensitivity of X-ray spectroscopy. We have recently demonstrated the potential of table-top X-ray absorption spectroscopy with a water-window high-harmonic source, observing the temporal evolution of unoccupied molecular orbitals and molecular shape resonances during chemical reactions [1]. Compressing the mid-infrared driving pulses to less than 2 optical cycles, we have demonstrated the extension of this light source to fully cover the oxygen K-edge [2]. Using the same technique, we have also demonstrated the generation of isolated attosecond pulses, which have established a new record of the shortest light pulses ever measured (43 attoseconds) [3]. Since the vast majority of chemical processes takes place in the liquid phase, the extension of attosecond spectroscopy to liquids is desirable. I will discuss the first observation of extreme-ultraviolet high-harmomic generation from liquids, achieved through the application of ultrathin (0.6-2 $\mu $m) flat microjets [4]. I will also present the extension of attosecond time-resolved spectroscopy from molecules [5] to liquids [6]. The time delays between photoemission from gaseous and liquid water range from 50-70 attoseconds and are shown to mainly originate from the solvation of water molecules, with liquid-phase electron scattering playing a minor role. These developments set the stage for attosecond time-resolved studies of molecular systems of chemical complexity. \textless br/\textgreater [1] Y. Pertot et al., \textit{Science }\textbf{355}, 264 (2017) [2] C. Schmidt et al., \textit{Opt. Exp. }\textbf{26}, 11834 (2018) [3] T. Gaumnitz et al., \textit{Opt. Exp }\textbf{25}, 27506 (2017) [4] T. T. Luu et al., \textit{Nature Communications }\textbf{9,} 3723 (2018) [5] M. Huppert et al., \textit{Phys. Rev. Lett.} \textbf{117}, 093001 (2016) [6] I. Jordan et al., \textit{submitted }(2019)