The fast and accurate analysis of chiral chemical mixtures is crucial for many applications but remains challenging. Here we use elliptically-polarized femtosecond laser pulses at high repetition rates to photoionize chiral molecules. The 3D photoelectron angular distribution produced provides molecular fingerprints, showing a strong forward-backward asymmetry which depends sensitively on the molecular structure and degree of ellipticity. Continuously scanning the laser ellipticity and analyzing the evolution of the rich, multi-dimensional molecular signatures allows us to observe real-time changes in the chemical and chiral content present with unprecedented speed and accuracy. We measure the enantiomeric excess of a compound with an accuracy of 0.4% in 10 min acquisition time, and follow the evolution of a mixture with an accuracy of 5% with a temporal resolution of 3 s. This method is even able to distinguish isomers, which cannot be easily distinguished by mass-spectrometry.

Controlling the polarization state of electromagnetic radiation enables the investigation of fundamental symmetry properties of matter through chiroptical processes. Over the past decades, many strategies have been developed to reveal structural or dynamical information about chiral molecules with high sensitivity, from the microwave to the extreme ultraviolet range. Most schemes employ circularly or elliptically polarized radiation, and more sophisticated configurations involve, for instance, light pulses with time-varying polarization states. All these schemes share a common propertythe polarization state of light is always considered as constant over one optical cycle. In this study, we zoom into the optical cycle in order to resolve and control a subcyle attosecond chiroptical process. We engineer an electric field whose instantaneous chirality can be controlled within the optical cycle, by combining two phase-locked orthogonally polarized fundamental and second harmonic fields. While the composite field has zero net ellipticity, it shows an instantaneous optical chirality which can be controlled via the two-color delay. We theoretically and experimentally investigate the photoionization of chiral molecules with this controlled chiral field. We find that electrons are preferentially ejected forward or backward relative to the laser propagation direction depending on the molecular handedness, similarly to the well-established photoelectron circular dichroism process. However, since the instantaneous chirality switches sign from one half cycle to the next, electrons ionized from two consecutive half cycles of the laser show opposite forward/backward asymmetries. This chiral signal, termed here as ESCARGOT (Enantiosensitive Sub-Cycle Antisymmetric Response Gated by electric-field rOTation), provides a unique insight into the influence of instantaneous chirality in the dynamical photoionization process. More generally, our results demonstrate the important role of sub-cycle polarization shaping of electric fields, as a new route to study and manipulate chiroptical processes.

Nonlinear multidimensional spectroscopy is ubiquitous in the optical and radio frequency regions as a powerful tool to access structure and dynamics. The extension of this technique into the extreme ultraviolet (XUV) region with attosecond pulses holds promise for probing electronic dynamics and correlations with unprecedented time and spatial resolution. In this work, we use noncollinear four-wave mixing of a weak XUV attosecond pulse train (11-17 eV) and few-femtosecond NIR pulses (800 nm) to spectroscopically and dynamically probe the dipole-forbidden double-well potential of the a'' 1∑+g electronic state of nitrogen. The results demonstrate optical coupling of the inner and outer limits of the initial XUV-prepared vibrational wave packet in the valence character b' 1∑+u state to the inner and outer wells, respectively, of the a'' 1∑+g double well state by 800 nm light. Two four-wave mixing schemes with different pulse timing sequences and noncollinear beam geometries are used (one NIR pulse collinear and one NIR pulse noncollinear versus both NIR pulses noncollinear to the XUV beam) to measure the a'' dark state energetic structure and to control the dynamical preparation and motion of a dark state wave packet by selective population of either the inner Rydberg or outer valence-character potential well. Experimental measurements of the a'' 1∑+g outer well vibrational spacing and anharmonicity closely match the values theoretically predicted for this previously unobserved state.

Nonlinear spectroscopies are utilized extensively for selective measurements of chemical dynamics in the optical, infrared, and radio-frequency regimes. The development of these techniques for extreme ultraviolet (XUV) light sources facilitates measurements of electronic dynamics on attosecond timescales. Here, we elucidate the temporal dynamics of nonlinear signal generation by utilizing a transient grating scheme with a subfemtosecond XUV pulse train and two few-cycle near-infrared pulses in atomic helium. Simultaneous detection of multiple diffraction orders reveals delays of ≥1.5 fs in higher-order XUV signal generation, which are reproduced theoretically by solving the coupled Maxwell–Schrödinger equations and with a phase grating model. The delays result in measurable order-dependent differences in the energies of transient light induced states. As nonlinear methods are extended into the attosecond regime, the observed higher-order signal generation delays will significantly impact and aid temporal and spectral measurements of dynamic processes.

The autoionization dynamics of the (2 P1/2)ns/d Rydberg states in krypton are investigated using spatially-isolated wave-mixing signals generated with a short train of subfemtosecond XUV pulses and noncollinear, few-cycle near infrared (NIR) pulses. Despite ubiquitous quantum beat oscillations from the XUV-induced coherences within the excited-state manifold, these wavemixing spectra allow for the simultaneous evaluation of autoionization lifetimes from a series of Rydberg states above the first ionization potential. Experimentally measured lifetimes of 22 ± 8 fs, 33 ± 6 fs, and 49 ± 6 fs for the wave-mixing signals emitting from the (2 P1/2)6d/8s, (2 P1/2)7d/9s, and (2 P1/2)8d/10s resonances compare favorably with lifetimes for the (2 P1/2)6d, 7d, and 8d Rydberg states determined from spectral linewidths. Analysis of the quantum beats reveals that the enhancement of wave-mixing pathways that couple the (2 P1/2)nd states to themselves leads to individual reporter state-dependent decays in the wave-mixing signals. The results demonstrate the promise of wave-mixing spectroscopies with subfemtosecond XUV pulses to provide valuable insights into processes governed by electronic dynamics. I.

We present a novel time-and phase-resolved, background-free scheme to study the extreme ultraviolet dipole emission of a bound electronic wavepacket, without the use of any extreme ultraviolet exciting pulse. Using multiphoton transitions, we populate a superposition of quantum states which coherently emit extreme ultraviolet radiation through free induction decay. This emission is probed and controlled, both in amplitude and phase, by a time-delayed infrared femtosecond pulse. We directly measure the laser-induced dephasing of the emission by using a simple heterodyne detection scheme based on two-source interferometry. This technique provides rich information about the interplay between the laser field and the Coulombic potential on the excited electron dynamics. Its background-free nature enables us to use a large range of gas pressures and to reveal the influence of collisions in the relaxation process.Transient Absorption Spectroscopy (TAS) in the extreme ultraviolet (XUV) range is a powerful technique for ultrafast dynamical studies, from the gas phase [1][2][3] to the solid state [4,5]. The recent progress of attosecond science has made the extension of TAS to the attosecond regime (ATAS) a reality. In the past few years, different configurations of ATAS experiments have emerged. In the first and most intuitive scheme, the XUV attosecond pulses probe a sample that has been pre-excited by an infrared (IR) pump laser pulse. This scheme was for instance used to follow the ultrafast coherent hole dynamics initiated in strong-field ionized krypton [1]. In the second arrangement, the XUV and IR pulses are temporally overlapped. The XUV absorption thus probes the IR-dressed atomic or molecular states, allowing the observation of light-induced states [6,7] as well as sub-cycle AC-Stark-shifts [8]. Finally, in what turned out to be the most widely used scheme, the XUV pulse comes first and serves as a pump, exciting a broadband superposition of quantum states through single-photon absorption. The wavepacket relaxes by coherently emitting XUV radiation (called XUV Free Induction Decay, XFID) that interferes with the incident light. A delayed IR laser field is used to follow or control the relaxation dynamics, such that these experiments can be seen as Transient Reshaping of the Absorption spectrum of the XUV light (TRAX) [9,10].In TRAX experiments, the delayed IR laser pulse has three main effects on the XFID emission ( Fig. 1) : (i) Damping of the emission by ionization of the excited states. This effect induces a lowering and a spectral broadening of the absorption features [2]. (ii) Field coupling of different electronic states, leading to population transfers. The resulting amplitude reshaping of the electronic wavepacket can be detected through temporal beatings in the absorption signal, as demonstrated in atoms [9,10] and molecules [11,12]. (iii) Phase shift induced by the Stark-shift of the excited states. This laser-induced phase was shown to enable full control over the absorption lineshapes, from Lorentz ...

The resonance-enhanced multiphoton ionization of chiral molecules by elliptically polarized laser pulses produces photoelectron angular distributions that are forward/backward asymmetric with respect to the light propagation axis. We investigate this...

After decades of supremacy of the Titanium:Sapphire technology, Ytterbium-based high-order harmonic sources are emerging as a promising alternative for experiments requiring high flux of ultrashort extreme ultraviolet (XUV) radiation. In this article we describe a versatile experimental setup delivering XUV photons in the 10–50 eV range. The use of cascaded high-order harmonic generation enables us to reach 1.9 mW of average power at 18 eV. Several spectral selection schemes are presented, to isolate a single high-harmonic or a group of them. In the perspective of circular dichroism experiments, we produce highly elliptical XUV radiation using resonant elliptical high-harmonic generation, and circularly polarized XUV by bichromatic bicircular high-harmonic generation. As an illustration of the capacities of the beamline, we focus the XUV beam in a coincidence electron–ion imaging spectrometer, where we measure the photoelectron momentum angular distributions of xenon monomers and dimers.