Capturing electron-driven chiral dynamics in UV-excited molecules.

Chiral molecules, used in applications such as enantioselective photocatalysis1, circularly polarized light detection2 and emission3 and molecular switches4,5, exist in two geometrical configurations that are non-superimposable mirror images of each other. These so-called (R) and (S) enantiomers exhibit different physical and chemical properties when interacting with other chiral entities. Attosecond technology might enable influence over such interactions, given that it can probe and even direct electron motion within molecules on the intrinsic electronic timescale6 and thereby control reactivity7-9. Electron currents in photoexcited chiral molecules have indeed been predicted to enable enantiosensitive molecular orientation10, but electron-driven chiral dynamics in neutral molecules have not yet been demonstrated owing to the lack of ultrashort, non-ionizing and perturbative light pulses. Here we use time-resolved photoelectron circular dichroism (TR-PECD)11-15 with an unprecedented temporal resolution of 2.9 fs to map the coherent electronic motion initiated by ultraviolet (UV) excitation of neutral chiral molecules. We find that electronic beatings between Rydberg states lead to periodic modulations of the chiroptical response on the few-femtosecond timescale, showing a sign inversion in less than 10 fs. Calculations validate this and also confirm that the combination of the photoinduced chiral current with a circularly polarized probe pulse realizes an enantioselective filter of molecular orientations following photoionization. We anticipate that our approach will enable further investigations of ultrafast electron dynamics in chiral systems and reveal a route towards enantiosensitive charge-directed reactivity.

Far-Field Petahertz Sampling of Plasmonic Fields.

The response of metal nanostructures to optical excitation leads to localized surface plasmon (LSP) generation with nanoscale field confinement driving applications in, for example, quantum optics and nanophotonics. Field sampling in the terahertz domain has had a tremendous impact on the ability to trace such collective excitations. Here, we extend such capabilities and introduce direct sampling of LSPs in a more relevant petahertz domain. The method allows to measure the LSP field in arbitrary nanostructures with subcycle precision. We demonstrate the technique for colloidal nanoparticles and compare the results to finite-difference time-domain calculations, which show that the build-up and dephasing of the plasmonic excitation can be resolved. Furthermore, we observe a reshaping of the spectral phase of the few-cycle pulse, and we demonstrate ad-hoc pulse shaping by tailoring the plasmonic sample. The methodology can be extended to single nanosystems and applied in exploring subcycle, attosecond phenomena.

Dispersion-engineered multi-pass cell for single-stage post-compression of an ytterbium laser.

Post-compression methods for ultrafast laser pulses typically face challenging limitations, including saturation effects and temporal pulse breakup, when large compression factors and broad bandwidths are targeted. To overcome these limitations, we exploit direct dispersion control in a gas-filled multi-pass cell, enabling, for the first time to the best of our knowledge, single-stage post-compression of 150 fs pulses and up to 250 µJ pulse energy from an ytterbium (Yb) fiber laser down to sub-20 fs. Dispersion-engineered dielectric cavity mirrors are used to achieve nonlinear spectral broadening dominated by self-phase modulation over large compression factors and bandwidths at 98% throughput. Our method opens a route toward single-stage post-compression of Yb lasers into the few-cycle regime.

Generation of deep ultraviolet sub-2-fs pulses.

We demonstrate the generation of few-cycle deep ultraviolet pulses via frequency upconversion of 5-fs near-infrared pulses in argon using a laser-fabricated gas cell. The measured spectrum extends from 210 to 340 nm, corresponding to a transform-limited pulse duration of 1.45 fs. We extract from a dispersion-free second-order cross-correlation measurement a pulse duration of 1.9 fs, defining a new record in the deep ultraviolet spectral range.

Determination of accurate electron chiral asymmetries in fenchone and camphor in the VUV range: sensitivity to isomerism and enantiomeric purity.

Photoelectron circular dichroism (PECD) manifests itself as an intense forward/backward asymmetry in the angular distribution of photoelectrons produced from randomly-oriented enantiomers by photoionization with circularly-polarized light (CPL). As a sensitive probe of both photoionization dynamics and of the chiral molecular potential, PECD attracts much interest especially with the recent performance of related experiments with visible and VUV laser sources. Here we report, by use of quasi-perfect CPL VUV synchrotron radiation and using a double imaging photoelectron/photoion coincidence (i(2)PEPICO) spectrometer, new and very accurate values of the corresponding asymmetries on showcase chiral isomers: camphor and fenchone. These data have additionally been normalized to the absolute enantiopurity of the sample as measured by a chromatographic technique. They can therefore be used as benchmarking data for new PECD experiments, as well as for theoretical models. In particular we found, especially for the outermost orbital of both molecules, a good agreement with CMS-Xα PECD modeling over the whole VUV range. We also report a spectacular sensitivity of PECD to isomerism for slow electrons, showing large and opposite asymmetries when comparing R-camphor to R-fenchone (respectively -10% and +16% around 10 eV). In the course of this study, we could also assess the analytical potential of PECD. Indeed, the accuracy of the data we provide are such that limited departure from perfect enantiopurity in the sample we purchased could be detected and estimated in excellent agreement with the analysis performed in parallel via a chromatographic technique, establishing a new standard of accuracy, in the ±1% range, for enantiomeric excess measurement via PECD. The i(2)PEPICO technique allows correlating PECD measurements to specific parent ion masses, which would allow its application to analysis of complex mixtures.

Catalytic enantioselective α-tosyloxylation of ketones using iodoaryloxazoline catalysts: insights on the stereoinduction process.

A family of iodooxazoline catalysts was developed to promote the iodine(III)-mediated α-tosyloxylation of ketone derivatives. The α-tosyloxy ketones produced are polyvalent chiral synthons. Through this study, we have unearthed a unique mode of stereoinduction from the chiral oxazoline moiety, where the stereogenic center alpha to the oxazoline oxygen atom is significant. Computational chemistry was used to rationalize the stereoinduction process. The catalysts presented promote currently among the best levels of activity and selectivity for this transformation. Evaluation of the scope of the reaction is presented.

Real-time observation of a correlation-driven sub 3 fs charge migration in ionised adenine.

Sudden ionisation of a relatively large molecule can initiate a correlation-driven process dubbed charge migration, where the electron density distribution is expected to rapidly move along the molecular backbone. Capturing this few-femtosecond or attosecond charge redistribution would represent the real-time observation of electron correlation in a molecule with the enticing prospect of following the energy flow from a single excited electron to the other coupled electrons in the system. Here, we report a time-resolved study of the correlation-driven charge migration process occurring in the nucleic-acid base adenine after ionisation with a 15-35 eV attosecond pulse. We find that the production of intact doubly charged adenine - via a shortly-delayed laser-induced second ionisation event - represents the signature of a charge inflation mechanism resulting from many-body excitation. This conclusion is supported by first-principles time-dependent simulations. These findings may contribute to the control of molecular reactivity at the electronic, few-femtosecond time scale.

Capturing roaming molecular fragments in real time.

Since the discovery of roaming as an alternative molecular dissociation pathway in formaldehyde (H2CO), it has been indirectly observed in numerous molecules. The phenomenon describes a frustrated dissociation with fragments roaming at relatively large interatomic distances rather than following conventional transition-state dissociation; incipient radicals from the parent molecule self-react to form molecular products. Roaming has been identified spectroscopically through static product channel-resolved measurements, but not in real-time observations of the roaming fragment itself. Using time-resolved Coulomb explosion imaging (CEI), we directly imaged individual "roamers" on ultrafast time scales in the prototypical formaldehyde dissociation reaction. Using high-level first-principles simulations of all critical experimental steps, distinctive roaming signatures were identified. These were rendered observable by extracting rare stochastic events out of an overwhelming background using the highly sensitive CEI method.

Tabletop imaging of structural evolutions in chemical reactions demonstrated for the acetylene cation.

The introduction of femto-chemistry has made it a primary goal to follow the nuclear and electronic evolution of a molecule in time and space as it undergoes a chemical reaction. Using Coulomb Explosion Imaging, we have shot the first high-resolution molecular movie of a to and fro isomerization process in the acetylene cation. So far, this kind of phenomenon could only be observed using vacuum ultraviolet light from a free-electron laser. Here we show that 266 nm ultrashort laser pulses are capable of initiating rich dynamics through multiphoton ionization. With our generally applicable tabletop approach that can be used for other small organic molecules, we have investigated two basic chemical reactions simultaneously: proton migration and C=C bond breaking, triggered by multiphoton ionization. The experimental results are in excellent agreement with the timescales and relaxation pathways predicted by new and quantitative ab initio trajectory simulations.