Real-time determination of enantiomeric and isomeric content using photoelectron elliptical dichroism

Conformer-Selective Photoelectron Circular Dichroism

Conformational flexibility and chirality both play a key role in molecular recognition. It is therefore very useful to develop spectroscopic methods that simultaneously probe both properties. It has been theoretically predicted that photoelectron circular dichroism (PECD) should be very sensitive to conformational isomerism. However, experimental proof has been less forthcoming and only exists for a very few favorable cases. Here, we present a new PECD scheme based on resonance-enhanced two-photon ionization (RE2PI) using UV/Vis nanosecond laser excitations. The spectral resolution obtained thereby guarantees conformer-selectivity by inducing resonant conformer-specific ππ* S1←S0 transitions. We apply this experimental scheme to the study of chiral 1-indanol, which exists in two conformers linked by a ring inversion and defined by the position of the hydroxyl group, namely axial and equatorial. We show that the PECD of the equatorial and axial forms considerably differ in sign, magnitude and shape. We also discuss the influence of the total ionization energy, vibronic excitation of intermediate and final states, and relative polarization of the excitation and ionization lasers. Conformer-specificity adds a new dimension to the applications of PECD in analytical chemistry addressing now the general case of floppy systems.

Determination of Enantiomeric Excess by Optofluidic Microlaser near Exceptional Point

Enantiomeric excess (ee) is an essential indicator of chiral drug purification in the pharmaceutical industry. However, to date the ee determination of unknown concentration enantiomers generally involves two separate techniques for chirality and concentration measurement. Here, a whispering-gallery mode (WGM) based optofluidic microlaser near exceptional point to achieve the ee determination under unknown concentration with a single technique is proposed. Exceptional point induces the unidirectional WGM lasing, providing the optofluidic microlaser with the novel capability to measure chirality by polarization, in addition to wavelength-based concentration detection. The dual-parameters detection of optofluidic microlaser empowers it to achieve ee determination of various unknown enantiomers without additional concentration measurements, a feat that is challenging to accomplish with other methods. Featuring the sensitivity enhancement and miniature structure of the WGM sensors, the obtained chiroptical response of the present approach is ≈30-fold higher than that of the conventional optical rotation-based polarimeter, and the reagent consumption is reduced by three orders of magnitude.

The importance of molecular axis alignment and symmetry-breaking in photoelectron elliptical dichroism

Photoelectron angular distributions (PADs) produced from the photoionization of chiral molecules using elliptically polarized light exhibit a forward/backward asymmetry with respect to the optical propagation direction. By recording these distributions using the velocity-map imaging (VMI) technique, the resulting photoelectron elliptical dichroism (PEELD) has previously been demonstrated as a promising spectroscopic tool for studying chiral molecules in the gas phase. The use of elliptically polarized laser pulses, however, produces PADs (and consequently, PEELD distributions) that do not exhibit cylindrical symmetry about the propagation axis. This leads to significant limitations and challenges when employing conventional VMI acquisition and data processing strategies. Using novel photoelectron image analysis methods based around Hankel transform reconstruction tomography and machine learning, however, we have quantified-for the first time-significant symmetry-breaking contributions to PEELD signals that are of a comparable magnitude to the symmetric terms in the multiphoton ionization of (1R,4R)-(+)- and (1S,4S)-(-)-camphor. This contradicts any assumptions that symmetry-breaking can be ignored when reconstructing VMI data. Furthermore, these same symmetry-breaking terms are expected to appear in any experiment where circular and linear laser fields are used together. This ionization scheme is particularly relevant for investigating dynamics in chiral molecules, but it is not limited to them. Developing a full understanding of these terms and the role they play in the photoionization of chiral molecules is of clear importance if the potential of PEELD and related effects for future practical applications is to be fully realized.

Analysis of the volatile monoterpene composition of citrus essential oils by photoelectron spectroscopy employing continuously monitored dynamic headspace sampling

A new photoelectron spectroscopic method permitting a quantitative analysis of the volatile headspace of several essential oils is presented and discussed. In particular, we focus on the monoterpene compounds, which are known to be the dominant volatile components in many such oils. The photoelectron spectra of the monoterpene constituents may be effectively isolated by accepting for analysis only those electrons that accompany the production of m/z = 136 ions, and by using low photon energies that restrict cation fragmentation. The monoterpene isomers are then identified and quantified by regression modelling using a library of terpene standard spectra. An advantage of this approach is that pre-concentration of the volatile vapour is not required, and all steps are performed at ambient temperature, avoiding the possible deleterious effects (such as isomerisation/decomposition) that may sometimes arise in gas chromatographic (GC) procedures. As a proof-of-principle demonstration, three citrus oils (lemon, lime, bergamot) are analysed with this approach and the results are compared with reported GC composition profiles obtained for these oils. Potential advantages of the methodology that include multiplex detection and real-time, in situ analysis are identified and discussed. Alternative and faster experimental implementations concerning laboratory-based ionization and detection schemes are proposed and considered, as is the possibility of a straightforward extension towards simultaneous determination of enantiomeric excesses.

Induced photoelectron circular dichroism onto an achiral chromophore

An achiral chromophore can acquire a chiral spectroscopic signature when interacting with a chiral environment. This so-called induced chirality is documented in electronic or vibrational circular dichroism, which arises from the coupling between electric and magnetic transition dipoles. Here, we demonstrate that a chiroptical response is also induced within the electric dipole approximation by observing the asymmetric scattering of a photoelectron ejected from an achiral chromophore in interaction with a chiral host. In a phenol-methyloxirane complex, removing an electron from an achiral aromatic π orbital localised on the phenol moiety results in an intense and opposite photoelectron circular dichroism (PECD) for the two enantiomeric complexes with (R) and (S) methyloxirane, evidencing the long-range effect (~5 Å) of the scattering chiral potential. This induced chirality has important structural and analytical implications, discussed here in the context of growing interest in laser-based PECD, for in situ, real time enantiomer determination.

Ultrafast control over chiral sum-frequency generation

We introduce an ultrafast all-optical approach for efficient chiral recognition that relies on the interference between two low-order nonlinear processes that are ubiquitous in nonlinear optics: sum-frequency generation and third-harmonic generation. In contrast to traditional sum-frequency generation, our approach encodes the medium's handedness in the intensity of the emitted harmonic signal, rather than in its phase, and it enables full control over the enantiosensitive response. We show how, by sculpting the sub-optical-cycle oscillations of the driving laser field, we can force one molecular enantiomer to emit bright light while its mirror twin remains dark, thus reaching the ultimate efficiency limit of chiral sensitivity via low-order nonlinear light-matter interactions. Our work paves the way for ultrafast and highly efficient imaging and control of the chiral electronic clouds of chiral molecules using lasers with moderate intensities, in all states of matter: from gases to liquids to solids, with molecular specificity and on ultrafast time scales.

Velocity-map imaging of photoelectron circular dichroism in non-volatile molecules using a laser-based desorption source

We present an initial demonstration of a velocity-map imaging (VMI) experiment using a back-irradiation laser-based desorption source directly integrated into the electrode assembly. This has the potential to greatly expand the utility of the popular VMI approach by permitting its use with high density plumes of non-volatile molecular samples. Photoelectron circular dichroism measurements on the phenylalanine molecule using 400 nm multiphoton ionization are used to illustrate this novel method, revealing forward-backward emission asymmetries on the order of 7%.

Carrier envelope phase sensitivity of photoelectron circular dichroism

We report on a combined experimental and numerical study of photoelectron circular dichroism (PECD) induced by intense few-cycle laser pulses, using methyloxirane as the molecular example. Our experiments reveal a remarkably pronounced sensitivity of the PECD strength of double-ionization on the carrier-envelope phase (CEP) of the laser pulses. By comparison to the simulations, which reproduce the measured CEP-dependence for specific orientations of the molecules in the lab frame, we attribute the origin of the observed CEP-dependence of PECD to the CEP-induced modulation of ionization from different areas of the wave functions of three dominant orbitals.

Tomographic reconstruction techniques optimized for velocity-map imaging applications

Examples of extracting meaningful information from image projection data using tomographic reconstruction techniques can be found in many areas of science. Within the photochemical dynamics community, tomography allows for complete three-dimensional (3D) charged particle momentum distributions to be reconstructed following a photodissociation or photoionization event. This permits highly differential velocity- and angle-resolved measurements to be made simultaneously. However, the generalized tomographic reconstruction strategies typically adopted for use with photochemical imaging-based around the Fourier-slice theorem and filtered back-projection algorithms-are not optimized for these specific types of problems. Here, we discuss pre-existing alternative strategies-namely, the simultaneous iterative reconstruction technique and Hankel Transform Reconstruction (HTR)-and introduce them in the context of velocity-map imaging applications. We demonstrate the clear advantages they afford, and how they can perform considerably better than approaches commonly adopted at present. Most notably, with HTR we can set a bound on the minimum number of projections required to reliably reconstruct 3D photoproduct distributions. This bound is significantly lower than what is currently accepted and will help make tomographic imaging far more accessible and efficient for many experimentalists working in the field of photochemical dynamics.

Femtosecond photoelectron circular dichroism of chemical reactions

Understanding the chirality of molecular reaction pathways is essential for a broad range of fundamental and applied sciences. However, the current ability to probe chirality on the time scale of primary processes underlying chemical reactions remains very limited. Here, we demonstrate time-resolved photoelectron circular dichroism (TRPECD) with ultrashort circularly polarized vacuum-ultraviolet (VUV) pulses from a tabletop source. We demonstrate the capabilities of VUV-TRPECD by resolving the chirality changes in time during the photodissociation of atomic iodine from two chiral molecules. We identify several general key features of TRPECD, which include the ability to probe dynamical chirality along the complete photochemical reaction path, the sensitivity to the local chirality of the evolving scattering potential, and the influence of electron scattering off dissociating photofragments. Our results are interpreted by comparison with high-level ab-initio calculations of transient PECDs from molecular photoionization calculations. Our experimental and theoretical techniques define a general approach to femtochirality.

Influence of the emission site on the photoelectron circular dichroism in trifluoromethyloxirane

We report a joint experimental and theoretical study of the differential photoelectron circular dichroism (PECD) in inner-shell photoionization of uniaxially oriented trifluoromethyloxirane. By adjusting the photon energy of the circularly polarized synchrotron radiation, we address 1s-photoionization of the oxygen, different carbon, and all fluorine atoms. The photon energies were chosen such that in all cases electrons with a similar kinetic energy of about 11 eV are emitted. Employing coincident detection of electrons and fragment ions, we concentrate on identical molecular fragmentation channels for all of the electron-emitter scenarios. Thereby, we systematically examine the influence of the emission site of the photoelectron wave on the differential PECD. We observe large differences in the PECD signals. The present experimental results are supported by corresponding relaxed-core Hartree-Fock calculations.

Absolute optical chiral analysis using cavity-enhanced polarimetry

Chiral analysis is central for scientific advancement in the fields of chemistry, biology, and medicine. It is also indispensable in the development and quality control of chiral compounds in the chemical and pharmaceutical industries. Here, we present the concept of absolute optical chiral analysis, as enabled by cavity-enhanced polarimetry, which allows for accurate unambiguous enantiomeric characterization and enantiomeric excess determination of chiral compounds within complex mixtures at trace levels, without the need for calibration, even in the gas phase. Our approach and technology enable the absolute postchromatographic chiral analysis of complex gaseous mixtures, the rapid quality control of complex mixtures containing chiral volatile compounds, and the online in situ observation of chiral volatile emissions from a plant under stress.

New opportunities for ultrafast and highly enantio-sensitive imaging of chiral nuclear dynamics enabled by synthetic chiral light

Synthetic chiral light [D. Ayuso et al., Nat. Photon., 2019, 13, 866-871] has opened up new opportunities for ultrafast and highly efficient imaging and control of chiral matter. Here we show that the giant enantio-sensitivity enabled by such light could be exploited to probe chiral nuclear rearrangements during chemical reactions in a highly enantio-sensitive manner. Using a state-of-the-art implementation of real-time time-dependent density functional theory, we explore how the nonlinear response of the prototypical chiral molecule H₂O₂ changes as a function of its dihedral angle, which defines its handedness. The macroscopic intensity emitted from randomly oriented molecules at even harmonic frequencies (of the fundamental) depends strongly on this nuclear coordinate. Because of the ultrafast nature of such nonlinear interactions, the direct mapping between the dissymmetry factor and the nuclear geometry provides a way to probe chiral nuclear dynamics at their natural time scales. Our work paves the way for ultrafast and highly efficient imaging of enantio-sensitive dynamics in more complex chiral systems, including biologically relevant molecules.

Photoelectron Circular Dichroism as a Signature of Subtle Conformational Changes: The Case of Ring Inversion in 1-Indanol

Chirality plays a fundamental role in the molecular recognition processes. Molecular flexibility is also crucial in molecular recognition, allowing the interacting molecules to adjust their structures and hence optimize the interaction. Methods probing simultaneously chirality and molecular conformation are therefore crucially needed. Taking advantage of a possible control in the gas phase of the conformational distribution between the equatorial and axial conformers resulting from a ring inversion in jet-cooled 1-indanol, we demonstrate here the sensitivity of valence-shell photoelectron circular dichroism (PECD) to both chirality and subtle conformational changes, in a case where the photoelectron spectra of the two conformers are identical. For the highest occupied orbital, we observe a dramatic inversion of the PECD-induced photoelectron asymmetries, while the photoionization cross-section and usual anisotropy (β) parameter are completely insensitive to conformational isomerism. Such a sensitivity is a major asset for the ongoing developments of PECD-based techniques as a sensitive chiral (bio)chemical analytical tool in the gas phase.

Elliptic dichroism in strong-field ionization of atoms subjected to tailored laser fields

The differential ionization rate for strong-field ionization by tailored laser fields of atomic systems averaged over the magnetic quantum number satisfies particular inversion and reflection symmetries. The symmetries of the elliptic-dichroism parameter, which is related to the change of sign of the ellipticity of the laser field, are considered in detail, with particular emphasis on high-order above-threshold ionization. The general results are illustrated by the examples of an elliptically polarized laser field and a bi-elliptical orthogonally polarized two-color (BEOTC) field. For the BEOTC field the differential ionization rate and the elliptic-dichroism parameter are investigated for the ω-2ω and ω-3ω field combinations and for various relative phases between the laser-field components. The inversion and reflection symmetries of the photoelectron momentum distribution in the polarization plane of the field depend on the parities of r and s in the rω--sω BEOTC field combination and on the relative phase between the field components. We suggest that, by analyzing the symmetry properties of the measured momentum distribution of the elliptic-dichroism parameter, one can identify the mechanism of strong-field ionization. If the rescattering mechanism is dominant one can use these distributions to obtain information about the atomic and molecular structure and dynamics.

Photoelectron elliptical dichroism spectroscopy of resonance-enhanced multiphoton ionization via the 3s, 3p and 3d Rydberg series in fenchone

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 photoelectron elliptical dichroism in the (2 + 1)-photon ionization of fenchone molecules, using wavelength tunable femtosecond UV pulses. We show that the photoelectron elliptical asymmetry is extremely sensitive to the intermediate resonant states involved in the ionization process, and enables electronic couplings to be revealed that do not show up so clearly when using circularly polarized light.

Assignment-free chirality detection in unknown samples via microwave three-wave mixing

Straightforward identification of chiral molecules in multi-component mixtures of unknown composition is extremely challenging. Current spectrometric and chromatographic methods cannot unambiguously identify components while the state of the art spectroscopic methods are limited by the difficult and time-consuming task of spectral assignment. Here, we introduce a highly sensitive generalized version of microwave three-wave mixing that uses broad-spectrum fields to detect chiral molecules in enantiomeric excess without any prior chemical knowledge of the sample. This method does not require spectral assignment as a necessary step to extract information out of a spectrum. We demonstrate our method by recording three-wave mixing spectra of multi-component samples that provide direct evidence of enantiomeric excess. Our method opens up new capabilities in ultrasensitive phase-coherent spectroscopic detection that can be applied for chiral detection in real-life mixtures, raw products of chemical reactions and difficult to assign novel exotic species.

Arbitrary image reinflation: A deep learning technique for recovering 3D photoproduct distributions from a single 2D projection

Many charged particle imaging measurements rely on the inverse Abel transform (or related methods) to reconstruct three-dimensional (3D) photoproduct distributions from a single two-dimensional (2D) projection image. This technique allows for both energy- and angle-resolved information to be recorded in a relatively inexpensive experimental setup, and its use is now widespread within the field of photochemical dynamics. There are restrictions, however, as cylindrical symmetry constraints on the overall form of the distribution mean that it can only be used with a limited range of laser polarization geometries. The more general problem of reconstructing arbitrary 3D distributions from a single 2D projection remains open. Here, we demonstrate how artificial neural networks can be used as a replacement for the inverse Abel transform and-more importantly-how they can be used to directly "reinflate" 2D projections into their original 3D distributions, even in cases where no cylindrical symmetry is present. This is subject to the simulation of appropriate training data based on known analytical expressions describing the general functional form of the overall anisotropy. Using both simulated and real experimental data, we show how our arbitrary image reinflation (AIR) neural network can be utilized for a range of different examples, potentially offering a simple and flexible alternative to more expensive and complicated 3D imaging techniques.

Detecting chirality in mixtures using nanosecond photoelectron circular dichroism

We report chirality detection of structural isomers in a gas phase mixture using nanosecond photoelectron circular dichroism (PECD). Combining pulsed molecular beams with high-resolution resonance enhanced multi-photon ionization (REMPI) allows specific isolated transitions belonging to distinct components in the mixture to be targeted.

Coincident measurement of photo-ion circular dichroism and photo-electron circular dichroism on 1-Phenylethylamine

Here we report the coincident measurement of the PICD and PECD effect in 1-Phenylethylamine upon multiphoton ionisation. Photo-ion circular dichroism (PICD) and photo-electron circular dichroism (PECD) are both methods to...

Trends in angle-resolved molecular photoelectron spectroscopy

In this perspective article, main trends of angle-resolved molecular photoelectron spectroscopy in the laboratory up to the molecular frame, in different regimes of light-matter interactions, are highlighted with emphasis on foundations and most recent applications.

Influence of Vibrational Excitation and Nuclear Dynamics in Multiphoton Photoelectron Circular Dichroism of Fenchone

We report photoelectron circular dichroism of S-(+)-fenchone enantiomers recorded with state-state vibrational level resolution using picosecond laser (2 + 1) resonance enhanced multiphoton ionization via 3s and 3p Rydberg intermediate states. The 3p state decays to the 3s state on a picosecond time scale so that, above the 3p Rydberg excitation threshold, ionization of vibrationally hot 3s states competes with direct 3p^-1 ionization. Complex vibronic dynamics of the 3p → 3s internal conversion weaken the Rydberg Δv = 0 propensity rule in both the 3p^-1 and 3s^-1 ionization channels. Large variations of the forward-backward chiral asymmetry factors are observed between the Δv = 0 and Δv > 0 vibrational transitions, including dramatic swings from up to ±17%. Such changes of sign indicate complete reversal of the preferred direction for photoelectron emission in the laboratory frame, associated with vibrational motion. These asymmetry switches easily exceed the amplitude and frequency of such vibrationally induced flips previously observed in single photon ionization.

Ultrafast relaxation investigated by photoelectron circular dichroism: an isomeric comparison of camphor and fenchone

We study the isomeric effects using time resolved photoelectron circular dichroism (TR-PECD). Using a (1 + 1') pump-probe ionisation scheme with photoelectrons collected by the velocity map imaging technique, we compare the relaxation dynamics from the 3s-Rydberg state in 1R,4R-(+)-camphor with the one in its chiral isomer, 1R,4S-(-)-fenchone [Comby et al., 2016, JPCL, 7, 4514]. Our measurements revealed a similar lifetime for both isomers. However, the circular dichroism in the photoelectron angular distribution decays exponentially in ∼730 fs from a +9% forward amplitude during the first hundreds of femtoseconds to reach an asymptotic -2% backward amplitude. This time-scale is drastically shorter than in fenchone. Our analysis allows us to evaluate the impact of the anisotropy of excitation; the relaxation dynamics, following photoexcitation by the linearly polarized pump, is then compared to that induced by a circularly polarized pump pulse (CPL). With such a CPL pump, we then retrieve time constants of our chiral observables similar to the ones recorded in fenchone. Quantum and classical simulations are developed and used to decipher the dependence of the PECD on the anisotropy of excitation and the spatial distribution of the 3s-Rydberg electron wavefunction. Our experimental investigations, supported by our simulations, suggest that varying the pump ellipticity enables us to reveal the breakdown of the Franck-Condon approximation.

Ultrafast circularly polarized pulses tunable from the vacuum to deep ultraviolet

We experimentally demonstrate the efficient generation of circularly polarized pulses tunable from the vacuum to deep ultraviolet (160-380 nm) through resonant dispersive wave emission from optical solitons in a gas-filled hollow capillary fiber. In the deep ultraviolet, we measure up to 13 µJ of pulse energy, and from numerical simulations, we estimate the shortest output pulse duration to be 8.5 fs. We also experimentally verify that simply scaling the pulse energy by 3/2 between linearly and circularly polarized pumping closely reproduces the soliton and dispersive wave dynamics. Based on previous results with linearly polarized self-compression and resonant dispersive wave emission, we expect our technique to be extended to produce circularly polarized few-fs pulses further into the vacuum ultraviolet, and few to sub-fs circularly polarized pulses in the near infrared.

Photoelectron circular dichroism of O 1s-photoelectrons of uniaxially oriented trifluoromethyloxirane: energy dependence and sensitivity to molecular configuration

The photoelectron circular dichroism (PECD) of the O 1s-photoelectrons of trifluoromethyloxirane (TFMOx) is studied experimentally and theoretically for different photoelectron kinetic energies. The experiments were performed employing circularly polarized synchrotron radiation and coincident electron and fragment ion detection using cold target recoil ion momentum spectroscopy. The corresponding calculations were performed by means of the single center method within the relaxed-core Hartree-Fock approximation. We concentrate on the energy dependence of the differential PECD of uniaxially oriented TFMOx molecules, which is accessible through the employed coincident detection. We also compare the results for the differential PECD of TFMOx to those obtained for the equivalent fragmentation channel and similar photoelectron kinetic energy of methyloxirane (MOx), studied in our previous work. Thereby, we investigate the influence of the substitution of the methyl group by the trifluoromethyl group at the chiral center on the molecular chiral response. Finally, the presently obtained angular distribution parameters are compared to those available in the literature.

Multi-photon above threshold ionization of multi-electron atoms and molecules using the R-matrix approach

We formulate a computationally efficient time-independent method based on the multi-electron molecular R-matrix formalism. This method is used to calculate transition matrix elements for the multi-photon ionization of atoms and molecules under the influence of a perturbative field. The method relies on the partitioning of space which allows us to calculate the infinite-range free-free dipole integrals analytically in the outer region, beyond the range of the initial bound wave function. This approach is valid for an arbitrary order, that is, any number of photons absorbed both in the bound and the continuum part of the spectrum (below- and above-threshold ionization). We calculate generalized multi-photon cross sections and angular distributions of different systems (H, He, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {H}_{{2}}$$\end{document}H2, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {CO}_{{2}}$$\end{document}CO2) and validate our approach by comparison with data from the literature.

Conformer-dependent vacuum ultraviolet photodynamics and chiral asymmetries in pure enantiomers of gas phase proline

Proline is a unique amino-acid, with a secondary amine fixed within a pyrrolidine ring providing specific structural properties to proline-rich biopolymers. Gas-phase proline possesses four main H-bond stabilized conformers differing by the ring puckering and carboxylic acid orientation. The latter defines two classes of conformation, whose large ionization energy difference allows a unique conformer-class tagging via electron spectroscopy. Photoelectron circular dichroism (PECD) is an intense chiroptical effect sensitive to molecular structures, hence theorized to be highly conformation-dependent. Here, we present experimental evidence of an intense and striking conformer-specific PECD, measured in the vacuum ultraviolet (VUV) photoionization of proline, as well as a conformer-dependent cation fragmentation behavior. This finding, combined with theoretical modeling, allows a refinement of the conformational landscape and energetic ordering, that proves inaccessible to current molecular electronic structure calculations. Additionally, astrochemical implications regarding a possible link of PECD to the origin of life's homochirality are considered in terms of plausible temperature constraints.

Self-referencing circular dichroism ion yield measurements for improved statistics using femtosecond laser pulses

The combination of circular dichroism with laser mass spectrometry via the measurement of ion yields is a powerful tool in chiral recognition, but the measured anisotropies are generally weak. The method presented in this contribution reduces the measurement error significantly. A common path optical setup generates a pair of counter-rotating laser foci in the interaction region of a time-of-flight spectrometer. As the space focus condition is fulfilled for both foci individually, this becomes a twin-peak ion source with well separated and sufficiently resolved mass peaks. The individual control of polarization allows for in situ correction of experimental fluctuations measuring circular dichroism. Our robust optical setup produces reliable and reproducible results and is applicable for dispersion sensitive femtosecond laser pulses. In this contribution, we use 3-methyl-cyclopentanone as a prototype molecule to illustrate the evaluation procedure and the measurement principle.

Strong Differential Photoion Circular Dichroism in Strong-Field Ionization of Chiral Molecules

We investigate the differential ionization probability of chiral molecules in the strong-field regime as a function of the helicity of the incident light. To this end, we analyze the fourfold ionization of bromochlorofluoromethane (CHBrClF) with subsequent fragmentation into four charged fragments and different dissociation channels of the singly ionized methyloxirane. By resolving for the molecular orientation, we show that the photoion circular dichroism signal strength is increased by 2 orders of magnitude.

Detecting handedness of spatially oriented molecules by Coulomb explosion imaging

We present a new technique for detecting chirality in the gas phase: Chiral molecules are spatially aligned in three dimensions by a moderately strong elliptically polarized laser field. The momentum distributions of the charged fragments, produced by laser-induced Coulomb explosion, show distinct three-dimensional orientation of the enantiomers when the laser polarization ellipse is rotated by a non-right angle with respect to the norm vector of the detector plane. The resulting velocity-map-image asymmetry is directly connected to the enantiomeric excess and to the absolute handedness of molecules. We demonstrated our scheme computationally for camphor (C₁₀H₁₆O), with its methyl groups as marker fragments, using quantum-mechanical simulations geared toward experimentally feasible conditions. Computed sensitivity to enantiomeric excess is comparable to other modern chiroptical approaches. The present method can be readily optimized for any chiral molecule with an anisotropic polarizability tensor by adjusting the polarization state and intensity profile of the laser field.

Closed-loop recycling of rare liquid samples for gas-phase experiments

Many samples of current interest in molecular physics and physical chemistry exist in the liquid phase and are vaporized for use in gas cells, diffuse gas targets, or molecular gas jets. For some of these techniques, the large sample consumption is a limiting factor. When rare, expensive molecules such as custom-made chiral molecules or species with isotopic labels are used, wasting them in the exhaust line of the pumps is quite an expensive and inefficient approach. Therefore, we developed a closed-loop recycling system for molecules with vapor pressures below atmospheric pressure. Once filled, only a few valves have to be adjusted, and a cold trap must be moved after each phase of recycling. The recycling efficiency per turn exceeds 95%.

Full-Stokes Polarization Perfect Absorption with Diatomic Metasurfaces

Metamaterial-based perfect absorbers provide efficient ways for selective absorption of light with both linear or circular polarizations. Perfect absorption for an arbitrary polarization requires the development of subwavelength structures absorbing efficiently elliptically polarized light, but they remain largely unexplored. Here, we design and realize experimentally novel plasmonic metasurfaces for full-Stokes polarization perfect absorption in the mid-infrared. The metasurface unit cell consists of coupled anisotropic meta-atoms forming a diatomic metamolecule. The designed plasmonic metastructures provide a strong field enhancement by at least 1 order of magnitude higher than conventional perfect absorbers. In experiment, our plasmonic metasurfaces demonstrate sharp differentiation of spectral responses for an arbitrary pair of orthogonal polarization states (linear, circular, or elliptical) providing perfect absorption for one polarization with strong reflection for its counterpart. Our results suggest a novel route for efficient control of light polarization in metasurfaces offering numerous potential applications ranging from thermal imaging to chiral molecule detection.

Artificial Neural Networks for Noise Removal in Data-Sparse Charged Particle Imaging Experiments

We present the first demonstration of artificial neural networks (ANNs) for the removal of Poissonian noise in charged particle imaging measurements with very low overall counts. The approach is successfully applied to both simulated and real experimental image data relating to the detection of photoions/photoelectrons in unimolecular photochemical dynamics studies. Specific examples consider the multiphoton ionization of pyrrole and (S)-camphor. Our results reveal an extremely high level of performance, with the ANNs transforming images that are unusable for any form of quantitative analysis into statistically reliable data with an impressive similarity to benchmark references. Given the widespread use of charged particle imaging methods within the chemical dynamics community, we anticipate that the use of ANNs has significant potential impact - particularly, for example, when working in the limit of very low absorption/photoionization cross-sections, or when attempting to reliably extract subtle image features originating from phenomena such as photofragment vector correlations or photoelectron circular dichroism.

Visible light photocatalysis in the late-stage functionalization of pharmaceutically relevant compounds

The late stage functionalization (LSF) of complex biorelevant compounds is a powerful tool to speed up the identification of structure-activity relationships (SARs) and to optimize ADME profiles. To this end, visible-light photocatalysis offers unique opportunities to achieve smooth and clean functionalization of drugs by unlocking site-specific reactivities under generally mild reaction conditions. This review offers a critical assessment of current literature, pointing out the recent developments in the field while emphasizing the expected future progress and potential applications. Along with paragraphs discussing the visible-light photocatalytic synthetic protocols so far available for LSF of drugs and drug candidates, useful and readily accessible synoptic tables of such transformations, divided by functional groups, will be provided, thus enabling a useful, fast, and easy reference to them.

Rapid chiral analysis based on liquid-phase cyclic chemiluminescence

Rapid chiral analysis has become one of the important aspects of academic and industrial research. Here we describe a new strategy based on liquid-phase cyclic chemiluminescence (CCL) for rapid resolution of enantiomers and determination of enantiomeric excess (ee). A single CCL measurement can acquire multistage signals that provide a unique way to examine the intermolecular interactions between chiral hosts and chiral guests, because the lifetime (τ) of the multistage signals is a concentration-independent and distinguishable constant for a given chiral host-guest system. According to the τ values, CCL allows discrimination between a wide range of enantiomeric pairs including chiral alcohols, amines and acids by using only one chiral host. Even the chiral systems hardly distinguished by nuclear magnetic resonance and fluorescence methods can be distinguished easily by CCL. Additionally, the τ value of a mixture of two enantiomers is equal to the weighted average of each enantiomer, which can be used for the direct determination of ee without the need to separate the chiral mixture and create calibration curves. This is extremely crucial for the cases without readily available enantiomerically pure samples. This strategy was successfully applied to monitoring of the Walden inversion reaction and analysis of chiral drugs. The results were in good agreement with those obtained by high-performance liquid chromatography, indicating the utility of CCL for routine quick ee analysis. Mechanism study revealed that the τ value is possibly related to the activity of the chiral substance to catalyze a luminol-H2O2 reaction. Our research provides an unprecedented and general protocol for chirality differentiation and ee determination, which is anticipated to be a useful technology that will find wide application in chirality-related fields, particularly in asymmetric synthesis and the pharmaceutical industry.

Using photoelectron elliptical dichroism (PEELD) to determine real-time variation of enantiomeric excess

In this work, the photoionization of chiral molecules by an elliptically polarized, high repetition rate, femtosecond laser is probed. The resulting 3D photoelectron angular distribution shows a strong forward-backward asymmetry, which is highly dependent not only on the molecular structure but also on the ellipticity of the laser pulse. By continuously varying the laser ellipticity, we can observe molecular and enantiomer changes in real time at a previously unseen speed and precision. The technique allows enantiomeric excess of a pure compound to be measured with a 5% precision within 3 s, and a 10-min acquisition yields a precision of 0.4%. The isomers camphor and fenchone can be easily distinguished, unlike with conventional mass spectrometry. Preliminary results for the pharmaceutically interesting ibuprofen are also given, showing the capability of photoionization as a means of distinguishing larger molecular systems.

High-resolution resonance-enhanced multiphoton photoelectron circular dichroism

Photoelectron circular dichroism (PECD) is a highly sensitive enantiospecific spectroscopy for studying chiral molecules in the gas phase using either single-photon ionization or multiphoton ionization. In the short pulse limit investigated with femtosecond lasers, resonance-enhanced multiphoton ionization (REMPI) is rather instantaneous and typically occurs simultaneously via more than one vibrational or electronic intermediate state due to limited frequency resolution. In contrast, vibrational resolution in the REMPI spectrum can be achieved using nanosecond lasers. In this work, we follow the high-resolution approach using a tunable narrow-band nanosecond laser to measure REMPI-PECD through distinct vibrational levels in the intermediate 3s and 3p Rydberg states of fenchone. We observe the PECD to be essentially independent of the vibrational level. This behaviour of the chiral sensitivity may pave the way for enantiomer specific molecular identification in multi-component mixtures: one can specifically excite a sharp, vibrationally resolved transition of a distinct molecule to distinguish different chiral species in mixtures.

Photoelectron circular dichroism in the multiphoton ionization by short laser pulses. III. Photoionization of fenchone in different regimes

Photoelectron circular dichroism (PECD) in different regimes of multiphoton ionization of fenchone is studied theoretically using the time-dependent single center method. In particular, we investigate the chiral response to the one-color multiphoton or strong-field ionization by circularly polarized 400 nm and 814 nm optical laser pulses or 1850 nm infrared pulse. In addition, the broadband ionization by short coherent circularly polarized 413-1240 nm spanning pulse is considered. Finally, the two-color ionization by the phase-locked 400 nm and 800 nm pulses, which are linearly polarized in mutually orthogonal directions, is investigated. The present computational results on the one-color multiphoton ionization of fenchone are in agreement with the available experimental data. For the ionization of fenchone by broadband and bichromatic pulses, the present theoretical study predicts substantial multiphoton PECDs.

Coincident measurement of photo-ion circular dichroism and photo-electron circular dichroism

Photo-ion circular dichroism (PICD) and photo-electron circular dichroism (PECD) have been measured for the first time simultaneously in a coincidence experiment detecting the chirality of R- and S-Methyloxirane.

Observation of Photoelectron Circular Dichroism Using a Nanosecond Laser

Photoelectron circular dichroism (PECD) is a fascinating phenomenon both from a fundamental science aspect but also due to its emerging role as a highly sensitive analytic tool for chiral recognition in the gas phase. PECD has been studied with single-photon as well as multi-photon ionization. The latter has been investigated in the short pulse limit with femtosecond laser pulses, where ionization can be thought of as an instantaneous process. In this contribution, we demonstrate that multi-photon PECD still can be observed when using an ultra-violet nanosecond pulse to ionize chiral showcase fenchone molecules. Compared to femtosecond ionization, the magnitude of PECD is similar, but the lifetime of intermediate molecular states imprints itself in the photoelectron spectra. Being able to use an industrial nanosecond laser to investigate PECD furthermore reduces the technical requirements to apply PECD in analytical chemistry.

Ultrafast electronic relaxations from the S3 state of pyrene

The ultrafast relaxation occurring in pyrene upon excitation at 4.68 eV was studied in a supersonic gas-jet fs pump–probe experiment.