Valence photoionization dynamics in circular dichroism of chiral free molecules: the methyl-oxirane

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.

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.

Ultrafast chirality: the road to efficient chiral measurements

Today we are witnessing the electric-dipole revolution in chiral measurements. Here we reflect on its lessons and outcomes, such as the perspective on chiral measurements using the complementary principles of "chiral reagent" and "chiral observer", the hierarchy of scalar, vectorial and tensorial enantio-sensitive observables, the new properties of the chiro-optical response in the ultrafast and non-linear domains, and the geometrical magnetism associated with the chiral response in photoionization. The electric-dipole revolution is a landmark event. It has opened routes to extremely efficient enantio-discrimination with a family of new methods. These methods are governed by the same principles but work in vastly different regimes - from microwaves to optical light; they address all molecular degrees of freedom - electronic, vibrational and rotational, and use flexible detection schemes, i.e. detecting photons or electrons, making them applicable to different chiral phases, from gases to liquids to amorphous solids. The electric-dipole revolution has also enabled enantio-sensitive manipulation of chiral molecules with light. This manipulation includes exciting and controlling ultrafast helical currents in vibronic states of chiral molecules, enantio-sensitive control of populations in electronic, vibronic and rotational molecular states, and opens the way to efficient enantio-separation and enantio-sensitive trapping of chiral molecules. The word "perspective" has two meanings: an "outlook" and a "point of view". In this perspective article, we have tried to cover both meanings.

Anharmonic Aspects in Vibrational Circular Dichroism Spectra from 900 to 9000 cm-1 for Methyloxirane and Methylthiirane

Vibrational circular dichroism (VCD) spectra and the corresponding IR spectra of the chiral isomers of methyloxirane and of methylthiirane have been reinvestigated, both experimentally and theoretically, with particular attention to accounting for anharmonic corrections, as calculated by the GVPT2 approach. De novo recorded VCD spectra in the near IR (NIR) range regarding CH-stretching overtone transitions, together with the corresponding NIR absorption spectra, were also considered and accounted for, both with the GVPT2 and with the local mode approaches. Comparison of the two methods has permitted us to better describe the nature of active “anharmonic” modes in the two molecules and the role of mechanical and electrical anharmonicity in determining the intensities of VCD and IR/NIR data. Finally, two nonstandard IR/NIR regions have been investigated: the first one about ≈2000 cm^–1, involving mostly two-quanta bending mode transitions, the second one between 7000 and 7500 cm^–1 involving three-quanta transitions containing CH-stretching overtones and HCC/HCH bending modes.

Continuum Electronic States: The Tiresia Code

A multicenter (LCAO) B-spline basis is described in detail, and its capabilities concerning affording convergent solutions for electronic continuum states and wavepacket propagation are presented. It forms the core of the Tiresia code, which implements static-DFT and TDDFT hamiltonians, as well as single channel Dyson-DFT and Dyson-TDDFT descriptions to include correlation in the bound states. Together they afford accurate and computationally efficient descriptions of photoionization properties of complex systems, both in the single photon and strong field environments. A number of examples are provided.

Photoelectron Circular Dichroism as a Probe of Chiral Hydrocarbons

The sensitivity of Photoelectron Circular Dichroism (PECD) in the angular distribution of photoelectrons, a recent chiral technique, to detect chirality in pure hydrocarbons is investigated in a number of benchmark molecules. It is found that a very large chiral signal is expected, surpassing most current examples, giving a sure fingerprint of absolute configuration. On the other hand, the sensitivity to specific isomers or closely related molecules is relatively modest.

Multi-electron excitation contributions towards primary and satellite states in the photoelectron spectrum

The computation of Dyson orbitals and corresponding ionization energies has been implemented within the Equation of Motion Coupled Cluster Singles, Doubles and Perturbative Triples (EOM-CC3) method. Coupled to an accurate...

PECD study of a single conformer molecule: a critical comparison of experiment and theory

In this work we address a specific experimental and theoretical quest on the influence of a conformational population in the modeling of PhotoElectron Circular Dichroism spectroscopy. In the last two...

Valence-shell photoelectron circular dichroism of ruthenium(III)-tris-(acetylacetonato) gas-phase enantiomers

Chiral transition-metal complexes are of interest in many fields ranging from asymmetric catalysis and molecular materials science to optoelectronic applications or fundamental physics including parity violation effects. We present here a combined theoretical and experimental investigation of gas-phase valence-shell photoelectron circular dichroism (PECD) on the challenging open-shell ruthenium(III)-tris-(acetylacetonato) complex, Ru(acac)₃. Enantiomerically pure Δ- or Λ-Ru(acac)₃, characterized by electronic circular dichroism (ECD), were vaporized and adiabatically expanded to produce a supersonic beam and photoionized by circularly-polarized VUV light from the DESIRS beamline at Synchrotron SOLEIL. Photoelectron spectroscopy (PES) and PECD experiments were conducted using a double imaging electron/ion coincidence spectrometer, and compared to density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. The open-shell character of Ru(acac)₃, which is not taken into account in our DFT approach, is expected to give rise to a wide multiplet structure, which is not resolved in our PES signals but whose presence might be inferred from the additional striking features observed in the PECD curves. Nevertheless, the DFT-based assignment of the electronic bands leads to the characterisation of the ionized orbitals. In line with other recent works, the results confirm that PECD persists independently on the localization and/or on the achiral or chiral nature of the initial orbital, but is rather a probe of the molecular potential as a whole. Overall, the measured PECD signals on Ru(acac)₃, a system exhibiting D₃ propeller-type chirality, are of similar magnitude compared to those on asymmetric-carbon-based chiral organic molecules which constitute the vast majority of species investigated so far, thus suggesting that PECD is a universal mechanism, inherent to any type of chirality.

Enantio-sensitive unidirectional light bending

Structured light, which exhibits nontrivial intensity, phase, and polarization patterns in space, has key applications ranging from imaging and 3D micromanipulation to classical and quantum communication. However, to date, its application to molecular chirality has been limited by the weakness of magnetic interactions. Here we structure light's local handedness in space to introduce and realize an enantio-sensitive interferometer for efficient chiral recognition without magnetic interactions, which can be seen as an enantio-sensitive version of Young's double slit experiment. Upon interaction with isotropic chiral media, such chirality-structured light effectively creates chiral emitters of opposite handedness, located at different positions in space. We show that if the distribution of light's handedness breaks left-right symmetry, the interference of these chiral emitters leads to unidirectional bending of the emitted light, in opposite directions in media of opposite handedness, even if the number of the left-handed and right-handed emitters excited in the medium is exactly the same. Our work introduces the concepts of polarization of chirality and chirality-polarized light, exposes the immense potential of sculpting light's local chirality, and offers novel opportunities for efficient chiral discrimination, enantio-sensitive optical molecular fingerprinting and imaging on ultrafast time scales.

Accurate Description of Photoionization Dynamical Parameters

Calculation of dynamical parameters for photoionization requires an accurate description of the initial and final states of the system, as well as of the outgoing electron. We show that using a linear combination of atomic orbitals B-spline density functional theory (DFT) method to describe the outgoing electron, in combination with correlated equation of motion coupled cluster singles and double Dyson orbitals, gives good agreement with experiment and outperforms other simpler approaches, like plane and Coulomb waves, used to describe the photoelectron. Results are presented for cross-sections, angular distributions, and dichroic parameters in chiral molecules, as well as for photoionization from excited states. We also present a comparison with the results obtained using Hartree-Fock and DFT molecular orbitals selected according to Koopmans' theorem for the bound states.

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.

Angular Distribution of Ion Products in the Double Photoionization of Propylene Oxide

A photoelectron-photoion-photoion coincidence technique, using an ion imaging detector and tunable synchrotron radiation in the 18.0–37.0 eV photon energy range, inducing the ejection of molecular valence electrons, has been applied to study the double ionization of the propylene oxide, a simple prototype chiral molecule. The experiment performed at the Elettra Synchrotron Facility (Trieste, Italy) allowed to determine angular distributions for ions produced by the two-body dissociation reactions following the Coulomb explosion of the intermediate (C₃H₆O)²⁺ molecular dication. The analysis of the coincidence spectra recorded at different photon energies was done in order to determine the dependence of the β anisotropy parameter on the photon energy for the investigated two-body fragmentation channels. In particular, the reaction leading to CH3+ + C₂H₃O⁺ appears to be characterized by an increase of β, from β ≈ 0.00 up to β = 0.59, as the photon energy increases from 29.7 to 37.0 eV, respectively. This new observation confirms that the dissociation channel producing CH3+ and C₂H₃O⁺ final ions can occur with two different microscopic mechanisms as already indicated by the bimodality obtained in the kinetic energy released (KER) distributions as a function of the photon energy in a recent study. Energetic considerations suggest that experimental data are compatible with the formation of two different stable isomers of C₂H₃O⁺: acetyl and oxiranyl cations. These new experimental data are inherently relevant and are mandatory information for further experimental and theoretical investigations involving oriented chiral molecules and linearly or circularly polarized radiation. This work is in progress in our laboratory.

Recovery of High-Energy Photoelectron Circular Dichroism through Fano Interference

It is commonly accepted that the magnitude of a photoelectron circular dichroism (PECD) is governed by the ability of an outgoing photoelectron wave packet to probe the chiral asymmetry of a molecule. To be able to accumulate this characteristic asymmetry while escaping the chiral ion, photoelectrons need to have relatively small kinetic energies of up to a few tens of electron volts. Here, we demonstrate a substantial PECD for very fast photoelectrons above 500 eV kinetic energy released from methyloxirane by a participator resonant Auger decay of its lowermost O 1s excitation. This effect emerges as a result of the Fano interference between the direct and resonant photoionization pathways, notwithstanding that their individual effects are negligibly small. The resulting dichroic parameter has an anomalous dispersion: It changes its sign across the resonance, which can be considered as an analogue of the Cotton effect in the x-ray regime.

Intense Vibronic Modulation of the Chiral Photoelectron Angular Distribution Generated by Photoionization of Limonene Enantiomers with Circularly Polarized Synchrotron Radiation

Photoionization of the chiral monoterpene limonene has been investigated using polarized synchrotron radiation between the adiabatic ionization threshold, 8.505 and 23.5 eV. A rich vibrational structure is seen in the threshold photoelectron spectrum and is interpreted using a variety of computational methods. The corresponding photoelectron circular dichroism-measured in the photoelectron angular distribution as a forward-backward asymmetry with respect to the photon direction-was found to be strongly dependent on the vibronic structure appearing in the photoelectron spectra, with the observed asymmetry even switching direction in between the major vibrational peaks. This effect can be ultimately attributed to the sensitivity of this dichroism to small phase shifts between adjacent partial waves of the outgoing photoelectron. These observations have implications for potential applications of this chiroptical technique, where the enantioselective analysis of monoterpene components is of particular interest.

Double photoionization of propylene oxide: A coincidence study of the ejection of a pair of valence-shell electrons

Propylene oxide, a favorite target of experimental and theoretical studies of circular dichroism, was recently discovered in interstellar space, further amplifying the attention to its role in the current debate on protobiological homochirality. In the present work, a photoelectron-photoion-photoion coincidence technique, using an ion-imaging detector and tunable synchrotron radiation in the 18.0-37.0 eV energy range, permits us (i) to observe six double ionization fragmentation channels, their relative yields being accounted for about two-thirds by the couple (C₂H₄⁺, CH₂O⁺) and one-fifth by (C₂H₃⁺, CH₃O⁺); (ii) to measure thresholds for their openings as a function of photon energy; and (iii) to unravel a pronounced bimodality for a kinetic-energy-released distribution, fingerprint of competitive non-adiabatic mechanisms.

Conformational effects in photoelectron circular dichroism

Photoelectron circular dichroism (PECD) is a novel type of spectroscopy, which presents surprising sensitivity to conformational effects in chiral systems. While classical photoelectron spectroscopy mainly responds to conformational effects in terms of energy level shifts, PECD provides a rich and detailed response to tiny changes in electronic and structural properties by means of the intensity dispersion of the circular dichroism as a function of photoelectron kinetic energy. In this work, the basics of PECD will be outlined, emphasizing the role of interference from the [Formula: see text] outgoing partial wave of the photoelectron in the PECD transition matrix element, which is responsible for the extreme sensitivity to conformational effects. Examples using molecular systems and interfaces will shed light on the powerful application of PECD to classical conformational effects such as group substitution, isomerism, conformer population and clustering. Moreover, the PECD results will be reported in challenging new fields where conformations play a key role, such as vibrational effects, transient chirality and time- resolved experiments. To date, PECD has mostly been based on synchrotron radiation facilities, but it also has a future as a table-top lab experiment by means of multiphoton ionization. An important application of PECD as an analytical tool will be reported. The aim of this review is to illustrate that in PECD, the presence of conformational effects is essential for understanding a wide range of effects from a new perspective, making it different from classical spectroscopy.

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

Photoelectron circular dichroism is a chiroptical effect able to distinguish isomers and to determine accurately the enantiopurity of chiral compounds.

VUV photodynamics and chiral asymmetry in the photoionization of gas phase alanine enantiomers

The valence shell photoionization of the simplest proteinaceous chiral amino acid, alanine, is investigated over the vacuum ultraviolet region from its ionization threshold up to 18 eV. Tunable and variable polarization synchrotron radiation was coupled to a double imaging photoelectron/photoion coincidence (i(2)PEPICO) spectrometer to produce mass-selected threshold photoelectron spectra and derive the state-selected fragmentation channels. The photoelectron circular dichroism (PECD), an orbital-sensitive, conformer-dependent chiroptical effect, was also recorded at various photon energies and compared to continuum multiple scattering calculations. Two complementary vaporization methods-aerosol thermodesorption and a resistively heated sample oven coupled to an adiabatic expansion-were applied to promote pure enantiomers of alanine into the gas phase, yielding neutral alanine with different internal energy distributions. A comparison of the photoelectron spectroscopy, fragmentation, and dichroism measured for each of the vaporization methods was rationalized in terms of internal energy and conformer populations and supported by theoretical calculations. The analytical potential of the so-called PECD-PICO detection technique-where the electron spectroscopy and circular dichroism can be obtained as a function of mass and ion translational energy-is underlined and applied to characterize the origin of the various species found in the experimental mass spectra. Finally, the PECD findings are discussed within an astrochemical context, and possible implications regarding the origin of biomolecular asymmetry are identified.

FTIR spectra and density functional theory P.E.D. assignments of oxiranes in Ar matrix at 12 K

The FTIR spectra of a series of oxiranes were studied in Ar matrix at 12K. The interpretation of the spectra was accomplished on the basis of density functional theory calculations employing the 6-311++G(3df,3pd) basis set with the B3LYP functional. Potential energy distribution was carried out for each molecule employing the B3LYP/6-311++G(3df,3pd) force field and a non-redundant definition of internal coordinates. The study of the FTIR spectra led to the reassignment of some vibrational modes of the molecules. The FTIR spectrum of trifluoroepoxypropane measured in Ar matrix and its assignment is reported for the first time.

Vibrationally induced inversion of photoelectron forward-backward asymmetry in chiral molecule photoionization by circularly polarized light

Electron-nuclei coupling accompanying excitation and relaxation processes is a fascinating phenomenon in molecular dynamics. A striking and unexpected example of such coupling is presented here in the context of photoelectron circular dichroism measurements on randomly oriented, chiral methyloxirane molecules, unaffected by any continuum resonance. Here, we report that the forward-backward asymmetry in the electron angular distribution, with respect to the photon axis, which is associated with photoelectron circular dichroism can surprisingly reverse direction according to the ion vibrational mode excited. This vibrational dependence represents a clear breakdown of the usual Franck-Condon assumption, ascribed to the enhanced sensitivity of photoelectron circular dichroism (compared with other observables like cross-sections or the conventional anisotropy parameter-β) to the scattering phase off the chiral molecular potential, inducing a dependence on the nuclear geometry sampled in the photoionization process. Important consequences for the interpretation of such dichroism measurements within analytical contexts are discussed.

Resonant circular dichroism of chiral metal-organic complex

A sizable enhancement of the circular dichroism in photoelectron spectroscopy has been measured and computed for the metal complex Δ-cobalt(III) tris-acetylacetonate highest occupied molecular orbital state in the region of the Co 3p→3d Fano resonance. In the resonance the dichroism reaches the maximum value of 5% and even changes its sign as compared to the direct photoionization channel. We ascribe this enhancement to electron correlation processes, namely, with the coupling between discrete excitations and the continuum, which is correctly described in the time dependent density functional theory (TDDFT) framework. These findings open new physical aspects of photoelectron circular dichroism that now can be interpreted not only via the simple direct ionization, but also through more complex electron correlation processes.

The valence electronic structure and conformational flexibility of epichlorohydrin

The electronic structure of epichlorohydrin is investigated in the whole valence region by a combined experimental and theoretical study. The issue of controversial assignments of the molecular electronic structure is here addressed. Photoelectron spectra (PES) and Threshold Photoelectron spectra (TPES) of room temperature molecules in the gas phase are recorded. Geometries and energies of the stable conformers due to internal rotation of the C-C-C-Cl dihedral angle, gauche-II (g-II), gauche-I (g-I), and cis, are calculated, and the effect of the conformational flexibility on the photoionization energetics is studied by DFT and 2h-1p Configuration Interaction (CI) methods. Strong breakdown of the Koopmans Theorem (KT) is obtained for the four outermost ionizations, which are further investigated by higher level ab initio calculations. The full assignment of the spectrum is put on a firm basis by the combination of experimental and theoretical results. The orbital composition from correlated calculations is found closer to the DFT orbitals, which are then used to analyze the electronic structure of the molecule. The Highest Occupied Molecular Orbital (HOMO) and HOMO--2 are n(O)/n(Cl) mixed orbitals. The nature of each valence MO is generally preserved in all the conformers, although the magnitude of the n(O)/n(Cl) mixing in HOMO and HOMO--2 varies to some extent with the C-C-C-Cl dihedral angle. The low energy part of the HOMO PE band is predicted to be substantially affected by the conformational flexibility, as experimentally observed in the spectra. The rest of the spectrum is described in terms of the dominant conformer g-II, and a good agreement between experiment and theory is found. The inner-valence PE spectrum is characterized by satellite structures, due to electron correlation effects, which are interpreted by means of 2h-1p CI calculations.

Circular Dichroism in Ion Yields in Multiphoton Ionization of (R)-Propylene Oxide Employing Femtosecond Laser Pulses

The feasibility of measuring circular dichroism (CD) in ion yields is demonstrated in multiphoton ionization employing femtosecond laser radiation for the first time. CD values for (R)-propylene oxide are reported at 738 nm, 810 nm and 878 nm. The data suggest the possibility of resonance contributions at 810 nm. At the other wavelengths the CD appears to originate from non-resonant multiphoton ionization. Possible implications of measuring non-resonant CD are discussed.

Electrostatic hexapole state-selection of the asymmetric-top molecule propylene oxide

Rotational state-selection of the asymmetric-top molecule propylene oxide was carried out using an electrostatic hexapole field of 85-cm length. Molecular beam intensities were monitored by a quadrupole mass spectrometer. It was found that beam intensities of molecular beams for pure propylene oxide and those seeded in He and in Ar increased with increasing hexapole voltages. The hexapole voltage dependence of the beam intensity, which is called the focusing curve, was interpreted by computer simulation of the trajectories of molecules in the hexapolar field due to the Stark effect, as a function of rotational temperatures of molecular beams. The calculated best fit focusing curves, when compared with the experimental results, demonstrated that the rotational temperatures, associated with the distribution of states of a given rotational angular momentum J, are similar to the translational temperatures. It was found that the M = 0 states (where M is the projection of J along the direction of the electrostatic field) and negative values of the pseudoquantum number tau of propylene oxide can be selected using our experimental setup. These results suggest that the hexapole electric field is a tool even for the selection of rotational and orientation states of asymmetric-top molecules.