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Physical chemistry, chemical physics 24(14), 8081 - 8092 (2022). doi:10.1039/D1CP05748K special issue: "Festschrift Ivan Powis: Advances in Molecular Photoelectron Spectroscopy: Fundamentals & Application", We present an experimental X-ray photoelectron circular dichroism (PECD) study of liquid fenchone at the C 1s edge. A novel setup to enable PECD measurements on a liquid microjet [Malerz et al., Rev. Sci. Instrum., 2022, 93, 015101] was used. For the C 1s line assigned to fenchone's carbonyl carbon, a non-vanishing asymmetry is found in the intensity of photoelectron spectra acquired under a fixed angle in the backward-scattering plane. This experiment paves the way towards an innovative probe of the chirality of organic/biological molecules in aqueous solution., Published by RSC Publ., Cambridge

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.

Linearly polarized synchrotron radiation has been used to record polarization dependent valence shell photoelectron spectra of imidazole in the photon energy range 21–100 eV. These have allowed the photoelectron angular distributions, as characterized by the anisotropy parameter β, and the electronic state intensity branching ratios to be determined. Complementing these experimental data, theoretical photoionization partial cross sections and β-parameters have been calculated for the outer valence shell orbitals. The assignment of the structure appearing in the experimental photoelectron spectra has been guided by vertical ionization energies and spectral intensities calculated by various theoretical methods that incorporate electron correlation and orbital relaxation. Strong orbital relaxation effects have been found for the 15a′, nitrogen lone-pair orbital. The calculations also predict that configuration mixing leads to the formation of several low-lying satellite states. The vibrational structure associated with ionization out of a particular orbital has been simulated within the Franck–Condon model using harmonic vibrational modes. The adiabatic approximation appears to be valid for the X 2A″ state, with the β-parameter for this state being independent of the level of vibrational excitation. However, for all the other outer valence ionic states, a disparity occurs between the observed and the simulated vibrational structure, and the measured β-parameters are at variance with the behavior expected at the level of the Franck–Condon approximation. These inconsistencies suggest that the excited electronic states may be interacting vibronically such that the nuclear dynamics occur over coupled potential energy surfaces.

International audience; Polarization dependent photoelectron spectra encompassing the outer valence orbitals of oxalyl chloride have been recorded in the photon energy range 19–91 eV. These have allowed photoelectron anisotropy parameters and branching ratios to be determined. Photoionization partial cross sections and photoelectron anisotropy parameters have been calculated with the Continuum Multiple Scattering – Xα approach. Four of the outer valence orbitals are predicted to possess a significant Cl 3p lone-pair character and have closely grouped binding energies. The photoionization dynamics of these four orbitals are predicted to be strongly affected by the Cooper minimum associated with the Cl 3p orbital in the isolated atom at photon energies around 40 eV. A comparison between the theoretical and measured photoelectron anisotropy parameters has enabled the molecular orbital sequence to be clarified. A doublet has been observed in the region of the photoelectron spectrum where a band due to the 5bu orbital might be anticipated. Our calculations indicate that the 6bu and 5bu orbitals are coupled. This coupling may account for the apparent lack of a pronounced Cooper minimum in the β-parameter associated with the nominal 6bu ionization and for the unexpected appearance of the adjacent photoelectron band, nominally associated with the 5bu orbital. The vertical ionization energy of the outermost 7ag orbital was experimentally determined to be 11.266 ± 0.005 eV.

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, besides a conformer-dependant cation fragmentation behaviour, we present experimental evidence of an intense and striking conformer-specific PECD, measured in the VUV photoionization of proline. This finding, combined with theoretical modelling, 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.

A fresh perspective on the interaction of electron and nuclear motions in photon induced dynamical processes can be provided by the coupling of photoelectron angular distributions and cation vibrational states in the photoionization of chiral molecules using circularly polarized radiation. The chiral contributions, manifesting as a forward-backward asymmetry in the photoemission, can be assessed using Photoelectron Circular Dichroism (PECD), which has revealed an enhanced vibrational influence exerted on the outgoing photoelectron. In this paper, we investigate the PECD of a rigid chiral monoterpene, 3-carene, using single-photon vacuum ultraviolet ionization by polarized synchrotron radiation and selecting energies from the ionization threshold up to 19.0 eV. By judicious choice of these photon energies, two factors that influence PECD asymmetry values, electron kinetic energy and ion vibrational level, can be effectively isolated, allowing a clear demonstration of the very marked vibrational effects. A slow photoelectron spectrum is used to examine the vibrational structure of the isolated outermost valence (HOMO) photoelectron band, and peak assignments are made with the aid of a Franck-Condon simulation. Together, these provide an estimate of the adiabatic ionization energy as 8.385 eV. The reported chiral asymmetry from the randomly oriented 3-carene enantiomers reaches a maximum of over 21%. Theoretical PECD calculations, made both for the fixed equilibrium molecular geometry and also modeling selected normal mode vibration effects, are presented to provide further insight.

The VUV absorption spectrum of fenchone is re-examined using synchrotron radiation Fourier transform spectrometry, revealing new vibrational structure. Picosecond laser (2+1) resonance enhanced multiphoton ionization (REMPI) spectroscopy complements this, providing an alternative view of the 3spd Rydberg excitation region. These spectra display broadly similar appearance, with minor differences that are largely explained by referring to calculated one- and two-photon electronic excitation cross-sections. Both show good agreement with Franck-Condon simulations of the relevant vibrational structures. Parent ion REMPI ionization yields with both femtosecond and picosecond excitation laser pulses are studied as a function of laser polarization and intensity, the latter providing insight into the relative two-photon excitation and one-photon ionization rates. The experimental circular-linear dichroism observed in the parent ion yields varies strongly between the 3s and 3p Rydberg states, in good overall agreement with the calculated two-photon excitation circular-linear dichroism, while corroborating other evidence that the 3pz sub-state plays no more than a very minor role in the (2+1) REMPI spectrum. Vibrationally resolved photoelectron spectra are recorded with picosecond pulse duration (2+1) REMPI at selected intermediate vibrational excitations. The 3s intermediate state displays a very strong Δv=0 propensity on ionization, but the 3p intermediate evidences more complex vibronic dynamics, and we infer some 3p→3s internal conversion prior to ionization.

The photoionization of four chiral bicyclic monoterpene isomers, α-pinene, β-pinene, 3-carene and sabinene — all commonly found constituents in essential natural oils — has been studied using synchrotron radiation and compared to recent findings for the cyclic isomer limonene. Slow photoelectron spectra (SPES) are recorded between threshold and an energy of 10.5 eV. In the case of limonene, α-pinene, and 3-carene, vibrational structure is observed in the ground ionic state and attributed to a C C double bond stretching in the cation, using Franck-Condon vibrational band simulations. The photoelectron circular dichroism (PECD) is examined for specific enantiomers of these terpenes, and vibrational modification of the forward-backward photoelectron asymmetry detected by PECD can be tentatively identified, even when the corresponding SPES is unstructured. Large chiral asymmetry factors are found at low binding energies for the pinenes and 3-carene, with α-pinene in particular displaying a 37% forward-backward photoelectron asymmetry, believed to be a record chiroptical asymmetry for randomly-oriented, non-interacting molecules.

Photoelectron angular distributions (PADs), ranging up to the maximum 6th order Legendre polynomial term set by the Yang theorem, have been recorded for the (2 + 1) resonance enhanced multiphoton ionization (REMPI) of two terpene isomers, 3-carene and α-pinene, employing femtosecond lasers and electron velocity map imaging detection. PAD measurements made with coincident photoion detection allow ion fragmentation effects to be assessed. Using circular polarization and enantiomerically pure samples the PAD measurements are extended to include chiral (odd) Legendre polynomial terms, and these are analysed and discussed as multiphoton photoelectron circular dichroism (MP-PECD). Comparisons are also made with single photon (synchrotron radiation) PECD of these compounds. Although for a given compound a common final cation state is reached, pronounced differences are observed between PECD and MP-PECD, and between the alternative identified REMPI intermediate states in the case of MP-PECD.

Valence photoelectron spectra and photoelectron angular distributions of trans-dichloroethene have been measured with vibrational resolution at photon energies between 19 eV and 90 eV. Calculations of photoelectron anisotropy parameters, β, and harmonic vibrational modes help provide initial insight into the molecular structure. The photon energy range encompasses the expected position of the atomic Cl 3p Cooper minimum. A corresponding dip observed here in the anisotropy of certain photoelectron bands permits the identification and characterization of those molecular orbitals that retain a localized atomic Cl character. The adiabatic approximation holds for the X 2Au state photoelectron band, but vibronic coupling was inferred within the A–B–C and the D–E states by noting various failures of the Franck–Condon model, including vibrationally dependent β-parameters. This is further explored using the linear vibronic coupling model with interaction parameters obtained from ab initio calculations. The A/B photoelectron band is appreciably affected by vibronic coupling, owing to the low-lying conical intersection of the A 2Ag and B 2Bu states. The C 2Bg band is also affected, but to a lesser extent. The adiabatic minima of the D 2Au and E 2Ag states are almost degenerate, and the vibronic interaction between these states is considerable. The potential energy surface of the D 2Au state is predicted to have a double-minimum shape with respect to the au deformations of the molecular structure. The irregular vibrational structure of the resulting single photoelectron band reflects the non-adiabatic nuclear dynamics occurring on the two coupled potential energy surfaces above the energy of their conical intersection.

The journal of chemical physics 149(9), 094304 (2018). doi:10.1063/1.5045640, Auger electron spectra following excitation or ionization of the I 3d level in CH$_3$I have been recorded with horizontally or vertically plane polarized synchrotron radiation. These spectra have enabled the Auger electron angular distributions, as characterized by the β parameter, to be determined. The I 3d photoionization partial cross section of CH3I has been calculated with the continuum multiple scattering approach, and the results show that in the photon energy range over which Auger spectra were measured, the I 3d cross section exhibits an atomic-like behavior and is dominated by transitions into the $εf$ continuum channel. In this limit, the theoretical value of the alignment parameter (A$_{20}$) characterizing the core ionized state in an atom becomes constant, independent of photon energy. This theoretical value has been used to obtain the Auger electron intrinsic anisotropy parameters $(α_2)$ from the β parameters extracted from our normal (non-resonant) molecular Auger spectra. The resulting anisotropy parameters for the M45N45N45 transitions in CH3I have been compared to those calculated for the corresponding transitions in xenon, and the experimental and theoretical results are in good agreement. Anisotropy parameters have also been measured for the M$_{45}$N$_1$N$_{45}$, M$_{45}$N$_{23}$N$_{45}$, and M$_{45}$N$_{45}$O$_{23}$ transitions. For the M$_{45}$N$_1$N$_{45}$ and M$_{45}$N$_{23}$N$_{45}$ Auger decays in CH$_3$I, the experimentally derived angular distributions do not exhibit the strong dependence on the final ionic state that is predicted for these transitions in xenon. Resonantly excited Auger spectra have been recorded at 620.4 and 632.0 eV, coinciding with the $I 3d_{5/2} → σ*$ and $3d_{3/2} → σ*$ transitions, respectively. The resulting Auger electron angular distributions for the M$_4$N$_{45}$N$_{45}$ and M$_5$N$_{45}$N$_{45}$ decays were found to exhibit a higher anisotropy than those for the normal process. This is due to the larger photo-induced alignment in the neutral core excited state. For a particular Auger transition, the Auger electron kinetic energy measured in the resonantly excited spectrum is higher than that in the normal spectrum. This shift, due to the screening provided by the electron excited into the $σ*$ orbital, has been rationalized by calculating orbital ionization energies of I 3d excited and I 3d ionized states in CH$_3$I., Published by American Institute of Physics, Melville, NY

International audience; The influence of vibronic coupling on the outer valence ionic states of cis-dichloroethene has been investigated by recording photoelectron spectra over the excitation range 19–90 eV using plane polarized synchrotron radiation, for two polarization orientations. The photoelectron anisotropy parameters and electronic state branching ratios derived from these spectra have been compared to theoretical predictions obtained with the continuum multiple scattering approach. This comparison shows that the photoionization dynamics of the à 2B2, B̃ 2A1, C̃ 2A2, and D̃ 2B1 states, all of which are formed through the ejection of an electron from a nominally chlorine lone-pair orbital, exhibit distinct evidence of the Cooper minimum associated with the halogen atom. While retaining a high degree of atomic character, these orbital ionizations nevertheless display clear distinctions. Simulations, assuming the validity of the Born-Oppenheimer and the Franck-Condon approximations, of the X̃ 2B1, à 2B2, and D̃ 2B1 state photoelectron bands have allowed some of the vibrational structure observed in the experimental spectra to be assigned. The simulations provide a very satisfactory interpretation for the X̃ 2B1 state band but appear less successful for the à 2B2 and D̃ 2B1 states, with irregularities appearing in both. The B̃ 2A1 and C̃ 2A2 state photoelectron bands exhibit very diffuse and erratic profiles that cannot be reproduced at this level. Photoelectron anisotropy parameters, β, have been evaluated as a function of binding energy across the studied photon energy range. There is a clear step change in the β values of the à 2B2 band at the onset of the perturbed peak intensities, with β evidently adopting the value of the B̃ 2A1 band β. The D̃ 2B1 band β values also display an unexpected vibrational level dependence, contradicting Franck-Condon expectations. These various behaviors are inferred to be a consequence of vibronic coupling in this system

Ionization of the I 3d, 4s, and 4p orbitals in methyl iodide (CH$_3$I) has been studied by using synchrotron radiation to measure the total ion yield and by recording photoelectron spectra with linearly polarized radiation in two polarization orientations. The complete photoelectron spectrum of CH3I has been recorded at several photon energies, and bands due to the C 1s, I 3d, 4s, 4p, and 4d atomic-like orbitals, as well as the molecular orbitals, have been observed and assigned. In the vicinity of the I 3d5/2 and 3d3/2 ionization thresholds at 626.8 and 638.3 eV, respectively, the ion yield displays weak structure in the pre-edge region due to transitions into valence or Rydberg states, and, at higher energies, a shoulder and a broad maximum attributed to the I 3d$_{5/2} → εf$ and the I 3d$_{3/2} → εf$ shape resonances, respectively. The absorption spectrum calculated using time-dependent density functional theory, within the Tamm-Dancoff approximation, has allowed assignments to be proposed for the valence and Rydberg states. The Stieltjes imaging technique has been used to simulate the absorption spectrum above the ionization threshold and indicates that transitions into the f(l = 3) continuum channel dominate. This conclusion has been corroborated by a Continuum Multiple Scattering–Xα (CMS–Xα) calculation. The asymmetric broadening of the photoelectron bands associated with the I 3d orbital, due to post collision interaction, is taken into account in our experimental analysis. Experimentally derived photoelectron anisotropy parameters for the I 3d orbital are in good agreement with the theoretical predictions obtained with the CMS–Xα approach. The I 3d shake-up/shake-off photoelectron spectrum has been recorded, and assignments have been proposed for several of the satellites. The M$_4$N$_{45}$N$_{45}$ and M$_5$N$_{45}$N$_{45}$ Auger electron yields have been measured, and that for the M$_5$N$_{45}$N$_{45}$ decay exhibits a maximum due to interchannel coupling between the 3d$_{5/2}$ and 3d$_{3/2}$ continua. The photoelectron band associated with the I 4p orbital has an unusual appearance. Based upon previous theoretical work for the analogous Xe 4p orbital, it appears that the initial I 4p$^{−1}$ hole state decays rapidly through Coster-Kronig and super-Coster-Kronig transitions. This leads to a redistribution of the spectral intensity associated with the I 4p orbital and results in a photoelectron spectrum containing a single structured band together with an extended continuum. Another continuum is observed on the high binding energy side of the peak due to the 4s orbital, and we assign this to super-Coster-Kronig transitions into the 4p$^{-1}$4d$^{−1}$ continuum.

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.

Dissociative ionization of 1,3-butanediol and 2,3-butanediol isomers is investigated using threshold photoelectron-photoion coincidence (TPEPICO) measurements scanned across a photon energy range of approximately 4 eV from threshold. Breakdown diagrams prepared from these internal energy-selected data display significant isomer-specific behavior. The observed fragment appearance energies are compared with calculated thresholds to allow identification, where possible, of the fragmentation pathways. In turn, these are rationalized by comparison with calculated parent ion structures. The roles of intermolecular H-bonding and weak one-electron C C bonding in the parent cations are discussed.

The valence shell ionization spectrum of pyridine was studied using the third-order algebraic-diagrammatic construction approximation scheme for the one-particle Green’s function and the outer-valence Green’s function method. The results were used to interpret angle resolved photoelectron spectra recorded with synchrotron radiation in the photon energy range of 17–120 eV. The lowest four states of the pyridine radical cation, namely, 2A2 (1a 2 −1 1a2−1 ), 2A1(7a 1 −1 7a1−1), 2B1(2b 1 −1 2b1−1), and 2B2(5b 2 −1 5b2−1), were studied in detail using various high-level electronic structure calculation methods. The vertical ionization energies were established using the equation-of-motion coupled-cluster approach with single, double, and triple excitations (EOM-IP-CCSDT) and the complete basis set extrapolation technique. Further interpretation of the electronic structure results was accomplished using Dyson orbitals, electron density difference plots, and a second-order perturbation theory treatment for the relaxation energy. Strong orbital relaxation and electron correlation effects were shown to accompany ionization of the 7a1 orbital, which formally represents the nonbonding σ-type nitrogen lone-pair (nσ) orbital. The theoretical work establishes the important roles of the π-system (π-π* excitations) in the screening of the nσ-hole and of the relaxation of the molecular orbitals in the formation of the 7a1(nσ)−1 state. Equilibrium geometric parameters were computed using the MP2 (second-order Moller-Plesset perturbation theory) and CCSD methods, and the harmonic vibrational frequencies were obtained at the MP2 level of theory for the lowest three cation states. The results were used to estimate the adiabatic 0-0 ionization energies, which were then compared to the available experimental and theoretical data. Photoelectron anisotropy parameters and photoionization partial cross sections, derived from the experimental spectra, were compared to predictions obtained with the continuum multiple scattering approach.

Electron-ion coincidence imaging is used to study chiral asymmetry in the angular distribution of electrons emitted from randomly-oriented enantiomers of two molecules, methyloxirane and trifluoromethyloxirane, upon ionization by circularly polarized VUV synchrotron radiation. Vibrationally-resolved photoelectron circular dichroism (PECD) measurements of the outermost orbital ionization reveal unanticipated large fluctuations in the magnitude of the forward-backward electron scattering asymmetry, including even a complete reversal of direction. Identification and assignment of the vibrational excitations is supported by Franck-Condon simulations of the photoelectron spectra. A previously proposed quasi-diatomic model for PECD is developed and extended to treat polyatomic systems. The parametric dependence of the electronic dipole matrix elements on nuclear geometry is evaluated in the adiabatic approximation, and provokes vibrational level dependent shifts in amplitude and phase, to which the chiral photoelectron angular distributions are especially sensitive. It is shown that single quantum excitation of those vibrational modes which experience only a relatively small displacement of the ion equilibrium geometry along the normal coordinate, and which are then only weakly excited in the Franck-Condon limit, can be accompanied by big shifts in scattering phase; hence the observed big fluctuations in PECD asymmetry for such modes.

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.

Enantiomers of the monoterpene limonene have been investigated by (2 + 1) resonance enhanced multiphoton ionization and photoelectron circular dichroism employing tuneable, circularly polarized femtosecond laser pulses. Electron imaging detection provides 3D momentum measurement while electron-ion coincidence detection can be used to mass-tag individual electrons. Additional filtering, by accepting only parent ion tagged electrons, can be then used to provide discrimination against higher energy dissociative ionization mechanisms where more than three photons are absorbed to better delineate the two photon resonant, one photon ionization pathway. The promotion of different vibrational levels and, tentatively, different electronic ion core configurations in the intermediate Rydberg states can be achieved with different laser excitation wavelengths (420 nm, 412 nm, and 392 nm), in turn producing different state distributions in the resulting cations. Strong chiral asymmetries in the lab frame photoelectron angular distributions are quantified, and a comparison made with a single photon (synchrotron radiation) measurement at an equivalent photon energy.

International audience; 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 (i2PEPICO) 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 i2PEPICO technique allows correlating PECD measurements to specific parent ion masses, which would allow its application to analysis of complex mixtures

We have measured the photoelectron circular dichroism (PECD) of single enantiomers of endoborneol in the photon region from 9.9 to 23.6 eV by combining circularly polarized synchrotron radiation and a velocity map imaging technique. A photoelectron spectrum and the state-selected fragmentation curves of this terpene were also recorded. Unlike previous case studies, the broad featureless transitions encountered here preclude an orbital by orbital analysis of the PECD, although semiquantitative features of the highest-occupied molecular orbital PECD are identified and compared to full calculations. Despite our inability to further identify individual orbitals experimentally, we show that we are able to unambiguously assign the absolute configuration by comparison with realistic simulated PECD spectra. Furthermore, the calculations predict that for electron kinetic energies above 5 eV; the contributions of individual conformers to the PECD are nearly identical. Should this observation apply to bigger biological systems, the analysis could be greatly simplified by recording high kinetic energy electrons. On the other hand the contributions of the different conformers to the slow electron PECD seem to vary more significantly, and we deduce, within the theoretical limitations, a plausible 1:1:1 distribution of the three identified conformers.

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim A competition between chiral characteristics alternatively attributable to either conformation or to absolute configuration is identified. Circular dichroism associated with photoexcitation of the outer orbital of configurational enantiomers of 1,3- and 2,3-butanediols has been examined with a focus on the large changes in electron chiral asymmetry produced by different molecular conformations. Experimental gas-phase measurements offer support for the theoretical modeling of this chiroptical effect. A surprising prediction is that a conformationally produced pseudo-enantiomerism in 1,3-butanediol generates a chiral response in the frontier electron dynamics that outweighs the influence of the permanent configurational handedness established at the asymmetrically substituted carbon. Induced conformation, and specifically induced conformational chirality, may thus be a dominating factor in chiral molecular recognition in such systems.

Simultaneous, enantiomer-specific identification of chiral molecules in multi-component mixtures is extremely challenging. Many established techniques for single-component analysis fail to provide selectivity in multi-component mixtures and lack sensitivity for dilute samples. Here we show how enantiomers may be differentiated by mass-selected photoelectron circular dichroism using an electron–ion coincidence imaging spectrometer. As proof of concept, vapours containing ∼1% of two chiral monoterpene molecules, limonene and camphor, are irradiated by a circularly polarized femtosecond laser, resulting in multiphoton near-threshold ionization with little molecular fragmentation. Large chiral asymmetries (2–4%) are observed in the mass-tagged photoelectron angular distributions. These asymmetries switch sign according to the handedness (R- or S-) of the enantiomer in the mixture and scale with enantiomeric excess of a component. The results demonstrate that mass spectrometric identification of mixtures of chiral molecules and quantitative determination of enantiomeric excess can be achieved in a table-top instrument., Detecting enantiomers in dilute mixtures of volatile organic compounds is a challenge. Here, the authors demonstrate a method to identify enantiomers and enantiomeric excess in a multi-component mixture containing two chiral species using laser mass spectrometry and photoelectron circular dichroism.

Valence and C1s core level photoelectron spectra of gaseous alanine have been recorded with synchrotron radiation. Using ab initio Green's Function calculations of the vertical outer valence ionization energies and CMS-Xα calculations of the orbital ionization cross-sections, it is possible to account well for the features of both the new hν = 92 eV valence photoelectron spectrum and also its differences with an earlier hν = 21.2 eV spectrum. Good agreement may be achieved by considering just the contribution of a single molecular conformation. This agrees with previous experimental findings, but conflicts with calculations which suggest that a range of molecular conformations would coexist in an equilibrium sample. A study of the valence photoelectron spectrum of the amino acid threonine complements that of alanine, but unlike the latter is limited by the effects of thermal decomposition of the sample. The C1s core level spectrum of alanine is reported and its peaks are assigned to ionization of the thre...

A pronounced vibrational state dependence of photoelectron angular distributions observed in chiral photoionization experiments is explored using a simple, yet realistic, theoretical model based upon the transiently chiral molecule H2O2. The adiabatic approximation is used to separate vibrational and electronic wavefunctions. The full ionization matrix elements are obtained as an average of the electronic dipole matrix elements over the vibrational coordinate, weighted by the product of neutral and ion state vibrational wavefunctions. It is found that the parity of the vibrational Hermite polynomials influences not just the amplitude, but also the phase of the transition matrix elements, and the latter is sufficient, even in the absence of resonant enhancements, to account for enhanced vibrational dependencies in the chiral photoionization dynamics.

The ionization energy of CF3+ is investigated by synchrotron radiation photoionization experiments using a skimmed molecular beam of CF3Br. Care is taken to eliminate contributions from higher energy second-order radiation from the monochromator gratings and to characterize the molecular beam. From the corrected 0 K CF3+ appearance energy of 12.07 ± 0.02 eV, we calculate the adiabatic CF3 ionization energy to be 9.04 ± 0.04 eV, a value in excellent agreement with recent calculations and a number of experiments. Apparent discrepancies with other, significantly lower, determinations of the ionization potential are discussed and explained − particularly those arising from a previous CF3Br molecular beam experiment − confirming a recently proclaimed consensus favoring a value as obtained here.

A theoretical treatment of the photoionization of the chiral amino acid l-alanine is presented. Particular attention is paid to a previously unobserved circular dichroism which should be detectable in the photoelectron angular distribution (CDAD) from randomly oriented molecules. Numerical estimates of this difference in the differential cross-sections for left- and right-circularly polarized light range as large as 40% of the mean cross-section. Three different low-energy conformational structures are considered. Further comparisons with the experimental photoelectron spectrum suggest, however, that only one dominates in the gas phase. This concurs with other experimental data but disagrees with conclusions drawn from previous molecular orbital calculations. The magnitude of the predicted CDAD effect, especially when ionizing skeletal bonding orbitals, is sufficient to suggest that it may provide an experimental means for successfully distinguishing optical and conformational isomers.

Multiphoton photoelectron circular dichroism (MP-PECD) has been observed as an asymmetry in the angular distribution of photoelectrons emitted in the ionization of pure enantiomers of the small chiral molecule methyloxirane using a femtosecond laser operated at 420 nm. Energetically, this requires the uptake of four photons. By switching the laser between left- and right-circular polarization, and observing the differences in the full three-dimensional electron momentum distribution recorded in an electron-ion coincidence technique, the chiral (odd) terms in the angular distribution expression can be isolated. Electron events can additionally be filtered by coincident ion mass, providing mass-tagged electron distributions and quantitative measures of the MP-PECD asymmetry that help characterize the different ionization channels. For the production of ground state parent cation, the magnitude of the mean chiral asymmetry is measured to be 4.7%, with a peak magnitude exceeding 10%(Figure Presented).

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., The Franck–Condon principle—frozen nuclear positions during electronic motion—is used to explain many physical phenomena. Garcia et al. show how this breaks down in a photoionized chiral molecule via the vibrational dependence of the photoelectron angular asymmetry in the laboratory frame.

We measured and calculated the PECD of three different halogenated chiral molecules in order to study the role of the localization of the initial orbital of the outgoing electron with respect to the chiral center.

In the present work the photoelectron circular dichroism of camphor has been theoretically studied using B-spline and continuum multiple scattering-Xalpha methods, and comparisons are made with available experimental data. In general, rather large dichroism effects have been found for both valence and core (O 1s, C 1s) photoionizations. The agreement between the two calculations reported here and previous experimental measurements for core C 1s data is essentially quantitative. For valence ionization satisfactory agreement between theory and experiment has been obtained and the discrepancies have been attributed to both exchange-correlation potential limitations and the absence of response effects in the adopted formalism. The calculations predict, moreover, important features in the cross-section profiles, which have been discussed in terms of dipole-prepared continuum orbitals.

The photoionization of two representative mono-terpenes (R-limonene and R-carvone) has been investigated using low energy synchrotron radiation in the region from the threshold for ionization of these molecules up to 11 eV. Data from threshold photoelectron spectra are interpreted with the aid of Outer Valence Green's Function (OVGF) calculations to identify the character of the outermost orbitals and improved ionization potentials are deduced. Fragmentation of the parent ions is studied using threshold photoelectron-photoion coincidence (TPEPICO) time-of-flight (TOF) mass spectrometry and breakdown diagrams identifying the major products in this near-threshold region are presented.

Previously reported dramatic changes in photoelectron angular distributions (PADs) as a function of photoelectron kinetic energy following the ionization of S1 p-difluorobenzene are shown to be explained by a shape resonance in the b(2g) symmetry continuum. The characteristics of this resonance are clearly demonstrated by a theoretical multiple-scattering treatment of the photoionization dynamics. New experimental data are presented which demonstrate an apparent insensitivity of the PADs to both vibrational motion and prepared molecular alignment, however, the calculations suggest that strong alignment effects may nevertheless be recognized in the detail of the comparison with experimental data. The apparent, but unexpected, indifference to vibrational excitation is rationalized by considering the nature of the resonance. The correlation of this shape resonance in the continuum with a virtual pi* antibonding orbital is considered. Because this orbital is characteristic of the benzene ring, the existence of similar resonances in related substituted benzenes is discussed.

The methyl iodide A-band photodissociation is investigated using a photofragment time-of-flight technique coupled with state specific detection of either the iodine atom or methyl radical dissociation products by multiphoton ionization. The method is quite general and allows the fragments' velocity and spatial distributions to be determined simultaneously with their rotational, vibrational and electronic excitation. It is shown that methyl product from two competing dissociation channels, one direct, the other necessitating an initial surface crossing, can be clearly distinguished by virtue of the characteristic translational excitation in each. The different rotational excitation which is observed in these two channels is discussed in terms of the dissociation dynamics in the methyl iodide A-band continuum.

We present new evidence for the existence of a near threshold π* shape resonance as a common feature in the photoionization of each isomer of difluorobenzene. Experimentally, this is revealed by significant changes in the anisotropy of the photoelectron angular distributions (PADs) following the ionization of the optically aligned S1 state of these molecules at varying photon energies. Continuum multiple scattering Xα calculations reproduce this behaviour well, and allow the visualisation of the continuum shape resonances. The resonances are unusually narrow in energy (

The valence shell photoelectron spectra of 2-chloropyridine and 3-chloropyridine have been studied both experimentally and theoretically. Synchrotron radiation has been employed to record angle resolved photoelectron spectra in the photon energy range 20–100 eV, and these have enabled anisotropy parameters and branching ratios to be derived. The experimental results have been compared with theoretical predictions obtained using the continuum multiple scattering Xα approach. This comparison shows that the anisotropy parameter associated with the nominally chlorine lone-pair orbital lying in the molecular plane is strongly affected by the atomic Cooper minimum. In contrast, the photoionization dynamics of the second lone-pair orbital, orientated perpendicular to the molecular plane, seem relatively unaffected by this atomic phenomenon. The outer valence ionization has been studied theoretically using the third-order algebraic-diagrammatic construction (ADC(3)) approximation scheme for the one-particle Green’s function, the outer valence Green’s function method, and the equation-of-motion (EOM) coupled cluster (CC) theory at the level of the EOM-IP-CCSD and EOM-EE-CC3 models. The convergence of the results to the complete basis set limit has been investigated. The ADC(3) method has been employed to compute the complete valence shell ionization spectra of 2-chloropyridine and 3-chloropyridine. The relaxation mechanism for ionization of the nitrogen σ-type lone-pair orbital (σN LP) has been found to be different to that for the corresponding chlorine lone-pair (σCl LP). For the σN LP orbital, π-π* excitations play the main role in the screening of the lone-pair hole. In contrast, excitations localized at the chlorine site involving the chlorine πCl LP lone-pair and the Cl 4p Rydberg orbital are the most important for the σCl LP orbital. The calculated photoelectron spectra have allowed assignments to be proposed for most of the structure observed in the experimental spectra. The theoretical work also highlights the formation of satellite states, due to the breakdown of the single particle model of ionization, in the inner valence region.