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. This article is part of the themed collection: Festschrift Ivan Powis: Advances in Molecular Photoelectron Spectroscopy: Fundamentals & Applicatio

Fourfold Differential Photoelectron Circular Dichroism

We report on a joint experimental and theoretical study of photoelectron circular dichroism (PECD) in methyloxirane. By detecting O 1s photoelectrons in coincidence with fragment ions, we deduce the molecule’s orientation and photoelectron emission direction in the laboratory frame. Thereby, we retrieve a fourfold differential PECD clearly beyond 50%. This strong chiral asymmetry is reproduced by ab initio electronic structure calculations. Providing such a pronounced contrast makes PECD of fixed-in-space chiral molecules an even more sensitive tool for chiral recognition in the gas phase

Photoelectron circular dichroism of O 1ss-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 coincidentelectron 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 results for 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 literature.Comment: 6 fig

Enantiosensitive Structure Determination by Photoelectron Scattering on Single Molecules

X-ray as well as electron diffraction are powerful tools for structure determination of molecules. Electron diffraction methods yield \r{A}ngstrom-resolution even when applied to large systems or systems involving weak scatterers such as hydrogen atoms. For cases in which molecular crystals cannot be obtained or the interaction-free molecular structure is to be addressed, corresponding electron scattering approaches on gas-phase molecules exist. Such studies on randomly oriented molecules, however, can only provide information on interatomic distances, which is challenging to analyse in case of overlapping distance parameters and they do not reveal the handedness of chiral systems8. Here, we present a novel scheme to obtain information on the structure, handedness and even detailed geometrical features of single molecules in the gas phase. Using a loop-like analysis scheme employing input from ab initio computations on the photoionization process, we are able to deduce the three dimensional molecular structure with sensitivity to the position individual atoms, as e.g. protons. To achieve this, we measure the molecular frame diffraction pattern of core-shell photoelectrons in combination with only two ionic fragments from a molecular Coulomb explosion. Our approach is expected to be suitable for larger molecules, as well, since typical size limitations regarding the structure determination by pure Coulomb explosion imaging are overcome by measuring in addition the photoelectron in coincidence with the ions. As the photoelectron interference pattern captures the molecular structure at the instant of ionization, we anticipate our approach to allow for tracking changes in the molecular structure on a femtosecond time scale by applying a pump-probe scheme in the future

Ab initio calculation of the photoionization cross sections and photoelectron angular distribution parameters of CH

Photoionization cross sections σnγ(ω) and photoelectron angular distribution parameters βnγ(ω) are systematically computed for the closed-shell molecules CH4, NH3, H2O and CO. Calculations are performed by the single center (SC) method based on the numerical integration of the coupled Hartree-Fock equations for a photoelectron in continuum. In the cases of the core nγ(1s) and valence nγ(2p) molecular orbitals, a good overall agreement between the present theory and experiment available in the literature is obtained. For the subvalence nγ(2s) molecular orbitals, agreement between the theory and available experiment is satisfactory, which can be attributed to an impact of many-electron correlations neglected in the present calculations

Multichannel single center method

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Electronic-state interference in the C 1 s excitation and decay of methyl chloride studied by angularly resolved Auger spectroscopy

International audienceResonant Auger (RA) decay spectra of carbon 1s excited CH3Cl molecules are recorded with angular resolutionusing linearly polarized synchrotron radiation. The selected photon energies corresponding to the C 1s → 8a1core to lowest unoccupied molecular orbital and C 1s → 4sa1, 4pe, and 4pa1 core to Rydberg excitations ofmethyl chloride are used and electrons in the binding energy range of 11–37 eV are detected. The vibrationallyunresolved RA electron angular distributions, recorded for participator Auger transitions populating the X, A,B, and C states of the CH3Cl+ ion, exhibit strong variations across the selected electronic resonances. Theseobservations are interpreted with the help of ab initio electronic structure and dynamics calculations, whichaccount for electronic-state interference between the direct and different resonant ionization pathways. Forspectator transitions, the theory predicts almost isotropic angular distributions with moderate changes of βparameters around zero, which is in agreement with the experimental observations

Observation of Nondipole-Induced Asymmetry in the Angular Emission Distribution of Photoelectrons from Fixed-in-Space CO Molecules

We investigate experimentally and theoretically the C and O 1s photoionization of fixed-in-space CO molecules at a photon energy of 905 eV. We find a significant dependence of the photoelectron angular distributions on the direction of propagation of the ionizing radiation. It results from an interplay of nondipole effects, on one hand, and molecular effects, on the other. The nondipole effects lead to an increase of the emission probability in the forward direction along the light propagation, and the photoelectron wave being scattered by the molecular potential gives rise to a strong peak in the direction of the atom neighboring the emitter site. These effects can either conspire or extenuate each other, depending on the photoelectron emission direction and molecular orientation in space

Observation of Nondipole-Induced Asymmetry in the Angular Emission Distribution of Photoelectrons from Fixed-in-Space CO Molecules

We investigate experimentally and theoretically the C and O 1s photoionization of fixed-in-space CO molecules at a photon energy of 905 eV. We find a significant dependence of the photoelectron angular distributions on the direction of propagation of the ionizing radiation. It results from an interplay of nondipole effects, on one hand, and molecular effects, on the other. The nondipole effects lead to an increase of the emission probability in the forward direction along the light propagation, and the photoelectron wave being scattered by the molecular potential gives rise to a strong peak in the direction of the atom neighboring the emitter site. These effects can either conspire or extenuate each other, depending on the photoelectron emission direction and molecular orientation in space

Angular emission distribution of O 1s photoelectrons of uniaxially oriented methanol

The angular distribution of O 1s photoelectrons emitted from uniaxially oriented methanol is studied experimentally and theoretically. We employed circularly polarized photons of an energy of hν = 550 eV for our investigations. We measured the three-dimensional photoelectron angular distributions of methanol, with the CH3–OH axis oriented in the polarization plane, by means of cold target recoil ion momentum spectroscopy. The experimental results are interpreted by single active electron calculations performed with the single center method. A comparative theoretical study of the respective molecular-frame angular distributions of O 1s photoelectrons of CO, performed for the same photoelectron kinetic energy and for a set of different internuclear distances, allows for disentangling the role of internuclear distance and the hydrogen atoms of methanol as compared to carbon monoxide