Co-reporter:Mohammad M. Rafiee Fanood, Maurice H. M. Janssen and Ivan Powis
Physical Chemistry Chemical Physics 2015 vol. 17(Issue 14) pp:8614-8617
Publication Date(Web):02 Mar 2015
DOI:10.1039/C5CP00583C
Limonene is ionized by circularly polarized 420 nm femtosecond laser pulses. Ion mass and photoelectron energy spectra identify the dominant (2 + 1) multiphoton ionization mechanism, aided by TDDFT calculations of the Rydberg excitations. Photoelectron circular dichroism measurements on pure enantiomers reveal a chiral asymmetry of ±4 %.
Co-reporter:Steven Daly, Ivan Powis, Maurice Tia, Gustavo A. Garcia, Laurent Nahon
International Journal of Mass Spectrometry 2015 Volume 376() pp:46-53
Publication Date(Web):15 January 2015
DOI:10.1016/j.ijms.2014.11.013
•Butanediol ion isomers display distinct fragmentation behavior.•Synchrotron radiation study of ion yields and appearance energies.•Comparison with CBS-QB3 calculated thresholds to identify fragmentation channels.•2,3-Butanediol ion readily cleaves extended C(2)C(3) bond.•H2O loss in 1,3-butanediol facilitated by intramolecular H-bonded structure.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 CC bonding in the parent cations are discussed.
Co-reporter:Ivan Powis, Steven Daly, Maurice Tia, Barbara Cunha de Miranda, Gustavo A. Garcia and Laurent Nahon
Physical Chemistry Chemical Physics 2014 vol. 16(Issue 2) pp:467-476
Publication Date(Web):12 Sep 2013
DOI:10.1039/C3CP53248H
A detailed study of the valence photoionization of small homochiral glycidol (C3O2H6) clusters is carried out with the help of circularly-polarized VUV synchrotron radiation by recording photoionization-based spectroscopic data detected by velocity map electron imaging with coincidence ion selection. We show that information on the stability of cationic as well as neutral chiral clusters can be obtained with enhanced sensitivity by examining the chiral fingerprint encapsulated in Photoelectron Circular Dichroism (PECD) spectra. In particular, by varying the clustering conditions we demonstrate that the PECD signal effectively carries the signature of the neutral precursor species, prior to any fragmentation of the ion, as may be inferred from the below-threshold monomer measurements (including ion imaging). Here the monomer's direct ionization channel is closed and the monomer ion hence must result exclusively as a fragment from dissociative ionization of the dimer (or higher) clusters. At higher photon energies, the mass-selection on the electron spectroscopy data, achieved through filtering the electron images in coincidence with selected ion masses, evidently succeeds in providing a degree of size-selection on the neutral clusters being ionized with, in particular, a clear differentiation of monomer and dimer PECD, showing the strong sensitivity of this chiroptical effect to the non-local long-range molecular potential.
Co-reporter:Mohammad M. Rafiee Fanood, Ivan Powis, and Maurice H. M. Janssen
The Journal of Physical Chemistry A 2014 Volume 118(Issue 49) pp:11541-11546
Publication Date(Web):November 17, 2014
DOI:10.1021/jp5113125
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%
Co-reporter:Gustavo A. Garcia, Laurent Nahon, Chris J. Harding and Ivan Powis
Physical Chemistry Chemical Physics 2008 vol. 10(Issue 12) pp:1628-1639
Publication Date(Web):31 Jan 2008
DOI:10.1039/B714095A
Photoionization of the chiral molecule glycidol has been investigated in the valence region. Photoelectron circular dichroism (PECD) curves have been obtained at various photon energies by using circularly polarized VUV synchrotron radiation and a velocity map imaging technique to record angle-resolved photoelectron spectra (PES). The measured chiral asymmetries vary dramatically with the photon energy as well as with the ionized orbital, improving the effective orbital resolution of the PECD spectrum with respect to the PES. Typical asymmetry factors of 5% are observed, but the peak values measured range up to 15%. The experimental results are interpreted by continuum multiple scattering (CMS-Xα) calculations for several thermally accessible glycidol conformers. We find that a nearly quantitative agreement between theory and experiments can be achieved for the ionization of several molecular orbitals. Owing to the sensitivity of PECD to molecular conformation this allows us to identify the dominant conformer. The influence of intramolecular hydrogen bond orbital polarization is found to play a small yet significant role in determining the chiral asymmetry in the electron angular distributions.
Co-reporter:Ivan Powis
Chirality 2008 Volume 20( Issue 9) pp:961-968
Publication Date(Web):
DOI:10.1002/chir.20537
Abstract
The technique of photoelectron circular dichroism (PECD) is introduced and illustrated by the presentation of results obtained for the C 1s core ionization of (+)-S-carvone enantiomers. Using circularly polarized ionizing radiation, large chiral effects in the angular distribution of photoelectrons emitted from a dilute, randomly oriented gas phase sample can be detected. This effect is predicted in the pure electric dipole approximation and is expected to be quite general. The forward–backward asymmetry regularly exceeds 10%, as demonstrated here for the carbonyl C 1s photoemission of carvone. Calculations are presented that reveal a pronounced dependence of this effect on molecular conformation. In the case of carvone the PECD associated with the C 1s electron localized at the carbonyl group varies with orientation of the isopropenyl tail group at the opposite end of the molecule. Comparisons of calculations with experiment confirm the molecular configuration and permit inferences to be drawn concerning the relative conformer populations in the experimental sample. Chirality, 2008. © 2008 Wiley-Liss, Inc.
Co-reporter:Ivan Powis ;Chris J. Harding Dr.;Gustavo A. Garcia Dr.;Laurent Nahon Dr.
ChemPhysChem 2008 Volume 9( Issue 3) pp:475-483
Publication Date(Web):
DOI:10.1002/cphc.200700748
Abstract
Photoelectron circular dichroism (PECD) is investigated in the valence ionization of selected fenchone enantiomers using a photoelectron imaging technique and circularly polarized synchrotron radiation. Theoretical modelling of the results using electron scattering calculations demonstrates that the observed chiral asymmetry in the photoelectron angular distributions depends strongly upon the final state scattering, and upon the quality of the molecular potential used for these calculations. However, very pronounced dependence on the orbital from which ionization occurs is also observed. Comparison with analogous results previously obtained for camphor reveals striking differences in the PECD, even when the ionizing orbitals are themselves left substantially unaffected by the changes in methyl groups’ substitution site. PECD measurements readily differentiate these molecules despite their very similar photoelectron spectra, demonstrating PECD to be a structurally sensitive probe.
Co-reporter:Gustavo A Garcia, Laurent Nahon, Ivan Powis
International Journal of Mass Spectrometry 2003 Volume 225(Issue 3) pp:261-270
Publication Date(Web):1 March 2003
DOI:10.1016/S1387-3806(03)00004-6
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.
Co-reporter:Emma E Rennie, Ivan Powis, Uwe Hergenhahn, Oliver Kugeler, Gustavo Garcia, Toralf Lischke, Simon Marburger
Journal of Electron Spectroscopy and Related Phenomena 2002 Volume 125(Issue 3) pp:197-203
Publication Date(Web):September 2002
DOI:10.1016/S0368-2048(02)00140-8
The valence photoelectron spectrum of camphor has been recorded with 95 eV synchrotron radiation, with better definition than previous He I spectra. The spectrum is interpreted by comparison with these He I results and with the aid of an outer-valence Green’s Function calculation of the orbital ionization energies. These calculations closely reproduce the observed vertical ionization energies in the outer valence region. A core level spectrum of the C 1s region (hν=357.9 eV) is also presented and reveals a marked shift of the carbonyl carbon relative to all others in the molecule.
Co-reporter:Jonathan G. Underwood and Ivan Powis
Physical Chemistry Chemical Physics 2000 vol. 2(Issue 4) pp:747-756
Publication Date(Web):15 Feb 2000
DOI:10.1039/A907905J
Photodissociation dynamics of CH3Br following excitation in the red wing (240–280 nm) of the Ã-band
absorption are investigated by state-selective resonance enhanced multiphoton ionization probing and
photofragment time-of-flight spectroscopy of Br and CH3
products. Evidence of significant methyl fragment
rotational excitation is found in the decay channels leading to Br(2P3/2) and to Br*(2P1/2). Vibrational
excitation is found in both channels, but is greater, with a population inversion, in that accompanying
formation of ground state Br. This latter channel displays an unexpectedly isotropic fragment angular
distribution, while the Br* channel has, overall, a parallel angular distribution especially for formation of the
lowest v2=0, 1 levels of CH3. These observations are shown to be consistent with a strong non-adiabatic
coupling between spin–orbit states of CH3Br with a distortion from C3v
geometry in the interaction region.
Co-reporter:Ivan Powis, Steven Daly, Maurice Tia, Barbara Cunha de Miranda, Gustavo A. Garcia and Laurent Nahon
Physical Chemistry Chemical Physics 2014 - vol. 16(Issue 2) pp:NaN476-476
Publication Date(Web):2013/09/12
DOI:10.1039/C3CP53248H
A detailed study of the valence photoionization of small homochiral glycidol (C3O2H6) clusters is carried out with the help of circularly-polarized VUV synchrotron radiation by recording photoionization-based spectroscopic data detected by velocity map electron imaging with coincidence ion selection. We show that information on the stability of cationic as well as neutral chiral clusters can be obtained with enhanced sensitivity by examining the chiral fingerprint encapsulated in Photoelectron Circular Dichroism (PECD) spectra. In particular, by varying the clustering conditions we demonstrate that the PECD signal effectively carries the signature of the neutral precursor species, prior to any fragmentation of the ion, as may be inferred from the below-threshold monomer measurements (including ion imaging). Here the monomer's direct ionization channel is closed and the monomer ion hence must result exclusively as a fragment from dissociative ionization of the dimer (or higher) clusters. At higher photon energies, the mass-selection on the electron spectroscopy data, achieved through filtering the electron images in coincidence with selected ion masses, evidently succeeds in providing a degree of size-selection on the neutral clusters being ionized with, in particular, a clear differentiation of monomer and dimer PECD, showing the strong sensitivity of this chiroptical effect to the non-local long-range molecular potential.
Co-reporter:Mohammad M. Rafiee Fanood, Maurice H. M. Janssen and Ivan Powis
Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 14) pp:NaN8617-8617
Publication Date(Web):2015/03/02
DOI:10.1039/C5CP00583C
Limonene is ionized by circularly polarized 420 nm femtosecond laser pulses. Ion mass and photoelectron energy spectra identify the dominant (2 + 1) multiphoton ionization mechanism, aided by TDDFT calculations of the Rydberg excitations. Photoelectron circular dichroism measurements on pure enantiomers reveal a chiral asymmetry of ±4 %.
Co-reporter:Gustavo A. Garcia, Laurent Nahon, Chris J. Harding and Ivan Powis
Physical Chemistry Chemical Physics 2008 - vol. 10(Issue 12) pp:NaN1639-1639
Publication Date(Web):2008/01/31
DOI:10.1039/B714095A
Photoionization of the chiral molecule glycidol has been investigated in the valence region. Photoelectron circular dichroism (PECD) curves have been obtained at various photon energies by using circularly polarized VUV synchrotron radiation and a velocity map imaging technique to record angle-resolved photoelectron spectra (PES). The measured chiral asymmetries vary dramatically with the photon energy as well as with the ionized orbital, improving the effective orbital resolution of the PECD spectrum with respect to the PES. Typical asymmetry factors of 5% are observed, but the peak values measured range up to 15%. The experimental results are interpreted by continuum multiple scattering (CMS-Xα) calculations for several thermally accessible glycidol conformers. We find that a nearly quantitative agreement between theory and experiments can be achieved for the ionization of several molecular orbitals. Owing to the sensitivity of PECD to molecular conformation this allows us to identify the dominant conformer. The influence of intramolecular hydrogen bond orbital polarization is found to play a small yet significant role in determining the chiral asymmetry in the electron angular distributions.
