Gustavo Garcia

Two-dimensional charged particle image inversion using a polar basis function expansion

We present an inversion method called pBasex aimed at reconstructing the original Newton sphere of expanding charged particles from its two-dimensional projection by fitting a set of basis functions with a known inverse Abel integral. The basis functions have been adapted to the polar symmetry of the photoionization process to optimize the energy and angular resolution while minimizing the CPU time and the response to the cartesian noise that could be given by the detection system. The method presented here only applies to systems with a unique axis of symmetry although it can be adapted to overcome this restriction. It has been tested on both simulated and experimental noisy images and compared to the Fourier-Hankel algorithm and the original Cartesian basis set used by [Dribinski et al.Rev. Sci. Instrum. 73, 2634 (2002)], and appears to give a better performance where odd Legendre polynomials are involved, while in the images where only even terms are present the method has been shown to be faster and simpler without compromising its accuracy.

Circular dichroism in the photoelectron angular distribution from randomly oriented enantiomers of camphor

Circular dichroism in the angular distribution of valence photoelectrons emitted from randomly oriented chiral molecules has been observed in experiments that use circularly polarized VUV synchrotron radiation. Photoionization of the outermost carbonyl oxygen lone pair electrons from pure enantiomers of the prototype chiral molecule camphor is shown to have an asymmetry in the forward–backward scattering of photoelectrons (relative to the propagation direction of the light beam) of magnitude approaching 3% at 9.2 eV photon energy. The asymmetry reverses on exchange of either the helicity of the radiation or of the molecular enantiomer, confirming theoretical predictions of an effect that arises in the pure electric-dipole approximation.

Photoelectron circular dichroism in core level ionization of randomly oriented pure enantiomers of the chiral molecule camphor

The inner-shell photoionization of unoriented camphor molecules by circularly polarized light has been investigated from threshold to a photoelectron kinetic energy of approximately 65 eV. Photoelectron spectra of the carbonyl C 1s orbital, recorded at the magic angle of 54.7 degrees with respect to the light propagation direction, show an asymmetry of up to 6% on change of either the photon helicity or molecular enantiomer. These observations reveal a circular dichroism in the angle resolved emission with an asymmetry between forward and backward scattering (i.e., 0 degrees and 180 degrees to the light beam) which can exceed 12%. Since the initial state is an atomiclike spherically symmetric orbital, this strongly suggests that the asymmetry is caused by final-state effects dependent on the chiral geometry of the molecule. These findings are confirmed by electron multiple scattering calculations of the photoionization dynamics in the electric-dipole approximation.

Valence and C 1s core level photoelectron spectra of camphor

Abstract 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 ( h ν=357.9 eV) is also presented and reveals a marked shift of the carbonyl carbon relative to all others in the molecule.

Advanced search for the origin of life's homochirality: asymmetric photon-induced processes on chiral compounds with far-UV circularly polarized synchrotron radiation

Assuming an extra-terrestrial formation of lifes molecular building blocks such as amino-acids, a possible abiotic explanation for the selection of the L enantiomers could be the exposure to an asymmetric bias such as far UV Circularly Polarized Light (CPL), during their journey towards Earth, inducing some enantiomeric excess (e.e) that could then be amplified on Earth via suitable autocatalytic mechanisms. Synchrotron Radiation (SR), with its intense flux and broad tunability, is a unique tool which mimics such an interstellar far UV CPL. We have recently employed it to study : (1) The irradiation of solid films of the amino acid D,L-leucine, i.e. under relevant astrophysical conditions. Starting from racemic D,L-leucine irradiated with CPL SR beam at 6.8 eV (182 nm), we have been able to induce by enantioselective photolysis an e.e. of 2.6 %, as measured by chiral-sensitive CG-MS analysis, in accordance with the CD spectrum recorded on the same type of sample. (2) CPL-induced gas phase photoionization of chiral molecules. By measuring the angular distribution of photoelectrons ejected from pure enantiomers, we observed a strong anisotropy (up to 16 %) in the forward/backward direction with respect to the light propagation axis. Because of momentum conservation, such an effect is accompanied by an asymmetric recoil of the corresponding ions that could lead to a high e.e. Future prospects on the new VUV SR beamline DESIRS at SOLEIL are presented.

Electron asymmetries in the photoionization of chiral molecules: possible astrophysical implications

ABSTRACT Photoelectron circular dichroism (PECD) is an intense orbital-specific chiroptical effect observed as asymmetries in the angular distribution of photoelectrons produced by photoionization of randomly oriented pure enantiomers with circularly polarized light. After a broad introduction placing this effect in the context of new physical chiral-sensitive methods, we review the main characteristics of PECD in terms of molecular photoionization dynamics. We stress also the analytical capabilities of PECD to retrieve enantiomeric excesses (e.es.) and to probe subtle details of the whole molecular potential, some of them exemplified by the showcase camphor and fenchone molecules. We then present the case of the amino acid alanine for which an interplay between PECD and conformer population is rationalized. Based on this study, we propose a photophysical astrophysical scenario for the origin of life’s homochirality, relying upon the asymmetry of the associated recoiling alanine parent ion that could lead at the relevant Lyman-α energy to an e.e. of up to 4% in a given line of sight, which appears independent of the temperature. In an attempt to generalize this scenario to other amino acids, new data on proline showing an e.e. of 12%, of the same sign as alanine, are also presented. Abbreviations ARPES: Angle-resolved photoemission; CMS-Xa: Continuum multiple scattering with Xa local-exchange potential; CPL: Circularly polarized light; CD: Circular dichroism; CSM: Circumstellar medium; DPI: Dissociative ionization; HHG: High harmonics generation; HOMO: Highest occupied molecular orbital; ISM: Interstellar medium; KE: Kinetic energy; MS: Mass spectrometry; MW: Microwave; PAD: Photoelectron angular distribution; PECD: Photoelectron circular dichroism; PECD-PICO: Photoelectron circular dichroism / photoion coincidence; PEPICO: Photoelectron / photoion coincidence; PES: Photoelectron spectrum; PV: Parity Violation; REMPI: Resonance-enhanced multi-photon ionization; RH: Resistive heating; TD: Thermodesorption; UV: Ultra-violet; VMI: Velocity map imaging; VUV: Vacuum ultra-violet. GRAPHICAL ABSTRACT

Valence-shell Photoelectron Circular Dichroism (PECD) on Chiral Halogenated Molecules in the Gas Phase

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.