Journal Club Meetings of 2017
||No Meeting: Holiday
||No Meeting: Network Meeting in Paris
"Time delay in molecular photoionization"
Date: 20th December 2017
Presented by: Jennifer Joseph
Journal Article: Hockett, P. et al., J. Phys.B: At. Mol. Opt. Phys., 2016, 49, 095602
Electron emission was considered to be instantaneous without associated delays. However in 1955, Wigner et al reported the theoretical possibility of having time delays (Wigner time delays) in such processes. Wigner time delay is related to the energy derivative of the scattering phase. It was only after the advent of attosecond laser sources that experiments studying electron dynamics and few measurements on time delays in photoionization were performed. Molecular photoionization being more complex, with strong spatial dependence is considered to be rich in information on the scattering potential. In this article, theoretical description of energy and angle resolved Wigner time delays associated with valence–single photon ionization of N2 and CO are discussed. Further, Wigner delays in the scattering theory perspective is presented along with possible experimental techniques that could be a window in accessing the delays associated with molecular photoionization.
"Molecular Frame Reconstruction Using Time-Domain Photoionization Intererometry"
Date: 11thOctober 2017
Presented by:Constant Schouder, Aarhus University
Journal Article: Marceau, C. et al., Phys. Rev. Lett., 2017, 119, 083401.
This study investigated how one can reconstruct the molecular frame Photo-Electrons Angular Distributions (PADS) using a boot strapping methodology.
The technique is based on the relation between the laboratory frame and the molecular frame. Both of these frames use an invariant set of elements which are related to the electronic transition between the ground state and the continuum in the electric dipole approximation.
This paper explains how to retrieve those elements in the laboratory frame, by inducing a dynamic in the system with the creation of a rotational wavepacket and using a fitting procedure. It will then be possible to calculate the PADS in the molecular frame.
"Observation of enhanced chiral asymmetries in the Inner-Shell photoionization of uniaxially orientated methyloxirane enantiomers"
Date: 19th July 2017
Presented by: Giammarco Nalin, ROENTDEK
Journal Article: Tia.M et al., J. Phys. Chem. Lett., 2017, 8, 2780.
When a chiral molecule is irradiated by circularly polarized light, the electron emission shows a slight asymmetry in the electron flux directed parallel/antiparallel with respect to the photon propagation direction. The latter effect has been observed in randomly oriented molecules as a consequence of switching the helicity of the light. The effect is known as Photo-electron Circular Dichroism (PECD) and it has been observed for many molecules. It usually becomes stronger for lower electron energies, as electrons are deflected strongly from the chiral potential. Most experiments have investigated PECD in the valence ionization regime and just a few in the inner-shell ionization one. For the latter regime, an inner-shell vacancy generated by the absorbed photon decays under emission of an Auger electron.
Consequently, the molecule undergoes coulomb explosion (CE). In the presented paper, this circumstance is utilised to post-orient one axis of the molecule in space. As a consequence, the PECD signal increased, for certain (partial)
orientations, quite significant.
The experiment has been performed using the well-established COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy) technique at the synchrotron facility SOLEIL (Saint-Aubin, France). The circularly polarized photon beam is perpendicularly intersected with a molecular gas jet of enantio-pure methyloxirane. Every hour, the light’s helicity was switched and both enantiomers (R and S forms) measured. The electron arm of the spectrometer was
optimised to measure electrons up to 550 eV (corresponding to Auger Electron from a decay of an O(1s-1)). The ion arm was designed in a time- and space- focussing geometry to have maximum momentum and mass resolution.
PECD is a promising technique for chiral recognition in racemic solutions. The interest of the community on this effect raised with the increasing availability of outstanding and reliable light sources like synchrotrons. Studying progressively heavier molecules leads to an increasing complexity of both theoretical models and experimental setup. This makes the topic particularly relevant the whole ASPIRE network.