Journal Club Meetings of 2019
||Dhirendra Singh and Giammarco Nalin
||Jennifer Joseph and Mikhail Osolodkov
||Jacqueline Arlt and Hassan Ganjitabar
Vibrationally Resolved PES and PECD
Date: 21st June 2019
Presented by: Hassan Ganjitabar
Journal Articles: G.A. Garcia et al., Nat Commun., 2013, 4, 2132; M.M.R. Fanood et al., Chemphyschem, 2018, 19(8), 921-933; H. Ganjitabar et al., J Mol Spectrosc, 2018, 353, 11-19.
Recently, the vibrational dependency of Photoelectron Circular Dichroism (PECD) has been reported [1, 2], showing that PECD has the potential to be used as a probe to the vibrational dynamics. In this ASPIRE journal club presentation a paper investigating the vibrational effects in single-photon ionisation regime was discussed .
The vibrationally resolved PES and PECD of five chiral members of the monoterpene family (α-pinene, β-pinene, limonene, 3-carene and sabinene) were discussed confirming the previously reported intensive modulation of PECD with vibrational structures in the X-band of the mentioned molecules. Furthermore, the details of the Frank Condon (FC) simulation, needed to assign the vibrational structures for all the molecules, was presented. Assigning the vibrational peaks in the PES helps to get the vibrational dependency of PECD understood.
 Gustavo A. Garcia, Laurent Nahon, Steven Daly & Ivan Powis; Nature Communications; 4: 2132 (2013).
 Mohammad M. Rafiee Fanood, Hassan Ganjitabar, Gustavo A. Garcia, Laurent Nahon, Stefano Turchini, and Ivan Powis; Chemphyschem; 19(8):921-933 (2018)
 Hassan Ganjitabar, Rim Hadidi, Gustavo A. Garcia, Laurent Nahon, Ivan Powis; Journal of Molecular Spectroscopy; 353:11-19 (2018).
"A 'circularisation' method to repair deformations and determine the centre of velocity map images"
Date: 21st June 2019
Presented by: Jacqueline Arlt
Journal Article: J.R. Gascooke, S.T. Gibson, and W.D. Lawrance, Chem. Phys., 2017, 147.
Distortions in velocity map images can occur, for example due to stray magnetic or electric fields, which cannot be fully eliminated. Hence, the analysis and interpretation of these images can be challenging. Photoelectron images are especially prone to distortion and the photoelectron spectra obtained through the images exhibit less resolution than the undistorted image would have. This problem can be solved by just analysing a wedge or a quadrant of the image. However, Gascooke et al. introduced an algorithm to restore a velocity map image’s circularity and thus provide a means to use the full image for the analysis.
The algorithm operates in the following manner: the image is transferred to polar coordinates and Abel inverted. The resulting r-θ-lines are fitted to a trigonometric series. If the image centre is incorrectly assigned, the multiplying factors of the trigonometric series do not converge to zero, approaching r=0. In this case, the previous steps are repeated with the corrected centre. With a correctly centred image, the obtained fitting is used to remove the distortion from the image and restore its circularity.
"Accuracy and presicion of the RABBIT Technique"
Date: 15th May 2019
Presented by: Mikhail Osolodkov
Journal Article: M. Isinger et al., Phil. Trans. R. Soc., 2019, 377: 20170475.
The reconstruction of attosecond beating by interference of two-photon transitions (RABBIT) technique is a powerful tool not only for the characterization of attosecond pulses, but also for accurate determination of the photoionization time delays in different targets. In this work the accuracy of the RABBIT technique is studied using numerical simulations. The effect of the spatial and temporal properties of the extreme ultraviolet (XUV) laser field on the precision of the method is studied. Such properties of the XUV field as spatial variation of the group delay over the interaction region in the detecting system, chirp rate and group delay dispersion are discussed. Also the effect of the experimental parameters, such as temporal jitter introduced by the instability of the pump-probe delay and background noise in the detected signal, is investigated. Based on the results of the simulations, the method achieved real measurements is estimated (5 – 10 as). The recommendations on the experimental parameters such as sampling step size and number of sampling points are proposed. This work provides important information on main factors limiting the temporal accuracy of the RABBIT technique that should be accounted for during the experiment.
"Orientation dependent stereo Wigner time delay and electron localization in a small molecule"
Date: 2nd May 2019
Presented by: Jennifer Joseph
Journal Article:J. Vos et al., Science, 2018, 360, 6395, 1326-1330.
The article investigates the time delay in molecular photoionization, in particular on the stereo Wigner time delay (SWTD), using a COLTRIMS set up and the interferometric XUV pump –IR probe RABBIT technique. The SWTD describes the relative time taken by the electron wavepacket (EWP) leaving the C-side and the O-side of the CO molecule. This measurement provides information on the localization of the EWP after ionization, in the presence of molecular potential. The dissociative state of CO producing C++ O+ e- channels is studied for i) parallel and ii) perpendicular orientation of the molecule with respect to the polarization axis. SWTDs for these two cases are measured and compared to the theoretical models (RD-RIS method and CWP method). Further investigation is done in the parallel orientation, where a delay difference of -165 attoseconds (as) is obtained at the low electron energy region and +30 as for higher electron energy region. This change in sign for SWTD is attributed to the bias in electron emission from the C side and the O side. For example for low energies dominance of 42Σ+ state, showing large asymmetry towards the C side results in the emission of an electron in the direction of the C side and a negative SWTD. Using the theoretical model, they establish that this is related to the localization of EWP within the molecule and determine the location from where the EWP leaves the molecule. The study serves as a benchmark in the investigation of time delays in molecular photoionization and also on the development of the theoretical model.
"Photoelectron angular distributions from strong-field ionization of oriented molecules"
Date: 25th February 2019
Presented by: Constant Schouder
Journal Article: L. Holmegaard et al., Nature Physics, 2010, 6, 428-432.
In this work, L.Holmegaard and group present the molecular frame photoelectron angular distributions (MFPADs) on two polar molecules, OCS and C7H5N. They access the molecular frame of both systems by combining alignment, induced from a non-resonant intense laser, and orientation coming from the static electric field used in their spectrometer. They probe the two molecules with circularly polarised 30-fs laser pulses. When the molecules are oriented, they can observe an anisotropy of the photoelectron distribution in the direction perpendicular to the fixed permanent dipole moment. Moreover, a depletion of the photoelectron signal in the molecular plane of C7H5N is measured when the molecule is 3D-aligned. A theoretical analysis based on tunnelling ionisation is able to reproduce the main features of the experimental results. One important element is the difference in energy between the ground state of both the neutral and the cation which is shown to change depending on the orientation of the system with respect to the static electric field. The depletion in the case of C7H5N is explained from the shape of the HOMO and HOMO-1. This technique can be extended to any polar molecules and for pump-probe experiment to possibly observe a geometry change thanks to the ionisation from the circularly polarised pulse.
"Probing ultrafast dynamics of chiral molecules using time-resolved photoelectron circular dichroism"
Date: 24th January 2019
Presented by: Dhirendra Singh
Journal Article: S. Beaulieau et al., Fraaday Discussions, 2016, 194, 325-348.
Measuring the ultrafast dynamics of chiral molecules in the gas phase has been a long standing and challenging quest of molecular physics. The main limitation to reach that goal has been the lack of highly sensitive chiroptical measurements. By enabling chiral discrimination with up to several 10% of sensitivity, photoelectron circular dichroism (PECD) offers a solution to this issue. In this article authors compare the PECD obtained with different light sources, from the extreme ultraviolet to the mid-infrared range, leading to different ionisation regimes: single-photon, resonance-enhanced multiphoton (MPPECD) above-threshold (ATI) and tunnel ionisation to probe the ultrafast relaxation of Fenchone molecules photoexcited in their first Rydberg states and the number of open channels depends on the ionisation regime, offering complementary pictures of the chiral molecules.
The advantage of quasi-circular HHG is that it delivers XUV pulses capable of producing a PECD signal by single-photon absorption. This enables straightforward comparison with spectroscopic studies carried out at synchrotrons and easier comparison with calculation and no effect of intermediate state.
In the ATI picture, the absorption of the fourth photon occurs when the electron is already slightly away from the ionic core, when most of the scattering into the molecular potential has happened. Since this scattering, and the associated interference of the outgoing partial waves, is what sets the value of the PECD, the MPPECD is mostly determined by the 2 + 1 process, and only slightly modified by the absorption of one additional photon.
Further, they did the time resolved measurements and showed that time-resolved PECD enables revealing dynamics much faster than the population decay of the Rydberg states, demonstrating the high sensitivity of this technique to vibronic relaxation.
"Ultrafast probing of core hole localization in N2"
Date: 29th January 2019
Presented by: Giammarco Nalin
Journal Article: M.S. Schöffler et al., Science, 2018, 320, 920-3.
In this work, M. S. Schöffler and collaborators show how it is possible to use an Auger electron as an ultrafast probe to indirectly locate the source of photoelectrons in a core photoionization process from the K-shell of an N2 molecule. The topological source of ejection of the photoelectron, and the consequent location of its hole left in the molecule, puzzled scientists up to the present times. The controversy is if the photoelectron is ejected from a molecular-like orbital (i.e. delocalised onto the molecule) or from an atomic-like orbital (i.e. localised more onto one of the two atoms). This work shed light on the question, noticing that the location of the hole depends upon the direction of ejection of the Auger electron. Consequently, the two electrons constitute an entangled Bell state.
The experimental data are compared with detailed simulations of angular distributions of both the photoelectron and the Auger electron. These simulations of the differential cross-sections of the photoelectron ejection followed by Auger relaxation process are computed to taking into account the overlapping of the intermediate N2+ states both in the delocalised assumption (just 1σg orbital taken in account) and in the localised one (sum of contribution for 1σg and 1σu orbitals). In the latter case, interference can break the symmetry of the angular distributions down.
The experiment has been performed using a COLTRIMS system; 4π sr collection for photoelectrons (9 eV), 1% sr for recoils (just the one roughly aligned with the photon direction are collected) and the Auger electron are reconstructed from momentum conservation (ca. 370 eV). CPL/R light has been used provided by the Advanced Light Source (ALS) synchrotron in Berkeley (CA, US).