Journal Cub Meetings of 2018
||No Meeting: Meeting in Brussels
||Federico Branchi and Rim Hadidi
||No Meeting: Holiday Break
||No Meeting: Meeting in Frankfurt
||Prabhash Prasannan Geetha and Ayse Duran
"Testbeam studies of a TORCH protoype detector"
Date: 29th November 2018
Presented by: Ayse Tuba Duran
Journal Article: Brook, N.H. et al., Nuclear Inst. and Methods in Physics Research, A, 2018, 908, 256-268.
In this paper, an overview of the TORCH (Timing Of internally Reflected CHerenkov photons) detector is given, a novel detector design that combines time of flight (TOF) and DIRC (Detection of Internally Reflected Cherenkov light) techniques to provide low-momentum regime particle identification. The current design of the TORCH detector is laid out in the context of the LHCb (Large Hadron Collider beauty) experiment, for which a potential application of the TORCH detector is proposed.
TORCH project is a collaboration between CERN, Photek Ltd and The Universities of Oxford and Bristol; a five-year R&D programme was initiated with Photek to address the issues of limitation on the MCP lifetime and restricted granularity of commercial devices, culminating in the construction of a small-scale prototype TORCH module. In testbeams at CERN, this prototype operated successfully with customised electronics and readout system. A full analysis chain has been developed to reconstruct the data and to calibrate the detector. A range of (83–115)±6 ps is measured for the single-photon time resolution of TORCH. Hence the single-photon timing performance is approaching the required 70 ps per photon. A full-scale TORCH module read out by ten MCP-PMTs (660 x 1250 x 10 mm3) is currently under construction and expected to be ready for testbeam operation in 2018.
"Imaging CF3I conical intersection and photodissociation dynamics with ultrafast electron diffraction"
Date: 8th November 2018
Presented by: Prabhash Prasannan Geetha
Journal Article: Yang. J. et al., Science, 2018, 361 (6397), 64-67.
In this work, J. Yang and group presents the simultaneous experimental characterization of both one-photon and two-photon excitation channels in isolated CF3I molecules using ultrafast gas-phase electron diffraction (UGED). From the two-photon excitation channels, they mapped out the real-space trajectories of a coherent nuclear wave-packet, which bifurcates onto two potential energy surfaces when passing through a conical intersection. In one-photon excitation channel, they resolved both the umbrella and the breathing vibrational modes in the CF3 fragment in multiple nuclear dimensions. The experimental results are validated with ab-initio multiple spawning (AIMS) simulations. This technique proves that UGED can track a nuclear wave packet with atomic spatial resolution during nonadiabatic processes involving conical intersection, measuring multidimensional nuclear geometry changes and simultaneously observing dynamics form different excitation channels. This approach opens a door for studying many important problems in fundamental photochemistry.
"Imaging the square of the correlated two-electron wave function of a hydrogen molecule"
Date: 26th July 2018
Presented by: Isabel Vela-Perez
Journal Article: M. Waitz et al., Nat. Commun., 2017, 8, 2266
The work presented in this publication shows a method to image the molecular wave function of hydrogen. To that aim, they access the initial electronic wave function of the one-electron H2+ molecular ion via high energy photoelectron angular distributions. This paper also shows how to disentangle the correlated H2 two-electron wave function.
The technique used is based on the coincident detection of the reaction fragments after high energy photofragmentation. Making use of high energy circularly polarized light and detecting the high energy electrons in the polarization plane, the authors demonstrate that the photoionization differential cross section in the electron emission direction (the so‑called molecular frame photoelectron angular distribution, MFPAD) is merely proportional to the square of the Fourier transform of the initial state.
Moreover, they rely on the detected kinetic energy release of the resulting ions to recognise the quantum state of the bound electron. Then, the projection of that identified state onto the state of the detected electron is used to study the two electron phase.
The dependence of the momentum distribution on the internuclear distance is also studied.
"Observation of enhanced chiral asymmetries in the Inner-Shell photoionization of uniaxially orientated methyloxirane enantiomers"
Date: 21st June 2018
Presented by: Rim Hadidi
Journal Article: Tia M. et al., J. Phys. Chem. Lett., 2017, 8, 2780.
PECD is sensitive to subtle changes of the molecular potential (shape and structure) both static and dynamic, It shows maximum values between 3% and 4% for the randomly oriented molecules. For non-racemic mixtures of chiral molecules: PECD is even observable after averaging over all possible molecular orientations, while for achiral molecules all asymmetries cancel out for a sample of randomly oriented molecules.
After ﬁxing the molecule in space,PECD occurring is strongly enhanced and could reach 100%.
In this paper, uniaxially oriented methyloxirane molecules were studied upon O(1s)-photoionization (hw = 550 eV) using the COLTRIMS-technique and a high-resolution spectrometer (3D focusing for electrons and ions). A Photoion-photoion Coincidence spectra (PIPICO) was recorded.
Analysis of different t molecular break-ups was performed in order to study PECD of fixed in space methyloxirane.
PECD obtained after ﬁxing the fragmentation axes in space parallel to the polarization planeshows much higher asymmetries. These results support the intuitive prediction that selecting a particular 3D orientation, rather than averaging over all orientations, enables to remove any cancellation that occurs due to compensation of the PECD for different molecular orientations
(PECD) can be signiﬁcantly enhanced by ﬁxing one molecular orientation: (fragmentation) axis in space, which supports the intuitive picture of the photoelectron scattering on the molecular potential being at the heart of the PECD effect.
"Molecular Orbital Imprint in Laser-Driven Electron Recollison"
Date: 21st June 2018
Presented by: Federico Branchi
Journal Article: Schell F., et al., Science Advances, 2018, 4, 5
The three-step-model is well established in the description of the interaction of intense femtosecond laser pulses with atom and molecules, which is the basis of attosecond physics. In the first step, the electron is removed by tunnel ionization; then, it propagates driven by the oscillating laser field and finally might re-collide with the parent ion.
A usual assumption is that, during the propagation step, information on the originating orbitals is “washed out”.
The work reported in this article aimed to test this assumption. Strong-field ionization and re-collision is investigated in 1,3-trans-butadiene. In this molecule, strong-field ionization mostly occurs from two orbitals, characterized by remarkably different electronic densities. Both experiments and theory were able to access the dynamics of electrons stemming from these different orbitals, separately, and show that the rescattering probability significantly differ; this leads to the conclusion that in the propagation step the initial molecular orbital structures are retained and even enhanced, rather than washed out. These findings have implications for the further development of attosecond science in molecular systems.
"Photoelectron Circular Dichroism (PECD)"
Date: 15th May 2018
Presented by: Hassan Ganjitabar
Journal Articles: Turchini,S., J Phys condens Matter, 2017, 29, 503001., Fanood M.M.R. et al., ChemPhysChem, 2018, 19, 921-933., Fanood, M.M.R. et al., J Chem Phys, 2016, 145, 124320., Kastner, A. et al., J Chem Phys., 2017, 147, 013926., Comby A., et al., J Phys Chem Lett., 2016, 7, 4514-4519.
Photoelectron circular dichroism (PECD) is the forward-backward asymmetry in the photoelectron angular distribution (PAD) when circularly polarized light (CPL) ionizes chiral molecules. The phenomena was initially predicted by Ritchie in 1976 and successful measurements were taken by Bo¨wering et al. in 2001 using synchrotron radiation. For a decade, people continued such PECD measurements using synchrotron radiation to establish the concept. Recently, a prospect is enhanced by doing multiphoton ionization (MPI) using laser light. The goal of the recent PECD measurements are to study the effect of different factors on the PECD. With MPI one can study factors like conformational effects , vibrational quantum states , intermediate state dependence [3, 4] and ultrafast dynamics .
In my presentation, I have referenced multiple studies which focus on the aforementioned factors. The papers which may be more interesting are those studying the role of intermediate state in a pump-probe ionization scheme. In this case, the pump wavelength can excite the molecules and at the same time align them, provided the matrix element for such an excitation possess some electric dipole moment. This could be a link between PECD measurements and molecular frame (MF) PAD.
1.Turchini, S., Conformational effects in photoelectron circular dichroism. Journal of Physics: Condensed Matter, 2017. 29(50): p. 503001.
2.M., R.F.M., et al., Intense Vibronic Modulation of the Chiral Photoelectron Angular Distribution Generated by Photoionization of Limonene Enantiomers with Circularly Polarized Synchrotron Radiation. ChemPhysChem, 2018. 19(8): p. 921-933.
3.Fanood, M.M.R., M.H.M. Janssen, and I. Powis, Wavelength dependent photoelectron circular dichroism of limonene studied by femtosecond multiphoton laser ionization and electron-ion coincidence imaging. The Journal of Chemical Physics, 2016. 145(12): p. 124320.
4.Kastner, A., et al., Intermediate state dependence of the photoelectron circular dichroism of fenchone observed via femtosecond resonance-enhanced multi-photon ionization. The Journal of Chemical Physics, 2017. 147(1): p. 013926.
5.Comby, A., et al., Relaxation Dynamics in Photoexcited Chiral Molecules Studied by Time-Resolved Photoelectron Circular Dichroism: Toward Chiral Femtochemistry. The Journal of Physical Chemistry Letters, 2016. 7(22): p. 4514-4519.
"Three-Dimensional Molecular Alignment Inside Helium Nanodroplets"
Date: 28th February 2018
Presented by: Jacqueline Arlt
Journal Article: Chatterley, A.S. et al., Phys. Rev. Lett., 2017, 119, 073202.
Embedding molecules in Helium nanodroplets is attractive, because the droplets provide a dissipative, superfluid environment that is transparent for photon energies up to 20 eV.
Aligning molecules in all three spatial dimensions is a means to achieve measurements in the molecular frame.
The work of this paper shows the three-dimensional spatial alignment of 3,5-dichloroiodobenzene (DCIB) molecules embedded in helium nanodroplets. If the non-axial recoil is taken into account, the alignment of DCIB inside helium droplets is slightly higher than the one achieved from DCIB as a free gas. Furthermore, regarding the time-dependence of the alignment, both traces follow the shape of laser pulse, but the alignment of the molecules inside helium nanodroplets is shifted to later times. This could provide more time for field-free measurements.