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Monday, 11 December 2017

13/12-2017 Ruaridh Forbes

Strong laser-field based methods such as high-harmonic generation and strong-field ionization (SFI) are considered novel probes of ultrafast molecular dynamics. Details of an experimental femtosecond time-resolved SFI study of the excited state dynamics of NO$_{2}$ using channel-resolved above-threshold ionization (CRATI) as the probe technique will be presented. CRATI makes use of PhotoElectron-PhotoIon COincidence (PEPICO) spectroscopy to study correlations in fragmentation dynamics in molecular systems. The use of PEPICO and covariance methods allows us to correlate ATI photoelectrons associated with a particular SFI electron orbital ionization channel. In disentangling the excited state dynamics of NO$_{2}$, the complex roles of one-photon excitation, multiphoton excitation to higher-lying neutral states, non-adiabatic dynamics and several neutral and ionic dissociation channels are examined. The results will likely have implications for all SFI based time-resolved studies in polyatomic molecules. 

Monday, 13 November 2017

Sunday, 23 April 2017

2/5-2017 Morgane: Electron Dynamics upon Ionization of Polyatomic Molecules: Coupling to Quantum Nuclear Motion and Decoherence

Knowledge about the electronic motion in molecules is essential for our understanding of chemical reactions and biological processes. The advent of attosecond techniques opens up the possibility to induce electronic motion, observe it in real time, and potentially steer it. A fundamental question remains the factors influencing electronic decoherence and the role played by nuclear motion in this process. Here, we simulate the dynamics upon ionization of the polyatomic molecules paraxylene and modified bis-methylene-adamantane, with a quantum mechanical treatment of both electron and nuclear dynamics using the direct dynamics variational multiconfigurational Gaussian method. Our simulations give new important physical insights about the expected decoherence process. We have shown that the decoherence of electron dynamics happens on the time scale of a few femtoseconds, with the interplay of different mechanisms: the dephasing is responsible for the fast decoherence while the nuclear overlap decay may actually help maintain it and is responsible for small revivals.

Monday, 20 February 2017

4/4-2017 [Provisional] Susan

28/3-2017 Tom Journal Club on double on ionization of molecules with attosecond pulses

14/3-2017 Daniel Schade: Reddragon booster amplifier

7/3-2017 Gediminas: Liquid jets are awesome

A time-resolved study of ultrafast phenomena in liquids is a very challenging yet potentially rewarding scientific goal. Circular liquid jets have been used in vacuum as targets for photoelectron spectroscopy [1-2] or alternatively thin-walled cells are used for X-rays absorption experiments [3]. Here we describe an alternative approach: a time-resolved soft X-rays absorption experiment using a thin liquid sheet target in vacuum to minimise absorption of soft X-rays in normal atmosphere or cell windows. The liquid sheet targets for transient absorption experiments have to be optically flat, very thin (on the order of few μm), stable and compatible with kHz light sources to achieve good signal-to-noise ratio. Optically flat liquid targets able to operate in vacuum would also have scope for many other applications: i) laser proton acceleration, ii) plasma mirrors for contrast enhancement or iii) timing tools for XFEL experiments. Colliding circular jet technology for the generation of flat liquid sheets is a potential solution to the problem [4] but can be expensive and difficult to use in practice due to a requirement to maintain alignment of two circular jets very precisely at a specific angle in vacuum. We propose to use a single-nozzle design, which completely eliminates this alignment problem. High stability, optically flat and very thin 1.6±0.1 μm liquid jet sheet targets of isopropanol were achieved in normal atmosphere (see Figure 1) and in medium vacuum (5×10-1 mbar) by using an innovative high resolution 3D-printed nozzles.

Figure 1: Liquid jet flow in normal atmosphere. Interference fringes of the jet were recorded by illuminating liquid jet with He-Ne laser.
Acknowledgments: This work was support by EPSRC Grant EP/I032517/1. The author would like to acknowledge Dr. Avi Braun (and Nanoplasmonics group) for training and access to 3D printer.

[1] B. Winter, M. Faubel, Chem. Rev. 2006, 106, 1176-1211.
[2] T. Gladytz, B. Abel, K. R. Siefermann, Phys. Chem. Chem. Phys.2015, 17, 4926-4936.
[3] J. J. Velasco-Velez, C. H. Wu, T. A. Pascal, L. F. Wan, J. Guo, D. Prendergast, and M. Salmeron, Science 2014, 346, 831-834.
[4] M. Ekimova, W. Quevedo, M. Faubel, P. Wernet, E. T. J. Nibbering, Struct. Dyn. 2015, 2, 0543101.

Tuesday, 10 January 2017

28/2-2017 Allan Journal club on "Time-resolved x-ray absorption spectroscopy with a water window high-harmonic source"

7/2-2017 Dane Austin Journal Club on Anisotropic high-harmonic generation in bulk crystals

Article at:

31/1-2017 Daniel Greening Journal Club

In this journal club I will discuss VUV RABBITT results off Ag(111) and Au(111) surfaces (R. Locher et al Optica 2.5 (2015)) and how this relates to the current VUV surface streaking experiment in H007.

24/11-2017 Daniel Walke LCLS journal club

In this journal club I will discuss an experiment in which I participated at LCLS in October, which aimed to explore possibilities for single shot temporal characterisation of hard (~5keV) X-ray pulses (duration ~15fs)  using a cross correlation with sub 10fs optical pulses in thin solids. My role was to support the development of a hollow core fibre system to deliver the short optical pulses.
In a suitable material,  absorption of an X-ray pulse will transiently decreases the optical transmission as electrons are promoted to the conduction band. This process can then be probed in time using a non-colinear laser pulse to map the time dependent transmission to a spatially dependent transmission, which is then can then be recorded by a high frame rate camera. A variant of this technique is already regularly used at LCLS to measure the arrival time of the FEL pulses relative to optical pulses (i.e in laser pump, X-ray probe experiments). This experiment aimed to study the time dependent optical response of various samples to the X-ray pulses, which is complicated by subsequent electronic processes such as cascaded Auger decay. This, together with knowledge of the laser pulse temporal profile,  would in principle allow the FEL pulse temporal profiles to be deconvolved from the spatially dependent transmission profiles on a single shot basis.

17/1-2017 Allan: High Harmonic Half-Cycle Cutoffs Beyond the Oxygen K-edge

Using high harmonic generation (HHG) to make attosecond pulses in the soft X-ray (SXR) for time-resolved X-ray spectroscopy measurements is a topic of great interest. Recent work has demonstrated the first CEP dependent harmonic generation into the so-called "water window" region between 284 eV and 540 eV [1]. This region is of interest because there are numerous absorption edges of interesting atoms, while water is largely transparent, allowing in principle for invivo imaging and spectroscopy. Other recent work includes improved X-ray generation up to 500 eV [2] and time-resolved measurements at the carbon K-edge (284 eV) [3].
I'll present our work on generation of CEP dependent HHG across the water window (implicitly, attosecond pulses). We have generated HHG from neon and helium up to maximum energy of 650 eV, and can move our cutoff energy (and thus, attosecond pulse) to any absorption edge in the water window. Our source also exhibits excellent spatial quality, as demonstrated by a SWORD measurement, suggesting the potential for imaging applications and even, intriguingly, X-ray non-linear optics. We also perform the first X-ray absorption measurement with an HHG source at the oxygen K-edge (540 eV).