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Department Locally-Sensitive & Time-Resolved Spectroscopy

The Liquid & Interfacial Dynamics with Ultrafast X-rays (LIDUX) Facility

Within the Liquid & Interfacial Dynamics with Ultrafast X-rays (LIDUX) group we are developing and applying state-of-the-art infrastructure to enable ultrafast time-resolution, bulk and interfacial condensed-phase electronic structure measurements. Through a combination of few-femtosecond time-resolution and the element-, electronic-state-, and chemical-environment-selectivity afforded by soft X-ray spectroscopy, table-top experimental probes of evolving molecular structure will be facilitated at the limiting temporal and spatial scales of chemistry. These probes will be applied to understand, and potentially control, the dynamics and mechanisms of (1) physicochemical and photobiological processes, (2) the early stages of homogeneous and heterogeneous photocatalysis, as well as (3) charge separation, recombination, transfer, and propagation phenomena in condensed phase samples.

Laboratory Overview

The realization of the LIDUX laboratory involves four principal challenges:

Laser Systems

We adopt and develop next-generation, high-average-power, high-repetition-rate ultrashort pulse laser systems with the aim of producing a spectroscopically useful source of attosecond to few-femtosecond duration soft X-ray pulses. Specifically, a 400 W, 1-ps-pulse-duration thin-disk Yb:YAG regenerative amplifier (RGA) laser is applied as an ideal pump laser source for a pair of few-cycle, optical parametric chirped pulse amplification (OPCPA) lasers operating in the near-infrared (NIR) and mid-infrared (MIR) spectral ranges. These laser systems and a range of non-linear optical techniques are utilized to produce tunable, inherently synchronized, few-femtosecond pulses spanning the MIR to soft X-ray spectral range. The generated pulses are ideally suited for utilization in high-time-resolution, high-data-acquisition-rate, pump-probe experiments.

Ultrashort Pulse VUV, EUV, & Soft X-ray Generation

We utilize gas-phase high harmonic generation (HHG) - driven by ultrashort driving laser pulses in the UV, NIR, or MIR spectral ranges – to produce ultrashort vacuum ultraviolet (VUV), extreme ultraviolet (EUV), or soft X-ray pulses. This non-perturbative, extreme non-linear optical phenomenon intrinsically generates spatially and temporally coherent ionizing radiation that is particularly well-suited to ultrafast time-resolved spectroscopy applications. With the aim of maximizing ultraviolet-to-VUV and MIR-to-soft-X-ray radiation conversion efficiencies and the associated ionizing radiation fluxes, we are actively researching optimum generating gas density profiles and driving laser conditions. Accordingly, our developed secondary (HHG) laser sources are expected to produce EUV and soft X-ray fluences of 1014-16 photons s-1 and 1010-11 photons s-1 in broadband spectral ranges spanning 8-100 eV and 100-700 eV, respectively.

Characterisation, Manipulation, & Utilisation of Ultrashort X-ray Pulses

By driving the HHG process with few-cycle laser pulses, we generate isolated attosecond pulses or few-femtosecond duration attosecond pulse trains. The time-domain structure of this radiation corresponds to respective continuous or comb-like profiles in the frequency domain. To analyse the temporal structure of such radiation, we utilize cross-correlation-based techniques; employing additional, pre-characterised few-cycle optical pulses. To analyse the spectral properties of the generated EUV and soft X-ray light, we employ photoelectron spectroscopy or single-shot, ultrabroadband EUV/soft X-ray spectrometer systems.

The generated and characterized ultrashort, ultrabroadband EUV and soft X-ray pulses are excellently suited to single-shot, time-resolved EUV and soft X-ray absorption spectroscopy (TR-XAS) experiments. By carefully ‘monochromatizing’ such light sources, femtosecond time-resolved EUV and soft X-ray photoelectron spectroscopy (TR-XPS) experiments will also be enabled. To facilitate such applications, the high-intensity HHG driving laser must be filtered from the useful VUV, EUV, or soft X-ray radiation and the generated ionizing radiation must be manipulated (at least refocused) for utilization in experiments. A primary challenge is achieving both of these things while maintaining ultrashort pulse durations and maximizing the transfer efficiency of the generated ionizing radiation to experiments. We are co-developing and applying novel, bespoke, high-efficiency X-ray optical assemblies with NP-AOS and FG-ANT to address these challenges and to bring the LIDUX facility into operation.

Soft X-ray Spectroscopy & the Push Towards Few-Femtosecond Time-Resolution

We have a keen interest in the electronic structure of liquid water and aqueous solutions within our research group and have a particular focus on developments and applications of the liquid microjet photoelectron spectroscopy (LJ-PES) technique. We enjoy a close collaboration with the research group of Bernd Winter (Fritz-Haber Institut, FHI, Berlin) and perform energy-resolved LJ-PES experiments at a number of synchrotron light sources (BESSY II, PETRA III, and SOLEIL). Following the LIDUX laboratory commissioning phase, a range of such energy-resolved LJ-PES experiments will be facilitated in-house in the VUV, EUV, and lower soft X-ray range. Furthermore, ultrafast TR-XPS and TR-XAS experiments will be enabled, allowing ultrafast physicochemical transformations and their associated mechanisms to be interrogated in real-time.

Upon completion, the LIDUX facility is expected to yield a flexible, table-top light source with the following characteristics:

  • Sub-15-fs, tunable pump pulses spanning 0.4-8.0 eV photon energies
  • Tunable 1 fs-15 fs duration probe pulses spanning 25-700 eV (inherently synchronised to the aforementioned pump pulses)
  • <25 µm EUV and soft X-ray focal spot sizes (at 1/e2)
  • 15 fs & 50 fs time resolution modes for TR-XPS with respective ~400 meV and ~150 meV energy resolutions and 1 fs to 3 ns delay ranges
  • Sub-10-fs time-resolution TR-XAS modes of operation with up to 2000 resolving powers (ΔE/E) and 1 fs to 3 ns delay ranges
  • 1014-108 (monochromatised) photons s-1 between 25-700 eV (dependent on the utilised driving laser)
  • 1012-13 (average) / 1022-23 (peak) photons s-1 mm-2 mrad-2 (0.1% bandwidth)-1 at 400 eV

Collectively, the LIDUX facility will offer a relatively inexpensive, more readily accessible alternative to X-ray free electron laser facilities in situations where linear, ultrafast time-resolution probe techniques (e.g. TR-XPS and TR-XAS) are required for the interrogation of condensed phase samples. The facility will also offer a higher time-resolution complement to BESSY VSR, albeit with lower average brilliance.

Timeline & Current Status

A staged commissioning approach is adopted in bringing the LIDUX facility into operation. First experiments are planned in 2020, initially with photon energies up to ~150 eV. Subsequent developments will facilitate experiments up to ~350 eV and then ~700 eV in 2021/2022 and 2022/2023, respectively.

With the bespoke laboratory space in place and majority of our laser systems now installed and tested, we are currently commissioning our EUV and lower soft X-ray range beam lines for use from 8-350 eV. These beam lines are expected to be upgraded to cover the 25-1000 eV energy range in 2022.