Institute for Electronic Structure Dynamics
The Ultrafast Laser Laboratory for Applied Sciences (ULLAS) Facility
Ultrafast EUV spectroscopy of liquids and materials
The study of photo-induced electronic and geometric-structural dynamics is key to developing an understanding of light-initiated physicochemical processes. Within the ULLAS group, we interrogate the ultrafast phenomena that occur in biologically-relevant, catalytic, and light-harvesting materials following photoexcitation, with the aim of determining photochemical, radiobiological, photocatalytic, and solar energy conversion mechanisms. Such determinations are ultimately needed to understand the elemental steps of chemistry, mitigate radiobiological damage and develop radiotherapies, develop selective and efficient chemical processes, and accelerate the development of novel energy materials with better conversion performance. Following photoexcitation, the early-time electron and associated geometric-structural dynamics occur on a femtosecond time scale, requiring application of ultrafast transient spectroscopies to track the resulting charge, energy, and potentially mass transfer pathways.
Methods
We develop and utilize extreme ultraviolet (EUV) photoemission spectroscopy (PES) techniques within our laboratory. Ground-state sample electronic structures are readily studied using our table-top, energy-tunable, monochromatised EUV laser source. Femtosecond-time-resolved experiments are also performed using the pump-probe technique. Typically, in such experiments, a femtosecond laser pulse in the UV, visible, or IR spectral range interacts with the sample to promote valence electrons from the ground state of the molecular or material sample to an electronically-excited state. An ultrashort EUV probe pulse is then used to ionize the excited sample at variable time delays, thereby mapping the electron density distribution among the excited states onto the ionization continuum. By recording the transient kinetic energy spectra of the ionized electrons and the photoelectron angular distributions at different pump-probe time delays, we can follow the electron population dynamics on an absolute energy scale with femtosecond temporal resolution. Furthermore, we can infer information about the correlated, underlying sample geometric structure.
An expansion of the laboratory capabilities is currently underway to additionally enable broadband, EUV absorption spectroscopy across a 10-140 eV spectral range. Ground-state and femtosecond-time-resolved experiments will be respectively enabled using EUV-probe-only and optical-pump-broadband-EUV-probe laser schemes.
The ULLAS Laser & EUV Generation Laboratory
Experimental setup
A femtosecond Ti:sapphire laser system delivers pulses of 25 fs minimum duration at an 800 nm wavelength and a repetition rate of 5 kHz. This laser is used to generate both the optical pump and EUV probe beams. The optical pump beam is generated from the laser harmonics and/or with the use of an optical parametric amplifier, which facilitates the tuning of the pump wavelength in a wide range between 190 and 2500 nm (6.53 and 0.50 eV). EUV light is produced by up-converting the laser frequency using the gas-phase, high-order harmonic generation (HHG) process.
A single-optic, zone-plate monochromator is used to select a desired harmonic in the 11 to 105 eV spectral range. Monochromatized photon fluxes between 1012 and 1010 s-1 are respectively realized at the sample at energies between 10 and 105 eV, with associated 100 to 500 meV bandwidths. The monochromator is designed to maximize photon transmission to experiments and minimize the temporal broadening of the energy-selected EUV pulses, yielding a pulse duration of ~45 fs [1]. The pump and probe beams are precisely focused in the experimental chamber, onto the sample, and in front of the entrance orifice to a photoelectron spectrometer. Liquid and solid samples can be studied using this setup. In the former case, liquid micro-jet techniques are applied to facilitate the high-vacuum conditions required for extended EUV-beam propagation and electron detection.
Schematic view of the experimental setup. An ultrashort-pulse (~25 fs), near-infrared (NIR) driving laser (1) is focused into a rare-gas-filled, differentially pumped high-harmonic generation (HHG) cell (2) to generate a frequency comb of extreme-ultraviolet (EUV), ultrashort pulses. The NIR driving beam is reflected from a thin-film metallic filter (3) to isolate the ultrashort EUV pulses. Single EUV harmonics of the NIR beam are selected using a user-selectable element of a reflective-zone-plate (RZP, 4a) array and focused onto a monochromatising slit assembly (5). Alternatively, the entire EUV spectrum can be reflected and focused through the slit assembly using a toroidal mirror (TM1, 4b). The resulting EUV beam is relay imaged onto the sample using a second toroidal mirror (TM2, 7). In time-resolved experiments, an optical pump beam (8) with variable arrival time is also focused onto the sample. Experiments are commonly performed on liquid-phase targets, delivered as continuously replenished, in-vacuum liquid microjets (9). The electrons generated at the laser-sample interaction region are collected and analysed by time-of-flight-based electron spectrometer systems (10). An EUV spectrometer is also under construction (11), which will be used to characterize the EUV generation, monochromatisation, and perform condensed-phase, ground-state and excited-state, transient EUV absorption spectroscopy experiments.
Photoelectrons are detected using angle-resolved or magnetic-bottle time-of-flight electron spectrometer systems. The former instrument offers moderate electron collection efficiencies (up to 0.2 Sr) and angle-resolved-electron-kinetic-energy analysis up to 600 eV via multiple detection modes, which allow energy resolutions, electron collection efficiencies, and simultaneously detected spectral bandwidths to be tailored to individual experiments. The latter offers significantly higher collection efficiencies (>6 Sr) up to ~40 eV electron kinetic energies, without angular resolution. Experimental time and energy resolutions between 50 to 60 fs and 150 to 500 meV are generally achieved, respectively, where the lower and upper limits are representative of experiments performed with low and high probe photon energies, respectively.
After installation of an additional zone-plate-bypass, toroidal-mirror optic in the EUV beamline, a high-efficiency EUV photon spectrometer system is being developed to characterize the EUV source, the monochromatised beamline output, and enable ground-state and ultrafast transient EUV absorption spectroscopy experiments with thin liquid flat jets and solid films.