Final Report Summary - SIXPAC & T-REX (Single-shot X-ray Pulse duration Acquisition (SiXPAc)<br/>&<br/>Time-Resolved X-ray spectroscopy on molecules (T-ReX))
Three-dimensional (3D) imaging techniques for the investigation of the spatial composition of crystalline materials and biological specimens have over the last decades been steadily refined to their current nano-scale resolution. However, nearly all of the methods relying on X-ray or electron 3D tomography up to date only deal with static samples and cannot convey simultaneous information about structural changes. One very promising approach in this direction is to make use of ultrabright state-of-the-art X-ray sources, so-called free-electron lasers (FELs), which are able to routinely produce coherent X-ray radiation with unprecedented intensities. Combining the high spatial resolution and the ultrashort duration of X-ray pulses for a full 4D characterization of the complex samples has been a long-standing goal of material and biological science.
Due to the ultrashort pulse durations of these FELs so far no measuring technique has been able to precisely determine the structure or the length of these pulses in the time domain, which is a necessary prerequisite for the successful implementation of the experiments mentioned above. In the first step of the envisioned project at the Linac Coherent Light Source (LCLS) we aim to demonstrate an experimental technique that will be capable of measuring the X-ray pulse durations for every single shot with a precision of a few 100 attoseconds utilizing photoelectron streaking spectroscopy.
The most important part of the project and prerequisite of any measurement with ultrafast temporal resolution therefore was the characterization of the FEL pulse duration and its substructure on a few-femtosecond or even attosecond time scale (SiXPAc). On this behalf the Marie Curie fellow has set up an experiment during the first 18 months at the LCLS relying on a flat-top streaking pulse shape with a carrier wavelength of ~6 µm. A beam time for this measurement was successfully implemented and conducted from May 30th to June 3rd 2014 at the atomic, molecular and optical (AMO) end station at LCLS (amoc8114).
The extensive data from this measurement is at the moment still being analysed in a major international collaboration between TUM, LCLS, University of Gothenburg, Sweden and Dublin University, Ireland. The results include the first direct characterization of the temporal structure of two-colour FEL pulses, for which the deployed method allows not only the determination of the duration of each of the two pulses independently, but also their temporal separation at the same time.
These streaking measurements will be compared to simultaneous pulse duration measurements with the XTCAV (1) to give insight into the relative merits of these complementary techniques. These findings may lead to assist the development of envisioned ultrafast (attosecond) FEL pulse shaping (2) and will pave the way to integrated online diagnostic tools for future ultrashort-pulse and ultrahigh-repetition rate X-ray laser systems.
In parallel, and as a further step to learn more about the temporal characteristics of FEL pulses in various modes of operation, the Marie Curie fellow has been awarded another beam time at the LCLS with a completely novel approach for FEL pulse structure characterization—that is the application of the attoclock technique (3) to the field of SASE free-electron lasers. This method of angular-resolved streaking is a combination of using a nearly circularly polarized IR dressing field and a multi-time-of-flight (TOF) detector that consists of a ringlike array of 16 TOF detectors arranged in a circle at right angles to the propagation direction of the FEL and the co-linear streaking beams (4) .
This beam time (amoh5215) has taken place from March 27th to March 31st 2015 again at the AMO experimental end station at LCLS. A group from DESY/XFEL from Hamburg, Germany (J. Grünert, J. Viefhaus, J. Buck) was supporting us with the multi-TOF detector and was also present during the measurement. For the first time, we were able to measure 16 simultaneous streaking spectra with various settings of the FEL. We were using different kinds of target atoms and also looking at distinct electronic states (Ne 1s, Ne KLL Auger & Xe 3d3/2) to compare the measurement at different electron kinetic energies and for different emission characteristics. In the last shift we used the FEL in a two-colour mode with variable relative delays. We hope to show that we can measure the individual durations, the respective photon energies and the temporal separation of the two X-ray pulses all at same time and on a single-shot basis, while the relative strength of the streaking effect gives information about the arrival time shift of the pulses with respect to the laboratory frame from shot to shot.
A highlight for the project has been the publication of the first direct measurement of few-femtosecond pulse durations at an FEL in the peer-reviewed scientific journal Nature Photonics by an international collaboration led by the Marie Curie fellow (5) . These results open the door to a plethora of new measurements at FELs that rely on few-femtosecond and even sub-femtosecond temporal resolution. The publication is attached to this summary in a pdf version and is in full accordance with the main goal of the Marie Curie project.
The second important control parameter for any pump/probe measurement is the relative timing of the two employed pulses, which should be known with a precision of the same order of magnitude as the duration of the laser pulses. This is especially difficult at a SASE FEL source due to the inherently stochastic nature of the X-ray pulse generation and the corresponding shot-to-shot timing jitter with respect to any external pump laser in the lab frame. Fortunately, just recently a novel technique for measuring this jitter with sub-femtosecond precision has been demonstrated at the LCLS. This exciting measurement scheme and evaluation principle has just been published in July 2014 in the high-impact journal Nature Photonics (6) with major contributions by the Marie Curie fellow.
As a preparatory test case for the project part of time-resolved X-ray spectroscopy on molecules (T-ReX),.an X-ray pump/X-ray probe experiment on the molecular dynamics of oxygen has been conducted in the AMO hutch (amoi0113) under the guidance of Dr. Coffee. The obtained data is currently also being analysed with the help of the Marie Curie fellow.
In addition the development of a suitable UV source for the pumping of the molecule under investigation has been driven forward. For this purpose, an existing regenerative amplifier laser system at the Accelerator Structure Test Area (ASTA) laboratory at SLAC, running at 1 kHz and 800 nm central wavelength, has been modified and extended with a hollow-core fiber (HCF) to broaden the spectrum and produce sub-ten femtosecond laser pulses with pulse energies between 500 µJ up to 1 mJ. The setup of a suitable low-harmonics generation vacuum chamber has been finished and all the necessary components for the UV generation and characterization have been identified and ordered. Since the original laser laboratory is no longer available, the actual build-up of the ultrashort UV pulse source will happen in a new lab at the SLAC site, which was not possible during the time of the fellowship. Nevertheless, the finalization of the setup will happen in permanent communication with the Marie Curie fellow and the design is being built on the concept developed for the Marie Curie project. It is planned for the Marie Curie fellow to visit the lab at SLAC and supervise the work in the critical phase of the setup.
For the upcoming round of experiments at LCLS beginning in March 2016, a UV pump/X-ray probe spectroscopy measurement has been devised and will be submitted, which will investigate the fundamental ionization dynamics of liquid water. In this proposal we take the dynamics of the so-called hydrated electron, a solvated electron in liquid water, as a prototypical example of a photo-chemical change and turn our attention to the ubiquitous chemical action of solute molecular interaction with the neighbouring radical and solvent environment. By photo-triggering a transient radical in water, followed by time-resolved Auger probing of the evolving solute–radical system, we will carry a traditionally gas-phase experimental paradigm into the solution phase.
In conclusion, two beam times for single-shot X-ray pulse duration acquisition (SiXPAc) have successfully been conducted at the LCSL during the Marie Curie fellowship. The first direct measurement of few-femtosecond and even sub-femtosecond pulse durations at an FEL have been published in the peer-reviewed scientific journal Nature Photonics by an international collaboration led by the Marie Curie fellow (5) . Another publication about a novel technique for measuring the shot-to-shot timing jitter of a SASE FEL with respect to any external pump laser in the lab frame with sub-femtosecond precision has been accepted by Nature Photonics (6) with major contributions by the Marie Curie fellow. For the project part of time-resolved X-ray spectroscopy on molecules (T-ReX), the basic setup of a suitable UV source for the pumping of the molecule under investigation has been developed and prepared. A proposal for a UV pump/X-ray probe spectroscopy measurement has been submitted to LCLS by an international academic collaboration with the Marie Curie Fellow as a Principal Investigator. By UV-triggering a transient radical in water, followed by time-resolved Auger probing of the evolving solute–radical system with an ultrashort FEL pulse, we plan to investigate the fundamental ionization dynamics of liquid water.
1. C. Behrens et al., Few-femtosecond time-resolved measurements of X-ray free-electron lasers. Nat. Commun. 5, 3762 (2014).
2. A. Zholents, W. Fawley, Proposal for Intense Attosecond Radiation from an X-Ray Free-Electron Laser. Phys. Rev. Lett. 92, 224801 (2004).
3. P. Eckle et al., Attosecond angular streaking. Nat. Phys. 4, 565–570 (2008).
4. E. Allaria et al., Control of the Polarization of a Vacuum-Ultraviolet, High-Gain, Free-Electron Laser. Phys. Rev. X. 4, 041040 (2014).
5. W. Helml, A. R. Maier & R. Kienberger et al., Measuring the temporal structure of few-femtosecond free-electron laser X-ray pulses directly in the time domain. Nat. Photonics. 8, 950–957 (2014).
6. N. Hartmann, W. Helml & R. N. Coffee et al., Sub-femtosecond precision measurement of relative X-ray arrival time for free-electron lasers. Nat. Photonics. 8, 8–11 (2014).
Due to the ultrashort pulse durations of these FELs so far no measuring technique has been able to precisely determine the structure or the length of these pulses in the time domain, which is a necessary prerequisite for the successful implementation of the experiments mentioned above. In the first step of the envisioned project at the Linac Coherent Light Source (LCLS) we aim to demonstrate an experimental technique that will be capable of measuring the X-ray pulse durations for every single shot with a precision of a few 100 attoseconds utilizing photoelectron streaking spectroscopy.
The most important part of the project and prerequisite of any measurement with ultrafast temporal resolution therefore was the characterization of the FEL pulse duration and its substructure on a few-femtosecond or even attosecond time scale (SiXPAc). On this behalf the Marie Curie fellow has set up an experiment during the first 18 months at the LCLS relying on a flat-top streaking pulse shape with a carrier wavelength of ~6 µm. A beam time for this measurement was successfully implemented and conducted from May 30th to June 3rd 2014 at the atomic, molecular and optical (AMO) end station at LCLS (amoc8114).
The extensive data from this measurement is at the moment still being analysed in a major international collaboration between TUM, LCLS, University of Gothenburg, Sweden and Dublin University, Ireland. The results include the first direct characterization of the temporal structure of two-colour FEL pulses, for which the deployed method allows not only the determination of the duration of each of the two pulses independently, but also their temporal separation at the same time.
These streaking measurements will be compared to simultaneous pulse duration measurements with the XTCAV (1) to give insight into the relative merits of these complementary techniques. These findings may lead to assist the development of envisioned ultrafast (attosecond) FEL pulse shaping (2) and will pave the way to integrated online diagnostic tools for future ultrashort-pulse and ultrahigh-repetition rate X-ray laser systems.
In parallel, and as a further step to learn more about the temporal characteristics of FEL pulses in various modes of operation, the Marie Curie fellow has been awarded another beam time at the LCLS with a completely novel approach for FEL pulse structure characterization—that is the application of the attoclock technique (3) to the field of SASE free-electron lasers. This method of angular-resolved streaking is a combination of using a nearly circularly polarized IR dressing field and a multi-time-of-flight (TOF) detector that consists of a ringlike array of 16 TOF detectors arranged in a circle at right angles to the propagation direction of the FEL and the co-linear streaking beams (4) .
This beam time (amoh5215) has taken place from March 27th to March 31st 2015 again at the AMO experimental end station at LCLS. A group from DESY/XFEL from Hamburg, Germany (J. Grünert, J. Viefhaus, J. Buck) was supporting us with the multi-TOF detector and was also present during the measurement. For the first time, we were able to measure 16 simultaneous streaking spectra with various settings of the FEL. We were using different kinds of target atoms and also looking at distinct electronic states (Ne 1s, Ne KLL Auger & Xe 3d3/2) to compare the measurement at different electron kinetic energies and for different emission characteristics. In the last shift we used the FEL in a two-colour mode with variable relative delays. We hope to show that we can measure the individual durations, the respective photon energies and the temporal separation of the two X-ray pulses all at same time and on a single-shot basis, while the relative strength of the streaking effect gives information about the arrival time shift of the pulses with respect to the laboratory frame from shot to shot.
A highlight for the project has been the publication of the first direct measurement of few-femtosecond pulse durations at an FEL in the peer-reviewed scientific journal Nature Photonics by an international collaboration led by the Marie Curie fellow (5) . These results open the door to a plethora of new measurements at FELs that rely on few-femtosecond and even sub-femtosecond temporal resolution. The publication is attached to this summary in a pdf version and is in full accordance with the main goal of the Marie Curie project.
The second important control parameter for any pump/probe measurement is the relative timing of the two employed pulses, which should be known with a precision of the same order of magnitude as the duration of the laser pulses. This is especially difficult at a SASE FEL source due to the inherently stochastic nature of the X-ray pulse generation and the corresponding shot-to-shot timing jitter with respect to any external pump laser in the lab frame. Fortunately, just recently a novel technique for measuring this jitter with sub-femtosecond precision has been demonstrated at the LCLS. This exciting measurement scheme and evaluation principle has just been published in July 2014 in the high-impact journal Nature Photonics (6) with major contributions by the Marie Curie fellow.
As a preparatory test case for the project part of time-resolved X-ray spectroscopy on molecules (T-ReX),.an X-ray pump/X-ray probe experiment on the molecular dynamics of oxygen has been conducted in the AMO hutch (amoi0113) under the guidance of Dr. Coffee. The obtained data is currently also being analysed with the help of the Marie Curie fellow.
In addition the development of a suitable UV source for the pumping of the molecule under investigation has been driven forward. For this purpose, an existing regenerative amplifier laser system at the Accelerator Structure Test Area (ASTA) laboratory at SLAC, running at 1 kHz and 800 nm central wavelength, has been modified and extended with a hollow-core fiber (HCF) to broaden the spectrum and produce sub-ten femtosecond laser pulses with pulse energies between 500 µJ up to 1 mJ. The setup of a suitable low-harmonics generation vacuum chamber has been finished and all the necessary components for the UV generation and characterization have been identified and ordered. Since the original laser laboratory is no longer available, the actual build-up of the ultrashort UV pulse source will happen in a new lab at the SLAC site, which was not possible during the time of the fellowship. Nevertheless, the finalization of the setup will happen in permanent communication with the Marie Curie fellow and the design is being built on the concept developed for the Marie Curie project. It is planned for the Marie Curie fellow to visit the lab at SLAC and supervise the work in the critical phase of the setup.
For the upcoming round of experiments at LCLS beginning in March 2016, a UV pump/X-ray probe spectroscopy measurement has been devised and will be submitted, which will investigate the fundamental ionization dynamics of liquid water. In this proposal we take the dynamics of the so-called hydrated electron, a solvated electron in liquid water, as a prototypical example of a photo-chemical change and turn our attention to the ubiquitous chemical action of solute molecular interaction with the neighbouring radical and solvent environment. By photo-triggering a transient radical in water, followed by time-resolved Auger probing of the evolving solute–radical system, we will carry a traditionally gas-phase experimental paradigm into the solution phase.
In conclusion, two beam times for single-shot X-ray pulse duration acquisition (SiXPAc) have successfully been conducted at the LCSL during the Marie Curie fellowship. The first direct measurement of few-femtosecond and even sub-femtosecond pulse durations at an FEL have been published in the peer-reviewed scientific journal Nature Photonics by an international collaboration led by the Marie Curie fellow (5) . Another publication about a novel technique for measuring the shot-to-shot timing jitter of a SASE FEL with respect to any external pump laser in the lab frame with sub-femtosecond precision has been accepted by Nature Photonics (6) with major contributions by the Marie Curie fellow. For the project part of time-resolved X-ray spectroscopy on molecules (T-ReX), the basic setup of a suitable UV source for the pumping of the molecule under investigation has been developed and prepared. A proposal for a UV pump/X-ray probe spectroscopy measurement has been submitted to LCLS by an international academic collaboration with the Marie Curie Fellow as a Principal Investigator. By UV-triggering a transient radical in water, followed by time-resolved Auger probing of the evolving solute–radical system with an ultrashort FEL pulse, we plan to investigate the fundamental ionization dynamics of liquid water.
1. C. Behrens et al., Few-femtosecond time-resolved measurements of X-ray free-electron lasers. Nat. Commun. 5, 3762 (2014).
2. A. Zholents, W. Fawley, Proposal for Intense Attosecond Radiation from an X-Ray Free-Electron Laser. Phys. Rev. Lett. 92, 224801 (2004).
3. P. Eckle et al., Attosecond angular streaking. Nat. Phys. 4, 565–570 (2008).
4. E. Allaria et al., Control of the Polarization of a Vacuum-Ultraviolet, High-Gain, Free-Electron Laser. Phys. Rev. X. 4, 041040 (2014).
5. W. Helml, A. R. Maier & R. Kienberger et al., Measuring the temporal structure of few-femtosecond free-electron laser X-ray pulses directly in the time domain. Nat. Photonics. 8, 950–957 (2014).
6. N. Hartmann, W. Helml & R. N. Coffee et al., Sub-femtosecond precision measurement of relative X-ray arrival time for free-electron lasers. Nat. Photonics. 8, 8–11 (2014).
