Workshop report ‘New instrument for ultrafast spectroscopy of small quantum objects at FLASH I and II’

Session 1: New opportunities at FLASH

Stefan Düsterer from DESY and Markus Gühr from Potsdam University organized a satellite workshop at the 2017 DESY user meeting. The purpose of the meeting was to inform the user community about the URSA-PQ instrument (Ultraschnelle Roentgenspektroskopie zur Abfrage der Photoenergiekonversion in Quantensystemen), which recently received funding from the German ministry for education and research. We want to address a maximal number of users with that instrument and thus keep it as flexible as possible under the baseline funding. We thus invited the user community to express their scientific ideas and specify instrument extensions or changes, if needed. From our point of view, the workshop was extremely successful. We received very interesting ideas and comments to the instrument design and work on incorporating those ideas.
The workshop was split into two parts. In the first session, staff scientists from FLASH talked about state of the art and foreseen development with the facility. The second session was devoted to the science case of the new instrument. We had reserved ample discussion time and further slots for short contributed talks.

URSA-PQ Workshop programme

FLASH FEL characteristics

The talk of S. Schreiber was devoted to accelerator technology at FLASH. He pointed out that the FLASH accelerator is feeding two undulators, one fixed gap undulator at FLASH I and a variable gap undulator at FLASH II. The electron beam energy is thus optimized for the desired FLASH I lasing wavelength which allows for tuning the FLASH II wavelength from the FLASH I wavelength to about triple of it. While tuning the wavelength at the first, fixed gap undulator is a tedious task, tuning at the variable gap undulator is much quicker. Changing wavelength in FLASH I means stopping FLASH II operation.

The nominal tuning range of FLASH I is 4 to 52 nm for FLASH II 4 to 90 nm in the fundamental. The record in shortest wavelength so far at FLASH II 3.5 nm (3.1 nm using undulator based frequency doubling), with few μJ pulse energy however. Both undulators allow for pulse energies up to a few 100 μJ for long pulses. For short pulses in the sub 20 fs regime a low buch charge has to be used, determining pulse energy on the single μJ level. A fast kicker allows to deliver separate electron bunches into the two undulators, thus very different pulse train length patterns can be realized in the two x-ray sources. Terahertz radiation is readily available at FLASH I (BL3), at the FLASH II facility this is planned for 2018.

Among the funded upgrade plans up to 2020 are two new accelerator modules allowing for a FLASH I fundamental wavelength down to 3.7 nm, a new injector laser, low charge diagnostics, polarization controlled photons for FLASH II and a fast arrival time stabilization on the 1 fs jitter level using warm cavities. The latter is particularly interesting for any time resolved experiment allowing for stable optical pump – x-ray probe experiments, if the optical pump laser delivery gets stabilized to the same level. As of now, the jitter stabilization works within a 20 to 30 fs window. Among the non funded upgrades, the variable gap undulator for FLASH I and a HGHG seeding scheme for FLASH are most interesting for the URSA-PQ project, although much of the seeding advantages are already under control with a timing jitter feedback. Seeding would allow for additional spectral stability, which would benefit absorption and photoelectron methods for probing.

Flash optical laser

I. Hartl pointed out that laser parameters are mostly driven by the user community, which is encouraged to deliver input (‘Our job is to make your experiment possible’). The most critical parameters to decide for the laser systems at FLASH I and II are available timing delay, pulse duration, power, energy and wavelength.

FLASH I currently houses 4 beamlines (BL 1 2 3 and PG 2) and the beam goes to just one of the experiments. One cannot switch between experiments quickly without changing delay zero. The burst mode laser is limited to 400 μsec of the 800 μsec of FLASH I pulse train, it delivers 20 µJ, at 100 fs pulse length with a wavelength of 800 nm. The single pulse laser delivers 1 pulse per macrobunch with a pulse energy of 10 mJ, 60 fs duration. It can be converted to 2mJ at 400 or 0.5 mJ at 266 nm. The timing jitter is 15 fs rms, the timing drift monitor correlates x-ray and optical pulse with streak camera. At FLASH I an OPA with tunable output will be realized, the laser team will focus activities on the CAMP endstation.

For FLASH II, an OPCPA with sub 20 fs pulses with 500 μJ at a repetition rate of 50 kHz is planned. Its center wavelength is 780 nm. At 500 kHz repetition rate, 50 μJ would be also possible. The laser is planned to be available at beamline FL24 in April 2018. An interesting option is the further use of the OPCPA pump laser with 4mJ and picosecond pulsewidth at 1030 nm wavelength. This could be used for molecular alignment in some cases. In addition, this pump laser pulse will be used for nonlinear conversion of the OPCPA pulses. For instance at the REMI beamline FL 26, mixing of the 800 nm femtosecond OPCPA output with the 1030 nm pump is planned, which will lead to 450 nm. At this point the question is if our new URSA-PQ instrument could fit on the floor behind the REMI. Tunable repetition rate, center wavelength and tripled output power are short to midscale upgrades to the OPCPA system.

The relay optics to the beamlines of FLASH II is transmissive, using long, chirped pulses. The dispersion will be compensated via a final compressor at the beamline. The incoupling window to the experiment needs to be clearly definded.

FLASH beamlines

S. Düsterer talked about the beamlines at the FLASH facility. In principle, the beamlines BL2,3,PG2 at FLASH I and FL24, FL23 at FLASH II offer space for the instrument. At those stations, a 3x4m footprint is available for new instrumentation. The monochromator beamline at FLASH I is PG2, at which the fundamental as well as the 3rd, 5th, 7th undulator harmonics are available. Two different resolutions of 1000 and 12000 can be selected. At FLASH II, a time compensating monochromator is planned to sit at FL23. For C edge spectroscopy, beamlines FL24-23 seem ideal because they are not carbon coated yet.

Split and delay units are (will be) available at BL2, PG2 (FL23, FL24). Those units are wavefront splitters and one can use different filter in the two paths so that one could pump with an x-ray fundamental and probe with the third undulator harmonic and vice versa. The FLASH I BL beamlines can already be operated with circular polarization, the best transmission is 10 % at the M edges of 3d transition metals. FLASH I uses a variable linespace grating in conjunction with a MHz camera to spectrally characterize single shots, whereas an inline photoelectron spectrometer is used at FLASH II.

The control system and DAQ is realized with DOOCS and jddd with lots of online viewing options, for motors, cameras etc. An online visualization using matlab is available. The data format is for the full dataset is hdf5, a quasi online analysis can be performed 5-10 min after run using the DAQ.

URSA-PQ instrument

M.Gühr presented the general outline and features of the new URSA-PQ instrument. He motivated the instrument design by different science cases from molecular photoprotection in gas phase nucleobases, over charge transfer in small molecules to nonlinear processes with isolated atoms. The baseline design allows for the following methods: Optical excitation – x-ray probe or vice versa (also x-ray – x-ray possible), transient photoelectron probing, transient Auger probing and transient absorption probing. An effusive capillary oven will allow for evaporation of molecules up to 200 C, experiments on gases as well as evaporated liquids can be accommodated using a heated gas needle as effusive source.

The instrument will consist of a main chamber and diagnostics chamber in line after the main chamber. Both will house manipulators with diagnostic options such as YAG screens, pinholes and fast diodes. The possibility of to install a jitter monitor in the diagnostics chamber for characterizing the optical-laser jitter will be evaluated. A first design of the main chamber suggests use of DN 200 flanges for instrumentation, major instrumentation is the magnetic bottle spectrometer which is has a 1.5 to 2 m long flight tube. Additional detection of ions using this flight tube was suggested in the presentation.

The talk ended by a discussion of the different probe techniques and implementation at the FLASH beamlines for realizing the experiments.

Session 2: Scientific opportunities opened by the URSA-PQ Instrument

After a discussion session and coffee break we had continued the workshop by three invited talks about science cases for the new instrument.

P. Wernet from the Helmholz Center Berlin talked about ‘Mapping chemical interaction dynamics with photoelectron spectroscopy at FLASH’. The motivation for observing photoinduced chemical dynamics in a time resolved way is given by the demand to efficiently and selectively convert solar energy into chemical energy. Wernet concentrated on metal containing molecules that are used for instance in dye sensitized solar cells as photosensitizers. In these complexes, the electron, lattice and spin dynamics contributes to the transformation of solar energy to charge transfer into a substrate. The iron pentacarbonyl (IPC) complex is an ideal testcase in order to establish the spectroscopy on metal-centered molecules.

The FLASH facility is ideally suited to investigate the dynamics of the metal-centered molecule. With its bright femtosecond pulses from 30 to 70 eV photon energy, it allows to perform absorption or photoelectron spectroscopy at the M edge of 3d transition metals. This allows a metal centered view on the whole photoenergy conversion. In addition, the process also allows for extremely high spin and oxidation state sensitivity.

The data on IPC resulted from previous FLASH beamtimes, in which IPC was excited with UV light accomplishing a photodissociation to iron tetracarbonyl (ITC). The M edge was found to be shifted by about 2.5 eV in ITC compared to IPC. Reasoning based on orbital arguments as well as complex simulations deliver an explanation for this shift. The M-edge probe process leaves a core hole at the center iron. The hole can be screened more effective at the ITC with respect to the IPC since valence electrons can localize at a position where the ligand is missing. Wernet pointed out that the whole ligand view on the dissociation process is missing, which would require core hole probing at the carbon or oxygen K edge, which however now becomes reality at FLASH. The new URSA-PQ instrument with its characterization tools for pump-probe experiments combined with the high efficiency magnetic bottle electron spectrometer provides an optimal environment for these type of experiments. According to Wernet, the challenges for this type of science lie in better time resolution in the 30 fs range, highly tunable optical excitation and increased repetition rate delivering better signal to noise ratio. These conditions are in part fulfilled with the planned OPCPA. Wernet suggests a series of systematic studies to discover trends in photoconversion of different metal complexes.

Raimund Feifel from Gothenburg University gave an introduction into correlation and covariance experiments in the first part of his talk. The magnetic bottle electron spectrometer is a rather ideal instrument for these types of correlated particle detection schemes, since it covers a large solid angle up to 4π. Feifel presented results from various free electron laser sources around the world to demonstrate the methods and various data analysis schemes. The correlated detection of electrons give insights into the details of the x-ray induced decay. In conjunction with strong x-ray pulses from free electron lasers nonlinear processes can be investigated with that powerful tool and one can for instance separate direct from sequential double ionization. In particular, Feifel advertized the spectroscopy of negatively charged ions in crossed beam geometry to gain insight into the exotic electronic structure of ions that are of importance in astrochemistry.

R. Feifel brought up an interesting extended opportunity for this new machine. Measuring ions and electrons in coincidence could reveal more details about the molecular behavior after photoexcitation. Once, the ions themselves carry useful information about the molecular geometry displayed in the fragmentation channel and the charge carried by the fragment. In addition, correlating the kinetic energy release of ions with the electron energy can lead to a detailed picture of the molecular potential energy surfaces. Feifel presented several possible geometries in which such a ion-electron coincidence detection could be accomplished. For one, the long time of flight tube for the electrons could be used to analyze the ion fragments, when extracting the ions at a later time after the electrons have been analyzed. The long time of flight is somehow sub-optimal for ion energy analysis and a devoted instrument either orthogonal to the electron flight path of even on the opposite side to the electron flight tube. This development would go beyond the baseline design and require additional mechanics, a detector with high voltage supplies and a nanosecond pulsed high voltage extraction field.

At the end of his talk, Feifel showed a very recent study of acetylacetone isomerization from the FERMI free electron laser in Italy. The molecule was excited with ultraviolet light to accomplish the structural change via excited states. The binding energy changes on the excited molecule are pronounced and a detailed analysis of the features is currently on the way.

Tommaso Mazza from the European XFEL talked about ‘Two color investigation of core hole relaxation dynamics in atoms and clusters at FLASH’. Mazza gave an introduction into some strong field physics concepts like ponderomotive potential and side band generation applied in his experiments. In the first part of the talk, Mazza concentrated on the effect of strong optical fields on x-ray induced core hole decay of krypton performed at FLASH. The core electron is initially excited into a valence resonance. From there, the atom can either decay by a resonant Auger process or be ionized with a strong infrared field. The strong field ionization is then followed by a normal Auger process of core-ionized krypton. Those competing processes are clearly resolved in the electron spectrum. Correlation of the emitted electrons with the ions would give additional opportunity to disentangle electron spectral features. As the strong laser field intensity increases one observes a broadening of the valence absorption resonance due to its decreased lifetime and in addition a ponderomotive shift. A systematic study of this phenomenon with a tunable OPCPA fundamental would open additional opportunities to study the phenomenon and utilize intermediate resonances to enhance the effect. One could apply the phenomenon for creating controlled spectral filtering for soft x-ray seeding schemes.

The second part of Mazza’s talk was devoted to core hole decay under plasmonic excitation of metal clusters. Those plasmonic resonances determine the optical properties of the cluster and make it suitable for strong and selective spectral markers. Preliminary results from FERMI demonstrate an influence of the plasmonic resonance on the core hole decay. Future beamtimes will be needed to reveal the physical mechanism behind the process. A cluster source will be available at the European XFEL SQS station and it is a matter of design compatibility if that source could work with the URSA-PQ instrument at FLASH.

Technical discussion

In discussion about technical details of the instrument invaluable input was made by the users which could directly be intergated into the design or which will be evaluated on the basis of technical and financial feasability. The most important are:

  • dimensions of CAMP in the most important directions and flanges
  • flight tube dimension – as long as possible for range and resolution
  • ion TOF parallel use with eTOF (switcher or additional flight tube)
  • diagnostics chamber strategy (highly divergent beam, refocusing after chamber?)
  • timing diagnostics (jitter tarcking, FEL pulse length)

From these, the first two could be directly integrated into the design resulting in three DN250 CF ports with camp dimensions ad a ~190 m long flight tube (see current design for details).