Ultrafast X-ray Photochemistry at European XFEL: Capabilities of the Femtosecond X-ray Experiments (FXE) Instrument

Round 1

Reviewer 1 Report

            The article presents the opportunities of the Femtosecond X-ray Experiments (FXE) instrument at the European XFEL based on the installed at present equipment. The authors give the overview of the possible and already performed Femtosecond-resolved x-ray experiments at other facilities and describe the present equipment at the European XFEL. The first results of x-ray emission spectra around the simultaneously recorded K-alfa and K-beta lines from the [Fe(bpy)₃]²⁺  complex, excited by femtosecond pulses of 400 nm laser, with ~ 125 fs  temporal resolution are presented. The WAXS pictures for 1, 10 and 50 ps delay times were obtained for the same object. The results demonstrate “the state of art” of the European XFEL at present.

            It is clear that the work presents the research frontiers in the synchrotron science and should be very interesting for readers.

 

However I would suggest some text corrections for better understanding.

 

From the Abstract and Conclusions I would rule out the sentences with general statements (like two first ones in the Abstract) and instead describe the first obtained results in more details. The general situation with FXE is more or less presented in the Introduction with good list of references. Besides I do not agree that the femtosecond X-ray experiments are “widely used”, they are only in fast progress. I would add to the description of Figs.3,4 that not only “the Kα and Kβ line strengths are not fully identical”, but also a short physical explanation of the reason why the excitations of the valence electronic shell change the Kα and Kβ emission spectra and what is the final (as well as the intermediate ones) state of the excited [Fe(bpy)₃]²⁺ complex. The expressions like “the laser pumped spectra” are slang. The axes of the map in Fig.3 A are not given. (The vertical one is the fluorescence photon energy and the horizontal one ?) I do not see the shaded regions in Fig. 3 (b) and (c)). I do not understand the comment in the caption to Fig.3 “Error bars, ±SD.” It is written that the difference scattering curves (in Fig.5) demonstrate “the influence of the solution heated by the laser pulse”. What is the concrete physical result? The comment in brackets “(1.6 K versus 4.6 K)“ should be explained more clear. The changeover to the another X-ray energy in the next paragraph (line 287) is unexpected. It is not clearly presented that the authors describe the results published in the another paper [50] based on the experiment performed at the ESRF. Probably these results are somehow not wanted here. Or they can be delivered in another way. I do not like the term ”nuclear structure” relative to the nuclear positions and field created by nuclei. Such expression is now used, but the nuclear physicists do not like it. The description of the scanning mode for the “femtosecond x-ray absorption spectroscopy at FXE”  is not clear: “The femtosecond time resolution can be preserved by adapting the laser time delay to the varying x-ray pulse arrival times during such monochromator scans which change the x-ray beam path length to the sample in a predictable way“. In what order the photon energy and the delay time will be varied? The references in the Introduction are not in the right order.

 

The article definitely should be published but with some text corrections.

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The introduction (lines 28-86) and section 3 (lines 173 - 213) are duplicate sections in terms of content and should therefore be merged. Also the citations are not ordered correctly in the sequence in which they appear. Parts of the FXE instrument were already presented in J. Synchrotron. Rad. 26 1432-1447 2019 (Ref 39.) and in this view Section 2 could be significantly shortened. For example the structure of the bunch trains, the beamline layout, spectrometers and detectors were already described.
The caption of figure 1 which contains quite some repetitions compared to the main text. In the X-ray emission spectroscopy part, RIXS and HERFD-XAS are discussed but not demonstrated experimentally. While it can be agreed that the Johann spectrometer might be more suitable than the von Hamos spectrometer for the second type of application, I do not agree that this is the case for the first type of application. Indeed, with a Johann spectrometer both the emission and the incidence energies need to be scanned and such work at an XFEL requires proper intensity normalization over a series of pulses. How can a Johann spectrometer then be more efficient than a von Hamos spectrometer for such a type of experiment? Indeed, the von Hamos spectrometer records a complete XES spectrum for each pulse with no moving components and should hence be more efficient in terms of number of pulses and time required to record an XES spectrum within a region of interest for a given incident photon energy. X-ray absorption spectroscopy is only introduced but not demonstrated. Here the authors could point out to works realized at different XFELs in both the monochromatic mode and the transmissive mode using 2 spectrometers. The latter approach, although using a different spectrometer, is demonstrated in Phys. Rev. B 92, 024108, 2015but not cited. The limitations of each mode should then be discussed if no experimental data is shown as the authors have sufficient expertise from their works at other facilities. For example, it can be guessed that the pulse power deposited on the sample and the time resolution which can be achieved are different. While emphasizing the benefit of combined X-ray spectroscopy and WAXS experiments and making a case for such studies in the Section 1 and 3, the authors do not demonstrate a corresponding experimental work. Proof-reading the manuscript to improve the readability (avoid long sentences containing multiple statements) and removing typos (line 136 “think”, page 7 “intermetitate”) would be beneficial to the manuscript.
In Figure 2 hardly anything can be recognized.
The abbreviations XTD, MLCT, HS are not introduced.
The benefit of a 500 µm thick sensor is repeated on line 136 and line 250.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

The authors present a very well-explained overview of "first experiments" using a new instrument which combines spectroscopic and scattering techniques at an XFEL. This is quite interesting work that would benefit from some minor revision, and a little more explanation of some of the techniques (and significance of the findings) would really help the non-expert.

I noticed a few English mistakes in the last paragraph of Section 1 (lines 78-87). "like e.g." (choose one), "in the fundamental" (fundamental what?), "in form of the" (I would just say "in the scientific..."). Line 136 says "think sensor" instead of "thick" Why did you specifically choose the Fe(bpy)3 complex for these experiments? Is it challenging to measure conventionally? Could you explain briefly the difference between the LPD and conventional x-ray detectors? I'm a little confused by Figure 5 (left) and the array of rectangular boxes. It looks like there are missing pixels and missing rectangles. And you say there is data from 200 shots in Figure 5 - do you effectively raster the detector (or the sample) so that each "pixel" is a given shot, or do you get multiple points in space per shot? PIN and APD detectors are not defined. Overall quality of the English is good but there are multiple spelling and grammar errors. Figure captions and legends ("Umpumped" in Fig 3) are particularly distracting.

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

The manuscript is a continuation of author's recently published work in developing Scientific instrument Femtosecond X-ray Experiments (FXE) at European XFEL. The authors demonstrated the capabilities of the instrument for monitoring photochemical reactions with femtosecond temporal resolution down to 125 fs by studying [Fe(bpy)3]2+ complex in aqueous solution. The authors also provide comparisons of the results obtained from their facility and other facility. The manuscript is detailed and well written. However, the following questions/comments should be addressed before publication.

 

Fig. 6 shows results obtained from ESRF. Why not showing the results collected at the XFEL? I would suggest removing the figure from the manuscript. It doesn’t add anything in showcasing the capabilities of the FXE instrument at XFEL There are several typos throughout the manuscript that need to be fixed. For example, in Line 160: conpressed should read compressed. Line 246: Fig. 3 (a) and (b) should be Fig. 3 (b) and (c)

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors made the essential corrections of the text and I guess the article can be published.

 

Only one question. Obviously that the Fig.3A is the detector image. The question was why the horisontal position of the Kalfa and Kbeta images is different?

Author Response

We thank the referee for his approval.

Concerning the question about the different horizontal positions of the Kalpha and Kbeta lines: this is due to the alignment of the chosen cylindrically focusing crystals (different for Kalpha and Kbeta lines), which can be slightly rotated horizontally for either emission line. Thus this is a geometric feature. The emission spectra are separated in the horizontal (focusing) direction intentionally. In this way the tails of the Kalpha and Kbeta spectra do not overlap along the energy dispersive direction (vertical on the image) and also the elastic scattering background can be easier corrected for for the two lines independently.

Reviewer 2 Report

The higher emphasis on experiments conducted and results obtained in the revised version of the manuscript was beneficial and the difference to Ref. 34 is in the terms of instrument presentation versus demonstration of experimental capabilities is obvious, as it was intended by the authors. Most of the issues raised were adressed by the authors.

However, a criticism from the first revision round has to be reiterated: the benefit of repeating similar arguments in the beginning of section 3 compared to the introduction is not clear (even the same literature is used). Indeed, the statements from lines 183-199 are still quite similar to the introduction (lines 63-87). The manscuript could be mroe concise if both parts were merged.

While the motivation to provide figure 2 is understandable, the layout and quality of the figure should be improved since even in the digital form the details provided are hard to recognize. This undermines the goal to provide a figure.

 

Author Response

We thank the referee for the valuable comments to help us improve the quality of this manuscript.

 

We have rephrased the lines 183-199 using "track changes". While the lines 63-87 describe the general methodology using different x-ray tools, the lines 183-199 now focus on the wavepacket dynamics observed in more recent studies.

 

We have modified Fig. 2 for clarity, as requested.