A Composite Velocity Map Imaging Spectrometer for Ions and 1 keV Electrons at the Shanghai Soft X-ray Free-Electron Laser
, Ruichang Wu 1,3, Yunfei Feng 1,3, Lifang Tian 1,3, Xiaohong Li 1,3, Jianye Huang 1,3, Zhi Liu 1,3 and Xiaojing Liu 1,3,*Round 1
Reviewer 1 Report
The manuscript reports on a new velocity map imaging spectrometer that can be operated up to 1 keV and can simultaneously measure the momentum distribution of electrons and ions. It is a useful technical development and the manuscript is well written. However, I wonder about two things:
- the resolution at 100 eV is 3 eV which sounds too much for many experiments. Normally, a sub-eV resolution is expected. A comment from the authors is expected.
- An ARTOF instrument, see for example https://scientaomicron.com/en/Components/Electron-Analysers/ARTOF-2, can operate up to 1 keV and provide a resolution of 0.1%. Is there any advantage of the proposed VMI over ARTOF for measuring the distribution of electrons?
Author Response
Thanks for the Referee’s effort, and we are delighted that the Referee considers the description of our composite VMI spectrometer as a useful technical development.
- Comment: “the resolution at 100 eV is 3 eV which sounds too much for many experiments. Normally, a sub-eV resolution is expected. A comment from the authors is expected.”
Answer: When electrons with the kinetic energy up to 100 eV can be measured, it means that all electrons with kinetic energy from 0 to 100 eV are measured within the whole 4\pi solid emission angle.
The best resolution we found is achieved by Cavanagh et al. PRA 76, 052708 (2007), who achieved dE/Ek = 0.38% at Ek = 0.9 eV. It is almost impossible to achieve such high resolution at Ek > 20 eV, because the voltage will be too high. If the electron kinetic energy is increased to about 20 eV, the typical energy resolution is about 1% https://www.velocitas-vmi.com/vmi-double-prime.html .
Of course, 3 eV resolution at 100 eV is not high to resolve closely spaced energy levels, e.g. vibration level, spin-orbit split, and so on, so it may not be suitable to high-resolution electron spectroscopy. But there are scientific cases in which the angular distribution is more important than the energy resolution, which is the scientific market that VMI spectrometer orients to. As a result, despite of its relatively low energy resolution, a VMI spectrometer is often selected instead of an ARTOF spectrometer.
- Comment: “An ARTOF instrument, see for example https://scientaomicron.com/en/Components/Electron-Analysers/ARTOF-2, can operate up to 1 keV and provide a resolution of 0.1%. Is there any advantage of the proposed VMI over ARTOF for measuring the distribution of electrons?”
Answer: Brochure from the same website reads that the energy resolution of 265 meV at 300eV kinetic energy with an angular range of \pm15 degree. Such condition is achieved at 2% energy window and 50 micron sample radius on the sample surface.
In comparison, CpVMI is characterized by measureing a whole 4pi, 100% energy window, 1500 micron sample radius and 1000 micron sample thickness.
ARTOF can not cover the angular range that is interesting for multiphoton ionization in a single measurement. We noticed that, due to a mistake, the description is dropped out of the introduction. We added it back.
We also add description of the “cookiebox” by Jens Viefhaus
Reviewer 2 Report
The manuscript describes a vital issue for the Free Electron laser community. The research is well designed in the manuscript, while minor modifications are required to improve the quality of the manuscript to reach the journal criteria. 1- Introduction: previous work needs to be presented with details to give the reader a general view of the research point.
2- Apparatus: the spic or maker of some devices is not given; please add the missing information.
3-Experimental results: Figure 3 without uncertainty error bars. Fig.3-b, the tendency of the energies resolution for 100 and 300 eV between 15 and 20 eV ion energy needs to be explained. The results in Figures 4 and 6 are presented as observations and not discussed in the manuscript. I am expecting a piece of experimental results to support the simulation or comparison with previous measurements, if any.
Author Response
We thank the Referee for valuing our paper as “a vital issue for Free Electron Laser community.”
- Comments: Introduction: previous work needs to be presented with details to give the reader a general view of the research point.
Answer: We fully agree on this issue, we strengthen the logic of the introduction part and add more references. Due to a mistake, ARTOF is not described in the introduction part. We added it back.
- Comments: Apparatus: the spic or maker of some devices is not given; please add the missing information.
Answer: We agree. The TMPs are from Pfeifer, NEG pump is from SAES, the ACP40 and ACP120 roots pumps are from Pfeifer, fast amplifer is from Roentdek, the HV popwer supply is from iseg, fs laser is from Coherent, and etc.
- Comment: Experimental results: Figure 3 without uncertainty error bars. Fig.3-b, the tendency of the energies resolution for 100 and 300 eV between 15 and 20 eV ion energy needs to be explained. The results in Figures 4 and 6 are presented as observations and not discussed in the manuscript. I am expecting a piece of experimental results to support the simulation or comparison with previous measurements, if any.
Answer: Figure 3 shows the relative energy resolution DE/E.We did not calculate DE’s uncertainty, as usually done in cited references, e.g. Kling2014, Rading2018, Garcia2005, and Skruszewicz2014. We think there might be a misunderstanding, most probably from our writing.
The tendency of ion energy resolution in Fig.3-b can be explained like this: Since we need to find a compromise between ion energy resolution and the zoom factor of ion image, as the electron arm is set for higher kinetic energy, the zoom factor increases, then the ion energy resolution decreases. As the focus surface is not planar \cite{Skruszewicz2014}, it is difficult to focus well for all radius. When the electron arm is set for 1000, 600, and 300 eV, the best ion focus is achieved at the outmost radius. When the electron arm is set at 100eV, it is easy to make the focus surface more planar. So the relative ion energy resolution depends less on the ion kinetic energy.
Added in the discussion four Figure 4: “the voltages are reduced to half of those in the simulation.” “It agrees well with simulation in Figure 3(a).”
We add a table in the manuscript to list the performance of VMI spectrometers.
