Charge Exchange Spectroscopy of Multiply Charged Erbium Ions

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors report in “Charge exchange spectroscopy of multiply charged erbium ions” the identification of multiple spectral lines in Er^3+. Such spectral lines are key ingredients to understand astrophysical processes and their potential sites, which are believed to be the origin of heavy elements in the universe. On the other hand, spectral lines for highly charged ions are scarcely found in data bases, in particular those exhibiting open shells due to the complexity of theory and experiment. 

In their approach, the authors produced multiply charged Er ions using an ECR source, selected them with a magnet, and subsequently investigated their light emission that was initiated by gas collisions. To this end they used a CCD camera attached to a spectrograph. 

The analysis of the spectra is convincing and the theory approach to identify the transitions seems to be suitable. A firm assignment of these transitions to certain level pairs is rather difficult in many cases due to the complexity of the electronic structure. 

The manuscript is well written, and the setup is explained in detail. The title is fine, and the conclusions are backed up by experiment and theory. In my view, this work merits publication. In the following, I will give some final suggestions for the authors to be considered before acceptance.

 

Major improvements: 

- I was a little surprised to see no form of schematic overview of the setup. I would encourage the authors to consider adding such a figure, which could for example include the sources, the magnet, the optics, relevant distances etc.

 

- I suggest the authors to add information on the focal length of the spectrometer, the length of the bundled optical fiber and on its transmission efficiency in the relevant wavelength range.

 

- Please indicate in which diffraction order the spectra shown in Fig.1/2 were measured?

 

- Please indicate the pressures used in the collision cell.

 

- Please indicate that nat. Er has been taken for the experiments if this is the case.

 

- The authors expect when the ionization potentials of the gaseous species (Ar, N2) are similar the electron capture processes in a highly charged projectile would be similar in a simplified picture of charge exchange process. The authors then conclude the photon emissions to be similar as well. I advise the authors to be more precautious with the latter statement as even if the same orbital was occupied during an EC process the interaction with Ar and N2 will be different leading eventually to different quenching channels and thus enabling cascades that were previously suppressed.

 

- Which version of the Cowan codes have been used?

 

- I suggest the authors to give more precise characteristics for line identification instead of using “target/projectile dependence”. One would be interested in knowing the boundary conditions in terms of SNR threshold to include/exclude certain peaks.

 

- Not all upper levels can be identified in Fig.3. Could the authors improve the figure and assign the levels?

 

- Could the authors indicate how they estimated the energy gain used in Eq.1?

 

 

Minor corrections:

- Line 4: “star merger called” -> “star merger event called” 

 

- Line 8/9 -> One may reformulate it like: “We report observed emission lines … and their identification based on theoretical estimates.”

 

- Line 15: “object” -> “transient”

 

- Line 27: delete “gasses” -> “gases”

 

- Line 42: “26 fibers each of 250”

 

- Line 53: “according to a simple” 

 

- Line 61: “spectra measured for different”

 

- Line 64+71: “N2 in different”

 

- Line 75: delete “can”

 

- Line 76: “can see” -> “conclude”

 

- Line 78: please reformulate the sentence!

 

- Line 79: “targets in” -> “targets that are produced in”

 

- Line 83: “can not see” -> “did not found”;  “from NIST” -> “tabulated in the NIST”

 

- Line 84: “available from” -> “available in”

 

- Line 85: “, we can consider” -> “with those measured, we consider”

 

- Line 91: “. and ” -> “ and that ”

 

- Tab. 1: Please define J, Delta E, lambdas! Are the given wavelengths air wavelengths?

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors After carefully reading the paper, I think the paper is good to publish as is. According to my experience in working on charge exchange reaction, most of the studies are state-selective measurement and X-ray. As from the fundamental point, the charge exchange can emit a broad spectrum. This paper extend the potentials of charge exchange reactiona and link to the neutral star merge event where the hot plasm ejections could have such processes even though there are few such astrophysical observation. To this end, it is good since such laboratory measurements can provide constraints on the future observation. It really help the both atomic physics community and astrophysical community. Finally, I agree to publish as it is now.