On the Ionization Tolerance of C₂₀ Fullerene in Ground and Excited Electronic States in Planetary Nebulae

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study investigates the structural stability and ionization tolerance of C20,  through quantum chemical methods, specifically Density Functional Theory (DFT). While the work is entirely theoretical, there are areas where it could be improved to ensure that the results are more robust and reliable. The author uses only one theoretical level (B3LYP) for all calculations (IP and frequencies); incorporating at least two different theoretical levels would increase confidence in the predictions and ensure the values fall within that range, which would significantly strengthen the study.

In section 2, the author may include references for B3LYP, and basis sets to make their work reproducible.

Since the calculations are already completed at B3LYP level, it would be good to provide rotational constants in the supplemental information. This data would be particularly useful for aiding the astrophysical detection of C20.

Finally, including a brief comparison with the stability and ionization behavior of other fullerenes, such as C60, would add valuable context and depth to the discussion.

Author Response

Please find our point-to-point replies to the comments in the attached pdf file.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The authors present a quantum chemical study using density functional theory (DFT) methods to investigate the ionization tolerance of C20 fullerene families in astronomical environments, such as the photoionized gas in planetary nebulae (PNe).

In their study, the authors theoretically demonstrate that C20 fullerene and its cations can survive extreme conditions, such as those present in planetary nebulae environments. They claim that these molecules can tolerate high levels of excitation and ionization, allowing the cations to exist long enough to be detectable. This represents a significant contribution to the field, given that it opens up opportunities for future PNe observations with the potential for new observational discoveries. Furthermore, the paper highlights the importance of theoretical investigations to guide astrophysical observations. Furthermore, the paper is well-written and well-organized.

I recommend accepting the paper for publication after addressing some minor comments. These suggestions are intended primarily to clarify and expand certain points and to incorporate more astrophysical context, addressing in particular the topic of planetary nebulae for future research.

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Abstract:

No comments.

1. Introduction:

-) Line 19:  I cannot access the paper by Torrisi et al. What temperature range referred in this context?, what hot is the plasma generated by the laser, and how closely does this temperature resemble the extreme conditions of the central stars in planetary nebulae?

-) Line 34:  I think it would be beneficial in this introductions indicate the wavelength range where these bands, C2 , C3 , C5 , C60 , C60 + and C70, are located in the spectra. This information could help the readers a better understand the detection methods and  the spectral features of these carbon clusters.

2. Quantum Chemical Computations:

 I believe, it will be useful for readers who could be not  familiarized with DFT to include a brief of how DFT works and its basic principles. Or alternative the authors could provide some key references to the readers to consults.

3. Discussion:
3.1. Ionisation Tolerance and Electronic Excitation

 -) Line 88: What happens to the molecule at q=+14? Could the ionization potential necessary to produce this caption disintegrate the molecule? This aspect is not clear and I think it should be addressed in the text. Additionally, this phrase "No stationary point on the Potential Energy Surface (PES) of C₂₀¹⁴⁺ at the singlet spin state could be found" also is not clear. It would be useful to explain what means in term of stability and or reactivity of the molecule. Does this imply that the q=+14 state is unstable or inaccessible?

-) Line 91:  This is aligned with my previous comments. Does this redistribution of the electrons density also occur at q=+14?

-) Line 132:  Table 3 indicate that caption are generated with high-energy photons. For instances, the charge as high as 13 + at a very high ionization potential (65.99 eV). Is there any idea in which zone of the PNe these captions are generated? I will think than in the more central part of the PNe. Additionally, considering that the conditions could vary among different PNe, the authors have any insight into how the distance from the central stars might affect the formation and preservation of these molecules or cations? I suspect that not all PNe will allow for the detection of this molecule. I mention this in the context of possible targets for follow-up observations.

4. Conclusion:

Line 166:  I think will be beneficial add a brief discussion on potential follow-up observational strategies for detecting these molecules in the future. For instance, what specific tecnique or instrument could be used, and which regions of the electromagnetic spectrum should the search be focused? A clue is provided in the introduction of this manuscript, where the authors mention, "These interesting astronomical sources [6] are one of the sources  of mysterious Unidentified Infrared Emission (UIE) bands attributed to the formation of complex organic molecules [7,8] including fullerenes [9,10] and fulleranes [11,12]." I believe expanding on this aspect would be relevant. And I want to hear more about this.
    

Author Response

Please find our point-to-point reply to the comments in the attached pdf file.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

With the revisions made, the paper has significantly improved and is suitable for publication.

Author Response

We appreciate your support of this work.

Reviewer 2 Report

Comments and Suggestions for Authors

Thanks to the authors for considering my suggestions and addressing all my doubts. I recommend this paper for publication. However, I have a few minor comments that I believe would be beneficial to address:

-) Line 22: Thank you for your reply. However, I still find the comparison between the temperature of the laser-generated plasma (348100K) and the conditions of the central stars of planetary nebulae confusing. The temperatures of the central stars of planetary nebulae are much lower (typically between 30000K and 200000K). While I understand that high-temperature plasma can provide insight into fullerene formation in the context of planetary nebulae, it would be helpful to clarify how this laser-generated plasma is considered to simulate the conditions of the central stars, given that the temperature of the plasma is much higher. Could you provide a more detailed explanation to clarify this point?

-) Line 175:  processes are listed in Table4, Their values are comparable to the ionization energies listed -> processes are listed in Table4. Their values are comparable to the ionization energies listed

 

Author Response

We appreciate the support of this work. Our replies are as follows:

Point 1: To make clear the difference between the kinetic energy of ions in the plasma and the macroscopic concept of the temperature we edited the sentence as follows:

Recently Torrisi et al identified the formation of this species in the hot plasma (30 eV kinetic energy equivalent to Boltzmann's temperature of 348100 K) of carbon atoms generated by a laser.

To address the comparison between the laboratory condition and center stars of PNs we edited the following sentence:

This condition resembles the physical conditions at subsurface regions of central stars in planetary nebulae.

This is to distinguish the star's blackbody surface temperature from its other regions.

Point 2: Corrected, as the respected reviewer commented.