Fragmentation of Multiply Charged C₁₀H₈ Isomers Produced in keV Range Proton Collision

Round 1

Reviewer 1 Report (New Reviewer)

I have only one question:

The efficiency is very low, as the author wrote

"the detection efficiency varies from ∼45 % for a single carbon ion to ∼20 % for an intact molecular ion"

?

More recent references should add to manuscript.

 

English language is fine.

Author Response

 

Reply to reviewer 1,

Thanks for the careful reading and helps to improve/enrich this manuscript further.. 

Reply to the comment:

1. I have only one question:

The efficiency is very low, as the author wrote

"the detection efficiency varies from ∼45 % for a single carbon ion to ∼20 % for an intact molecular ion"

?

More recent references should add to manuscript.

Reply : As far as our understanding, this detection efficiency is not very low, as the maximum detection efficiency of channel plate detector is typically 60%. The reference cited in this context  is also quite relevant, as it is occasionally cited by several authors in recent works.

 

Reviewer 2 Report (New Reviewer)

The authors present results of a complete experimental investigation of the fragmentation of multiply charged azulene / naphthalene C₁₀H₈ ions after collision with protons at the keV energy range. This paper continues and extend previous studies performed by the authors in the past.

I find the manuscript well written and full of interesting results regarding the process under study. My only objections for a straight acceptance of this work in its present status are mainly minor and are listed below. I would urge the authors to address them:

1.- I have problems to understand what the authors means with "cross sections for the two isomers" in Section 4.2 Do they refer to the ion count quantities shown in Figure 1? I do not understand what is the connection between the mass spectra and the cross sections. If I am not wrong this is the only time they refer to "cross sections" but I do not why.

2.- This is surely only due to something quite clear to the authors and perhaps some other readers but I have to confess that I would need to be further explained why complete dehydrogenation is seen in Figure 5. Perhaps some comments pointing out this in the text would help. 

3.- Use of acronyms looks a bit chaotic along the text. "ToF" is not defined (but used) in the abstract; "UV" but specially "VUV" could be defined (as in Ref. [11]); the same happens for "ECR", "LEIBF", "IAUC"; "2D" is defined twice; "DIBs" is defined but never used afterwards..

4.- The authors should include the number of the reference instead of just the surname of the first author. That happens quite often beyond pg 6;

5.- There is a repetition of "the" in line 124 pg. 3;

6.- "Equation 2" should be "Equation 1" in lines 140 and 182;

7.- Labels for the fragments are too small in Figs. 3, 4, 6 and 7

8.- There is a typo "Strcture" in Table 2

9.- I guess "coulomb" should be in Capital "Coulomb" and "Extreme" could go as "extreme"  in pg. 1

Author Response

Reply to reviewer 2,

Thank you for the careful reading and inputs to this manuscript. 

Reply to the comments:

1.- I have problems to understand what the authors means with "cross sections for the two isomers" in Section 4.2 Do they refer to the ion count quantities shown in Figure 1? I do not understand what is the connection between the mass spectra and the cross sections. If I am not wrong this is the only time they refer to "cross sections" but I do not why.

Reply: Sorry for this mistake from our side and the term “cross section” is now replaced by “ion yield in the mass spectrum”. The newly constructed sentence in this regard is pasted here

“But, the fragments in the region m/q 98 (\ce{C8H_m+}\, where 0<m<10) and 88 (\ce{C7H_m+}, where 0<m<10) have similar yield for two isomers in the mass spectrum”.

2.This is surely only due to something quite clear to the authors and perhaps some other readers but I have to confess that I would need to be further explained why complete dehydrogenation is seen in Figure 5. Perhaps some comments pointing out this in the text would help. 

Reply: the mass at m/q 120 ( in fig. 5a) corresponds to fully dehydrogenated nph+/az+

3.- Use of acronyms looks a bit chaotic along the text. "ToF" is not defined (but used) in the abstract; "UV" but specially "VUV" could be defined (as in Ref. [11]); the same happens for "ECR", "LEIBF", "IAUC"; "2D" is defined twice; "DIBs" is defined but never used afterwards..

Reply: Thanks for mentioning this, all are corrected.

4.- The authors should include the number of the reference instead of just the surname of the first author. That happens aquite often beyond pg 6;

Reply: ref. no. added

5.- There is a repetition of "the" in line 124 pg. 3;

Reply: corrected

6.- "Equation 2" should be "Equation 1" in lines 140 and 182;

 Reply: thank you for the mention, corrected.

7.- Labels for the fragments are too small in Figs. 3, 4, 6 and 7

Reply: font size increased.

8.- There is a typo "Strcture" in Table 2

Reply: corrected

9.- I guess "coulomb" should be in Capital "Coulomb" and "Extreme" could go as "extreme"  in pg. 1

Reply: done

Reviewer 3 Report (New Reviewer)

Review Report:

In their manuscript Vinitha et al. demonstrated the fragmentation of multiply charged naphthalene and azulene produced by collision with keV protons. The fragmentation mainly produces H⁺/C⁺ and CH₃⁺ which occur via two different pathways. Their results are also corroborated by the DFT calculations.

This work is important in the context of understanding fragmentation dynamics of PAHs under extreme conditions related to the ISM. Although the manuscript is thorough, well-written and fits with the journal scope, I expect some discussion on various ionization processes that could take place in the extraterrestrial environment. Therefore, I recommend publication of the manuscript after some modifications. My comments are copied below. I use the following abbreviations, P-page number and L-line number.

Comments:

1. P1-Introduction: The authors are asked to discuss different possible ionization routes, such as electron ionization, Penning ionization, H abstraction, and so on, under extreme conditions along with references (2018 ApJ 865 114; DOI 10.3847/1538-4357/aad462).

2. P2- L50: Larger fragments can also stay in high vibrational state as confirmed by the broadened IR spectra (See the reference: Phys. Chem. Chem. Phys., 2020, 22, 17275). I encourage the authors to include such discussion as well because this is already experimentally proved.

3. P2-L56: The authors are asked to discuss the basis of selecting this level of theory.

4. P3-L95: Mass-to-charge ratio is typically represented as m/z followed by without any “=” sign.

5. I would appreciate the optimized cartesian coordinates in the manuscript or in the supporting information.

Author Response

Reply to reviewer 3,

Thanks for all the suggestions for improving the manuscripts.

Reply to the comments:

1. P1-Introduction: The authors are asked to discuss different possible ionization routes, such as electron ionization, Penning ionization, H abstraction, and so on, under extreme conditions along with references (2018 ApJ865114; DOI 10.3847/1538-4357/aad462).

Reply: Thank you very much for the reference, but our study here is mainly focussing on the dissociation of highly charged nph+/az+ ions by energetic collisions. If we clearly understand your comment, the ionisation processes mentioned  are not directly relevant to this work, however,  they are important to refer when discussing the general aromatic chemistry in many interstellar regions.

2. P2- L50: Larger fragments can also stay in high vibrational state as confirmed by the broadened IR spectra (See the reference: Phys. Chem. Chem. Phys., 2020, 22, 17275). I encourage the authors to include such discussion as well because this is already experimentally proved.

Reply: Once again we thank the referee for mentioning interesting work. However, here we simply planned to predict stable geometries  of mono- and di- cations of C9H5 fragments ( based on the energy), which are found to be stable in our experimental time scale. We do not wish to indicate that the larger fragment is actually cold but as stated, it is produced closer to the equilibrium configuration compared to smaller fragment. We have tried our best to make this point clearer to the reader in the revised version [Page 2, Line 49 & Page 9, Line 256]. More detailed conclusions require further study.

3. P2-L56: The authors are asked to discuss the basis of selecting this level of theory.

Reply:Ok, It is discussed now in the revised version [P2, Line 57]

4. P3-L95: Mass-to-charge ratio is typically represented as m/z followed by without any “=” sign.

Reply: corrected

5. I would appreciate the optimized cartesian coordinates in the manuscript or in the supporting information.

Reply: We believe the computation itself is rather standard. Moreover the structures are also relatively simpler. Hence we wish to keep the details of the computational aspects very simple here. 

Round 2

Reviewer 3 Report (New Reviewer)

I still believe that the authors should include all the optimized cartesian coordinates in their manuscript as the structural parameters do get changed with charges even though the studied systems are simple. 

I still find that different ionization routes of the aromatic molecules need to be discussed in the present manuscript as the authors are ionizing their system which is followed by dissociation with proper references.

 

Author Response

Dear Reviewer, 

We have made the following changes. 

1) Details of the calculated structures are listed, including the coordinates in a supplementary note.

2) Reference is made to the supplementary note in the calculations sections. 

3) Table 2 caption is corrected appropriately.

Thank you. 

Author Response File: Author Response.docx

Round 3

Reviewer 3 Report (New Reviewer)

I recommend publication of the manuscript after second revision.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

In this manuscript, Vinitha et al present experimental results from studies of keV proton impact on two C₁₀H₈ isomers. The charged fragmentation products are analyzed by means of multi-coincidence time-of-flight mass spectrometry, and some DFT calculations have been performed to interpret the results. The experiments appear to have been carefully carried out providing new results that may be of interest for the broad astropah community in particular.  However, some of the main conclusions do not appear to be supported by the experimental results or by the theoretical results: 

 

“ we report a process of super-dehydrogenation of PAHs in single collision condition with a detectable intensity of total dehydrogenation for nph and az.”

 

This is concluded from the results shown in Fig. 5, i.e. from coincidences between protons and fragment ions with a time-of-flight close to that of the intact parent ion. The distribution in Fig 5a extends from an apparent mass of about 120 amu/e up to that that of the intact ion (128 amu/e) with a minimum at the center (at 124 amu/e) for which there is no explanation provided in the manuscript. It is not clear to me how the authors can rule out that what is seen in Fig. 5 is an instrumental effect. For instance a kinetic energy release distribution where protons that are emitted perpendicular to the spectrometer axis are predominantly lost (not collected) and thus not detected in coincidence with the resulting intact fragment ion? The latter would in such a case instead contribute to the single stop spectrum at a time of flight corresponding to 124 amu/e. Furthermore, the fact that there is a significant contribution at 128 amu/e is not consistent with the emission of a proton from the parent ion (m/q=128 amu/e) as they have the same mass. Thus, it seems that the conclusion that super-dehydrogenation is observed cannot be made based on the experimental time-of-flight distribution alone. Support from theory showing that fully dehydrogenated species may survive on the experimental timescales would have been a convincing argument, especially if fragment ions having m/q=124 will not survive.  

 

“One such mechanism of CH3+ elimination is found to progress via a common isomer of nph and az”

 

This seems to be concluded based on the similar kinetic energy release values measured for emission of CH₃⁺ from the two parent isomers and from the calculated structures and energies of two additional isomers that may be intermediates along the reaction pathways. However, the kinetic energy release distributions may be very similar for both isomers although the reaction pathways are different. It is the highest barrier (transition state) that will determine the kinetic energy release and that point on the potential energy surface need to be found for each isomer in order to make any conclusions regarding common pathways. 

 

Furthermore, I have the following more specific comments: 

 

- Line 26: What is meant by multi-stage VUV absorption?

- Line 55: It is stated that the cc-pVDZ basis set was used, but the actual method used is not mentioned (DFT? and in such a case which functional). Furthermore, were frequency calculations performed to ensure that local minima were found (i.e. not transition states) and for zero-point energy corrections? If not, I strongly recommend the authors to do that

- Line 85: How were the detection efficiencies determined for single carbon atoms and intact molecular ions, and how do they relate to the collection efficiencies? That is, are some of the light fragmentation products that are emitted perpendicular to the spectrometer axis lost (see above)

- Line 90: Why did the authors decide to use five different projectile energies and add the results rather than collecting more data for one collision system and hence the same energy transfer distributions?

- Figures 1,3,4,7. The labels, C_n⁺, appear to be misleading, shouldn’t it rather be C_nH_m⁺?

- Line 138: I find it confusing to call a fragmentation channel “carbon conserving” as the number of carbon atoms is always conserved in the present experiments.

- Line 163: This is consistent with the theoretical results in https://doi.org/10.1063/1.3541252

- Line 202: “In a laboratory setup, it is much easier to study the dissociation process and then try to  understand the associative process, than to experimentally look for the associative reaction itself. We believe that an investigation of the parent conformers and associated reverse barriers will add significant value to the understanding of the methyl addition processes in PAHs.” I doubt that one can learn something about methyl addition to neutral or singly charged PAHs by studying Coulomb explosions of multiply charged PAHs as the interaction dynamics for such systems are completely different. 

- Line 223-224: What is CD³⁺ elimination and what is a nph-d₈ cation?

- Figure 8, caption: Please clarify as it is difficult to understand

- Line 228: “Our KER measurements for both nph and az target for CH3+ +C9H5+ channel was found to be 2.9 eV.” I assume that the kinetic energy release distributions have been measured rather than a single value? In such a case is 2.9 eV representing the mean or most probable value? And how do the distributions compare for the two isomers (see my comment above)?

- Line 247: “Though the exact formation process of trication is not known as yet, it can be assumed that a similar amount of internal energy can be available for the trications at the time of formation.” Not clear to me how one can assume that.

- Line 259: “We consider here the same set of structures for mono- and dications of C9H5.” How can the authors rule out that other isomers come into play, for such small systems one would expect the charge to play an important role for the stabilities.

- Figure 10. How can the authors be sure that pure carbon molecules (C_n⁺) are detected in coincidence with C⁺ and not with hydrocarbons ions (C_nH_m⁺) ?

 

Based on these comments and the large number of typos (see below), I do not recommend this work to be published in its present form.

 

The quality of English language is ok, but there are so many typos that I find it difficult to believe that the authors have carefully read the final version of the manuscript before submission. It is not my task as referee to correct all typos, so here I just highlight a few examples:

Line 98: "neural" ->"neutral"

Line 117 "C_8Hm^+" -> "C_8H_m^+" and "C_7Hm+" -> "C_7H_m^+"

Line 118 "multifragementation" ->  "multifragmentation"

Line 120 "C10H8+" -> "C_10H_8" 

Line 225: "approximately 1" - > "approximately 1 [add unit]"

Line 226: "2KER" -> ?

Line 243: "13in" ->?

The latter two are examples were it is not possible to follow the text due to typos, which is particularly disturbing. 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors have performed collision-induced dissociation of naphthalene and azulene by swift proton impacts. To my knowledge, this work presents new results worthy of publication. However, I found the manuscript hard to follow. The quality of the figures needs to be improved.

Multiply charged ions in keV range collision are produced with substantial amounts.

 

Abstract line 7: "The most probable..." This sentence is a typical case of weird scientific argumentation. I understand that computation can help to predict the probability of occurrence of an event, but the second part of the sentence deals with experimental stuff that is used for observation (not prediction) of an event.

Section 1: introduction is good.

Section 2: computational detail. this section provides the minimum needed information about the molecular structure calculations performed to support the interpretations in the following of the paper.

Section 3: Experimental details and analysis.

line 63: "The emitted electron...". This suggests that one electron (and only one) is emitted for each collision. To my opinion, there should be a distribution of the number of emitted electrons from zero to several ones (maybe 3 or 4) depending, e.g., on the impact parameter, the deposit of energy on the target.

lines 68 to 70: It is not very clear which signal is used as a comment stop in the TOF measurement. Is it the delayed projectile signal? the delayed electron signal? the delayed coincidence signal between those signals?

 

lines 78 to 80: "The recorded data was filtered and cleaned..." This sentence is not clear to me. Is this a post-experiment data treatment? Is this an electronic noise reduction system used during the experiment? Anyhow, please explain exactly what is done and what are the implications of this treatment.

lines 84 to 85: I am very surprised that the detection efficiency of the MCP was so low. There are tricks that allow for much higher detection efficiency for C+ or H+. Do the given numbers include collection efficiency? If so, then it is necessary to write collection-detection efficiency. Anyhow, my advice would be to improve this point for future work because it is very important for studying the correlation between fragments.

4. Results and discussions

4.1 Normalization process -> OK

4.2. Single hit analysis

line 118: "Roughly 5% of triply charged parent monocations are detected in case of two isomers." I don't understand why this sentence is here and what supports this statement. Maybe it is here as a support of the next sentence. Another way to support the next statement would be to state that the small charged fragments are likely to come from multiply charged targets that undergo multiple dissociations that produce small charged fragments. In such a case the energy difference between the parent multiply charged isomer has probably less importance. In the case of multiply charged C10H8 isomers, I would suggest, naively, considering that dissociative states could be populated so that there is no chance for isomerization prior to dissociation.

lines 123-123 : "The sharp peaks at m/q=20 and 40 are due to Ar in the background". Please add the word "gas" after "background". Was Ar added in a controlled way? I mean if the Ar gas partial pressure could be known or kept the same in both experiments with nph and az, the Ar+ and Ar2+ peaks could be used for intensity normalization. 

4.3 Coincidence analysis for double hit data

line 125: I suggest modifying the beginning of the sentence: "The use of a multi-hit time digitizer ..." -> "The multi-hit time digitizer used  in the present experiment..." because it is possible to find multi-hit devices that can count many more hits per event.

lines 128-129: I don't understand the sentence: "The correlation patterns are found similar for two of the isomers". How many isomers are considered here? What are they? Which are the two that give similar correlation patterns? Or maybe the sentence should be: "The correlation patterns are found similar for the two isomers", where the two isomers are az and nph. This needs to be clarified.

Figure 2 and all other correlation maps : The resolution (in pixels) of the figure is not very good. Please consider improving the quality because it is difficult to read the subscripts and superscripts. Moreover it would also help to reader to chang the scale (channel no.) to m/q, or at least to add m/q as a secondary scale (I know this is not linear, but it is normally not so difficult to do so)

Line 142: It is a pity that the tail is not much discussed in the paper. If the intensity follows an exponential decay, it can be related to the internal energy after proton impact.

4.4 H+ coincidence

This section is very hard to follow. I'm not sure about the correctness of the interpretation of the observations made on figure 3.

lines158-159: " We detected a strong H+ correlation with all other smaller mass fragments, which might be due to the highest detection efficiency of H+". What means "smaller mass fragments"? Smaller than what? Not H+, I guess... As far as I understand (or guess), H+ islands appear with high intensity in the 2D-correlation map because it has the highest detection efficiency, so it has more chance to be detected together with another charged fragment.

line 163: "with charge state greater than 3". Does this include 3 or not? I believe it should.

lines 164-167: I think it is not that easy to compare counts of the second hit in coincidence with H+ with counts of the single hits because it is highly dependent on the detection efficiency. If we believe in the value given above in the text then some probability calculations can be made. What is the chance of detecting C3Hm+ when the corresponding H+ is detected or not? Can several H+ fragments be detected in a single event? Is H+ correlated with itself (not shown in the map)?

On the other hand, provided that detection efficiency is not much different for the different carbon clusters, it is possible to compare the total intensity of the carbon clusters. Then, the C3Hm dominates and the two sentences "this is because... ionization potential" (lines 166-168) are sound.

line 173: "...the relatively small coincidence with super-dehydrogenated..." The words "relatively small" may mislead the reader who may think the surprising fact is the small intensity, whereas the surprising is that this process exists.

4.5. C+, CH+, CH + coincidence  ->OK

4.6 CH3+ 

line 210 : typo error superscript -> C_9H_5^2+

lines 224-225: "The experimental KER reported by them was approximately 1... " Is is 1 eV? Add unit. Same unit is missing at the end of the sentence.

line 243 : "This process is found to deposit internal energy of about 13in the resulting dication"  What means 13in ? I suppose it is "...13 eV in..." What is supporting this statement?

4.7. Multihit analysis in coincidence with CH3 +

The arguments in this section are hard to follow. It seems to me that the reasoning of the authors considers that C+ or CH3+ is emitted first and that the remaining fragment dissociates afterward. I don't know what is supporting this. The fact that the lightest fragment is detected first does not mean it is emitted first. Consequently, the whole discussion in this section seems highly speculative to me except if the authors bring or recall strong evidence that other order of fragment emissions can be neglected. 

 

5.conclusions -> OK

 

My conclusion about this paper is that I didn't enjoy very much the reading because of many typo errors, low-quality figures, and of hard-to-follow argumentation.

From a scientific point of view, there are a few interesting results that are worth publishing,e.g., the observation of super-dehydrogenated fragments.

The experimental data supporting the discussion are suffering from several drawbacks such as low detection efficiency, the presence of acetone pollution for az, no possibility for charge balance of the collision, nor for energy balance. Discussions would be much easier if the initial charge and internal energy of the target prior ist dissociation could be known in the experiment. Otherwise, discussions remain highly speculative.

The comparison between nph and az most often leads to confirming that when high amount of energy is given to the molecule, it can explore all available isomers prior dissociation, independently from the initially chosen isomer. In a recent paper lee et al. (J. Chem. Phys. 158, 174305 (2023) https://doi.org/10.1063/5.0147456) have shown that even for lower internal energy, the authors explain clearly why the behavior of nph are az are very similar, even at much lower internal energy than the present study. 

The language quality is uneven depending on the sections. There are too many typo errors to report them all here. The authors should proofread very carefully before sending the manuscript to the editor.

 

For instance: all the figure captions start with "Figura" instead of "Figure".

Please, be careful about the use of subscripts in molecule formulae, for example, there are several such errors from lines 117 to 120

line 119: typo - multifragemntation

typo error in the legend of figure 3

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Title: Fragmentation of multiply charged C10H8 isomers produced in keV range proton collision
===============================================================================

Comments to Author(s):

In this article, the author studies the Fragmentation of multiply charged C_10H_8 isomers produced in keV range proton collision in view of their fundamental molecular dynamics. This aspect is assessed with the help of a multi-hit analysis of daughter ions in coincidence with the elimination of ions. The authors found super-dehydrogenation of naphthalene and azulene targets, with evidence of complete dehydrogenation in a single collision. Secondly, the production of H+, C+, CH+, and CH2+ are related, whereas CH3+ is produced exclusively by a different mechanism, which needs further investigation to understand the underlying theoretical and experimental aspects.


The manuscript is well-written, and the results are intriguing. I recommend its publication.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf