Dissociative Ionization of Molecular CF2Br2 under 800 and 400 nm Intense Femtosecond Laser Fields
Round 1
Reviewer 1 Report
Brief Summary
The authors describe the ionization of gaseous CF2Br2 by intense femtosecond laser pulses of two different wavelengths (800 nm & 400 nm) and the spectrally and spatially resolved measurement of the ensuing dissociation process. The resulting singly charged ionic products of the reaction after excitation are described and conclusions on the specific relaxation pathways are drawn. These findings are supported by calculations.
Broad Comments
The authors give a thorough overview of the current state of research for the relevant scientific area and the significance of the presented new results in this context. The cited literature is fitting to the topic and covers some of the main recent contributions to the field. The applied techniques and methods are suitable for the investigation of the posed research question, and their implementation is mostly described in an accessible way.
The quality of the presentation is adequate and the methodology is clearly scientifically sound. The structure of the paper is well chosen, but at some points a little difficult to follow for the reader. Also some spelling errors and ambiguous expressions should be corrected. The figures are on the whole informative and placed at the appropriate positions in the manuscript. In some respects, for instance in the interpretation of the physical mechanism for some of the different dissociative products and how these findings could be used to manipulate chemical reactions for specific purposes, the authors could provide more details to explain their results and to substantiate the conclusions.
Overall, the findings are clearly described, provide some novel aspects and are arrived at in a scientifically and methodologically well comprehensible way. The paper is thus recommended to merit publication in Applied Sciences with minor revisions.
Specific Comments
Line 137: Why does the Coulomb explosion only depend on the intensity and not the photon energy in this case? A short explanation to elucidate this would be helpful here.
Line 141: The appearance of the fragment Br2+ is just mentioned here, but no explanation is given to its generation pathway here or elsewhere. Since this is a major difference between the 800 nm and the 400 nm case, this should be treated more thoroughly in the manuscript.
Line 156: The meaning and definition of 'translational kinetic energy release' in this context should be explained in more detail, especially in comparison to the 'kinetic energy release'.
Lines 156 to 158: Here, some spelling errors make the understanding of the paper difficult ('transnational' instead of 'translational', production' instead of 'product'?). These need to be corrected throughout the paper.
Figure 4: The different channel numbers given in the paper (1–6) should be marked in the figure.
Line 210 and following: In this paragraph I would suggest to reverse the order, similar to the way it was done for the ionization in the 800 nm case. Put the different channels first and then describe the exact pathway for the dissociation dynamics.
Line 228: Amend '... the appearance of the KER hump (0.81 eV) for CF2Br+ in 400 nm laser fields ...
Lines 234–236: This is one of the main claims of the paper and should be given more consideration. How could one use and influence this dependence? For what purposes? Is there a way to steer the outcome more specifically to a single desired channel out of all the possibilities for the employed wavelength?
Figure 7: The coloring for this figure is a bit misleading. The two green arrows and the two green lines, don't fit together. Also, labels according to the discussed channels would help. The AC region is also hard to see.
Line 264/Line 283: These channels should be defined in the text with respect to the respective dissociation products and the angular distributions shown in Figure 6.
Author Response
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Author Response.docx
Reviewer 2 Report
The paper by Liu et al presents a combined experimental and theoretical study of the dissociative photoionisation of CF2Br2 in strong femtosecond laser field, both at 800 and 400nm. The dibromodifluoromethane is recognized as of interest for its inert and harmless behavior in refrigeration and as fire retardant, but is susceptible to photofragment under UV irradiation, liberating harmful bromine in the stratosphere. In that sense, it may interest the applied physical chemistry community. However, the relevance of femtosecond studies to applications is, in our opinion, farfetched.
The manuscript is poorly written, the level of English totally appalling, with the consequence that miscomprehension is inevitable. It is in the own interest of the authors to consider a professional translation if they want their interesting work to be properly disseminated and appreciated.
Some mistakes are particularly disturbing, and go beyond insufficient command of English:
Lines 155 and 167: Transnational kinetic energy (politics?) is discussed instead of translational (motion).
Lines 157-158: production a and b instead of products a and b.
Line 158: Va is (we guess) the measured velocity of one of the fragments. If true, then equation 3 is wrong (factor Ma/Mb missing (see Eq. 2 of Liu et al, Molecules 2018, 23, 3096).
Line 250: “However, the feather of Br+ ion is anisotropic.” What the feather is remains to be defined.
Lines 272-275: “Actually, the similar phenomenon has been observed in Ref [7], the isotropic angular distribution of Cl+ ions in channel(1,1)1 is attributed to the wave packet oscillation and AC coupling(avoided crossing) between excited states.” AC coupling is not used to describe non-adiabatic coupling or electronic coupling at avoided crossings. Reference 7 does not mention that concept, but instead describes an optical transition between bound and dissociating potentials. Reference 18 never mentions “AC coupling” either.
Lines 258 and 302: Electron wave packets (electron motion) are discussed instead of vibrational wavepackets (nuclear motion). That mistake was also present in reference 18.
References to the authors own work lack page numbers (refs. 7 and 11)
We therefore recommend the paper in its present form to be rejected, and resubmitted once it has gone through a profound revision. We believe it contains interesting material that deserves a more careful and professional presentation.
Author Response
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Reviewer 3 Report
The work from Botong Liu et al. focuses on the dissociative ionization of CF2Br2 by strong 800 nm and 400 nm pulses, investigated via time-of-flight spectrometry and dc-sliced ion imaging. The time-of-fight data at 400 nm reveal two products unobserved at 800 nm, namely the parent ion and the molecular bromine cation, suggesting the presence of a new elimination channel. The authors then detail the yield vs intensity of the ionization products at 800 nm across the range where the Keldysh parameter crosses unity for the sample under consideration, moving from the multi-photon ionization regime to field ionization. They finally discuss the new elimination channel at 400 nm, based on the ionic angular distributions and supporting numerical calculations of PESs. As the authors point out in the introduction, dissociative ionization of CF2Br2 by strong laser fields have received little attention and could be of interest for the specialized community. As such, I think the data could be worth publication. I instead have major concerns about the tentative explanations of the reaction pathways leading to the final dissociation products, both in the 800 nm and 400 nm cases: 1- The experiments at 800 nm are conducted at a Keldysh parameter close to 1 (thus, by definition in a regime in which the ponderomotive energy is comparable to the ionization potential), i.e. at intensities where the field dressing of the electronic states of the molecule is so strong that a field-free representation of the PESs is hardly an appropriate landscape for deriving the dynamics (as a recent reference, consider for example K. Amini, et al., “Imaging the Renner–Teller effect using laser-induced electron diffraction”, Proceedings of the National Academy of Sciences, 116(17), 8173-8177, (2019)). In this sense, for instance, the appearance potential energies shown in Table 1 don't take into account the ponderomotive shifts of the various energy levels. 2- The concern above is enhanced by the pulse duration of 70 fs exploited in the experiments. Given that relaxation dynamics of highly exited strong-field ionized small molecules frequently happen in a sub-100 fs time scale, particularly in the presence of conical intersections and avoided crossings (see for instance H. Timmers, et al., “Disentangling conical intersection and coherent molecular dynamics in methyl bromide with attosecond transient absorption spectroscopy”, Nature communications, 10(1), 1-8, (2019)), I would think that most of the dynamics leading to the products described here start and even develop for tens of fs inside the laser field, making a full time dependent laser-matter treatment important to understand them. 3- In general, I find it very ambitious to draw conclusions on ultrafast relaxation dynamics involving non adiabatic couplings and even excited in a strongly non perturbative manner without an actual time resolved experiment. Many different research fields and pump-probe techniques are exactly aimed at this, from the hhg-based ones mentioned in the introduction, to ultrafast electron diffraction. These conceptual issues add to the very low readability of the text, which really needs improvements in most periods regarding the language. The introduction fails to put the work in a proper context, and the conclusions are not robust to me, for the reasons stated above.Author Response
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Round 2
Reviewer 2 Report
Despite their efforts, the authors did not bring the paper to a level that is appropriate for publication.
1) We note persistent mistakes in the treatment of kinetic energies, and question the velocity slicing method for its overestimation of low KER contributions, as clearly visible in all spectra. Large peaks appear higher in energy in the kinetic energy spectra recorded at 800 nm, which can only arise due to Coulomb explosion. The intensity at which mass spectra and VMI images have been recorded is well above the kink in the MPI yields.
2) The calculated KER (not defined) and TER given in Table 2 are not consistent with the now correct equation 4. The KER should be defined as ½ m_a (V_a)^2. With this definition, TER = (m_a+m_b)/m_b KER, producing TER=0.236, 0.290 and 0.197 eV for channels (1) to (3), respectively. Applying the same calculation to the 0.81 eV hump in the CF2Br+ distribution yields TER = 2 eV, fully compatible with the interpretation given further in the text.
3) “The parent ion CF2Br2+ in Figure 1(b) indicates that most ions CF2Br2+ are populated in the bound excited state, and the lifetime of the parent ions in these states are much longer than the detection time (e.g. flight-time of ions) during the experiment.” In fact, almost none of them survive, hence the minuscule peak observed.
4) The level of English and/or physics is still not acceptable:
‘muti-excited states’; ‘And the avoided curve crossing between the excited states were strongly correlated.’; ‘the air pressure inside the source chamber’ (carrier gas is not air); ‘we set the operating period of the pulse valve (general valve, Parker) to 100 Hz’ (frequency!); ‘the relax force constant’ (relaxed force constant); ‘CF2Br and Br in CF2Br2+ share out nearly the positive charge.’; ‘CF2Br is more easily to lose the ‘redundant electron’’ (what is meant here?); ‘distribution of ion CF2Br+ is isotropic structure’; ‘Then the evolution of the vibrational wave packets undergoes on the corresponding PES of the excited states, and eventually dissociates into fragments. Meanwhile, the redundant energy in the dissociative states release’, etc.
Author Response
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Reviewer 3 Report
The new version of the manuscript is strongly improved, both from the language point of view and the consistency between results and conclusions.
The authors have replied to my conceptual points.
I believe there are still a number of minor language issues, after the correction of which I consider the work suited for publication:
1- line 35 "During the low laser intensity...": clarify the sentence.
2- line 48 "have": has.
3- line 105 "are only shown up": only appear.
4- line 108 "has involved": is involved.
5- line 137-141: the two periods are quite obscure both grammatically and conceptually.
6- line 176: "between above two channels": between the above two channels.
7- line 192: "It is clearly that...": The elimination reaction only occurs in the 400 nm laser field.
Author Response
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