Peer review of: Strong and selective isotope effect in the vacuum ultraviolet photodissociation branching ratios o (2019)

Reviewers' comments:  

 

Reviewer #1 (Remarks to the Author):  


The authors present measured branching ratios for CO dissociation at VUV wavelengths (92.8 to 94.0 
nm). They demonstrate preferential formation of 13C(1D) for a couple of the bands in this region, 
and argue that such a preference for 13C formation may be important for understanding the 
difference in the 12C/13C ratio for the Sun versus planetary materials. They apply a simple diabatic 
model to explain the preferential formation of 13C(1D) in their experiments. They then expand their 
diabatic arguments to O isotopes branching ratios in CO dissociation.  


The measurements presented seem to be of good quality. However, they of a small subset of CO 
bands in a moderately congested region of the CO spectrum. Since C(1D) formation is energetically 
allowed up to ~ 100 nm, it would make vastly more sense to present experimental results for 
isotopic branching ratios for some of the bands between 94 and 100 nm. It is impossible to draw 
conclusions about the importance of the 1.2 nm region they have studied for the simple reason that 
ii contributes only a small fraction to the total rate of CO photodissociation.  


Second, the rather extensive discussion of O isotopes fractionation, and the experiments of the 
Thiemens group, is completely unsupported by the data presented. If the authors wish to make a 
meaningful contribution to the understanding O isotope fractionation in CO dissociation, then they 
should perform experiments using C16O, C17O, and C18O. The discussion of O isotopes they have 
presented is merely conjecture, and does not acknowledge the role of other factors such as 
linewidth for the short wavelength CO bands. Line overlap has an important effect on O isotope 
fractionation.  


For the above two reasons I cannot recommend this paper for publication in its present form.  

Additional comments:  

 

1. The excited state C'1Sigma+(v'=7) is referred to repeatedly throughout the text. Is this a new state 
assignment (and if so please give the citation)? Usually this state is simply the 1Sigma+(v'=2) state.  

 

2. The potential energy curves in Figure 1b are not really useful in this cartoon format. A more 
accurate and properly labeled set of PECs is needed.  

 

Reviewer #2 (Remarks to the Author):  

I recommend the paper for publication with only minor revisions.  

The new results – concerning the strong isotope-dependence of photodissociation branching ratios 
in carbon monoxide – are relevant to ongoing studies of predissociation mechanisms in CO. In 
addition to the intrinsic merit of a more detailed understanding of the interactions of high-lying 
electronic states in CO, there are important astrophysical applications that rely on a quantitative 
understanding of predissociation in CO and its isotopologues. This paper highlights the previously 
under-appreciated role of branching ratio differences among the CO isotopologues as a possible 
explanation for the strong fractionation signatures of carbon and oxygen isotopes in the solar 
system. The qualitative analysis presented in the paper – a coupled-channel model of diabatic states 
leading to indirect predissociation – is a sensible explanation for the trends seen in the experimental 
results. The experimental methods are state-of-the-art and the level of experimental detail 
presented is appropriate.  

Below I have indicated some suggested minor revisions, but there is no need for further review.  

1. page 1, title: please replace “on the vacuum…” with “in the vacuum…”  

 
2. page 2, line 41: in the introductory discussion, the term “To test the self-shielding models” will be 
confusing to anyone who is not intimately familiar with the works being cited. The authors should 
define this term – a single sentence would help to clarify the introductory remarks.  


3. page 7, line 200: the phrase “the substitution of 12C by 13C must have dramatically changed the 
relative coupling strengths among different electronic states” should be re-phrased. As the authors 
later mention, the coupling between electronic states is independent of isotope. It is the coupling 
between individual rovibronic levels that depends on isotope (not the coupling between electronic 
states) – because of the energy shifts that the authors describe.  

4. Uncertainty estimates are included in the results presented in Table 1, however there is no 
discussion whatsoever about uncertainties in the text. The authors should include a brief description 
of the uncertainty levels in the text itself and how they came to determine those uncertainties.  


5. The grammar in the manuscript needs a careful review by the authors and/or the editors of the 
journal. Multiple awkward phrasings detract from the quality of the science being presented.   

Regards,  


Reviewer #3 (Remarks to the Author):  

The authors report on a compelling experimental study that characterizes a strong and selective 
isotopic effect of photodissociation branching ratios of carbon monoxide in the VUV absorption 
range. They use VUV excitation generated by two photon resonant enhanced four wave mixing 
coupled to a time-slice velocity-map ion imaging apparatus (TSVMI) to measure the branching ratios 
in the Rydberg 4p(2), 5p(0) and 5s(0) complex region for three indirect predissociative pathways for 
the two isotopomers : 12C16O or 13C16O : one leading to the atoms in their ground triplet 
(3P)states, one leading to C(1D) + O(3P) and one leading to C(3P) and O(1D). They report on a 
significant isotopic effect for the predissociation channel leading to C(1D) + O(3P) for the absorption 
bands 1Π(v´=2) and C´ 1Σ+(v´=7). For these bands, 12C16O primarily dissociates in the lowest 
channel while 13C16O dissociates to the next channel producing to C(1D) atoms. Significant 
variations of the branching ratios are also reported for the other absorption bands in this spectral 
region. To explain the strong experimental effect, the authors adopt the explanation proposed in ref 
34 : due to the mass effect, different vibronic states are excited in the two isotopomers : in the case 
when the predissociation process is indirect and involves an electronic coupling between a Rydberg 
and a valence state, the ladder of rovibrational states in the two electronic states are shifted by the 
change in reduced mass, more in the case of the shallow valence state, which leads to different 
efficiencies of the electronic coupling for the two isotopomers due to the fact that accidental 
resonances occur at slightly different energies in the two isotopomers. As noted already in ref 34 
and reinstated by the authors, this mechanism is general. In this sense, I recommend that the 
authors tone down the qualification ‘new’, ‘novel’, ‘for the first time’, that they use in several places 
in the manuscript. It is very important to document experimental the ubiquity of the mechanism 
proposed in ref 34 for N2 in case of indirect predissociation for other diatomic molecules but it can 
no longer be qualified to be novel. The authors might want to consider to also refer to more recent 
work of the authors of ref 34, for example J. S. Ajay, K. G. Komarova, F. Remacle and R. D. Levine, 
Proceedings of the National Academy of Sciences, 2018, 115, 5890.  

The paper is clearly written and reports on important experimental results that will stimulate more 
theoretical work. I recommend its publication in nature communication.