Peer review of: Revealing electronic state-switching at conical intersections in alkyl iodides by ultrafast XUV trans (2020)

REVIEWER COMMENTS 
Reviewer #1 (Remarks to the Author): 

 
This manuscript reports what is stated in the title, for two specific alkyl iodides – i-propyl iodide and 
t-butyl iodide. I am torn in my assessment. The work is noteworthy in that it provides (further) 
illustration of the insights into the photodissociation dynamics of small gas phase molecules that can 
be achieved using cutting-edge time resolved pump-probe spectroscopy methods (here using XUV 
absorption as the probe). Against this, I perceive two rival viewpoints. First, the opportunities 
afforded by this type of experiment have already been demonstrated, quite spectacularly in some 
cases, in other recent publications from the Berkeley group – which include papers describing 
studies of the simplest alkyl iodide (methyl iodide, ref. 36) and a somewhat richer variant, allyl 
iodide (ref. 40). Second, as the authors note in their introduction, the UV photochemistry of the alkyl 
(and related) iodides has already been much studied. What new insights are revealed in this 
manuscript? The key features, e.g. excited state potentials, relative transition dipole strengths, 
parent molecule excited state to atomic product state correlations, are all understood reasonably 
well and I/I* product branching ratios have been measured (often at several wavelengths) and 
rationalised in terms of branching at a conical intersection between the 3Q0 and 1Q1 potential 
energy surfaces – as summarised in the Introduction. Real time observation of the bifurcation at this 
conical intersection would be noteworthy but, as the authors point out (p. 3), ‘The determination of 
time constants characterising the evolution of the 3Q0 and 1Q1 features is currently limited by the 
temporal resolution of the experiments and is therefore not provided here’. Hence my dilemma: 
Those familiar with the field might well enjoy reading another well-written description of an 
application of an impressive new technology but will find few surprises in the results, while the more 
general chemical physics oriented reader could have already gained a similar overview from reading 
prior works from this group. 

There is no doubt the science merits publication; my uncertainty is whether this particular work 
meets the stated criterion for publication in Nature Communications – namely that ‘it represents an 
advance in understanding likely to influence thinking in the field.’ But, given my expectation that the 
work will be published in some appropriate journal, I offer the following comments that the authors 
may wish to consider: 

p. 2. The statement ‘For few-carbon containing alkyl iodides, excitation to 3Q0 comprises 70-80% of 
the oscillator strength of the A band’ may need qualifying. For comparison with the present data, 
one is interested in the partial absorption cross-sections to the various excited states at the pump 
wavelength of interest. If transitions to excited states other than 3Q0 in the present study really 
were responsible for 20-30% of the total oscillator strength, should they not be evident in the zero 
time delay transients? 

Fig. 2. The authors might reflect on the colour scheme used to display in panels (a) and (b) (pale to 
dark with increasing time delay). This choice ensures that the dominant features are associated with 
the asymptotic I and I* atoms, yet the point of primary novelty is surely the variation in the transient 
spectra at short time delays – which is currently hard to make out. Might a multi-colour scheme 
allow clearer visualisation of the early time data? 

p. 5 contains two curious sentences: ‘The modified CH3I transients qualitatively reproduce …….. with 
excellent agreement’; and ‘Future experiments with even shorter pump pulses …… will be the 
subject of a future study.’ 

p. 9. D.M.N. initials sequenced wrongly in line 3 of Author Contributions. 




Reviewer #2 (Remarks to the Author): 

This paper presents a very nice time-resolved study of the photodissociation of isopropyl- and t-
butyl-iodide into an alkyl radical and an iodine atom in the I or I* spin-orbit state. Ultrafast transient 
absorption spectroscopy is used to observe passage through a conical intersection along the 
dissociation coordinate. The switch between potential surfaces leads to a switch in the absorption 
spectrum of the dissociating molecule as well as that of the ultimate fragments. What is great about 
the paper is that it is very clearly written, the effects are clearly visible and (mostly) readily 
understood by the reader, the theory presents some confirmation of the model, and the whole thing 
could be used as a terrific textbook example of the effects of conical intersections on 
photodissociation dynamics. The demonstration of the experimental approach, especially when 
combined with the theory, clearly suggests that it will have substantial long term value, and I am 
sure it will lead to lots of exciting avenues of research. I am not sure one could point to any one 
thing in the results on alkyl iodide photodissociation where you could say, these results demonstrate 
something that nobody knew before, but the example is so nice and the technique is so powerful 
that I believe that the paper should be accepted for publication in Nature Communications. As I said, 
the paper is very clearly written, and could be published as is. I just have a couple points the authors 
might want to consider. 


1. On p. 3 the authors say, "the 3Q0 state is observed primarily undergo strong transitions to the 
(4d3/2)-1σ* state…, whereas the 1Q1 state primarily undergoes strong transitions to the (4d5/2)-
1σ* state…" This makes it sound like the selectivity just happens to be there, but it seems like there 
must be a simple atomic picture that shows why this is true (and certainly the calculations reproduce 
it). Maybe they could put something in the supplementary material about this, and then change the 
text to make it sound like they know why it is that way? 


2. The pictures used in the paper show the motion along the reaction coordinate, but of course, 
even in the photodissociation of methyl iodide, the motion along other normal modes affects the 
branching and product distributions (e.g., the calculations of Guo and Schatz). The authors mention 
in the discussion that probing near the carbon edge could provide insight into those dynamics, which 
would be really neat. It would be interesting to understand the extent to which such motions 
responsible for the changing branching ratios in different alkyl iodides. Certainly, the translational 
energy distributions for both samples show broad distributions (see Refs. 29 and 30), reflecting 
significant excitation of the radical vibrational modes. In any case, in the presence of all these other 
degrees of nuclear motion, it is interesting that the passage through the conical intersection still 
happens so fast. 

 
3. I probably missed something in the explanation, but in Figure 2c, the population in 1Q1 evolves 
smoothly into the feature associated the free I atoms, but for the feature associated with 3Q0, there 
is a minimum of density in the evolution from the 3Q0 feature to the free I* atoms. This is 
reproduced in the simulation, but I am not quite sure why there is this minimum. Does it just reflect 
the very short period of time at the corresponding geometry? Why is it absent, or at least much less 
obvious, for the 1Q1 component? 

These points are more curiosities, and I do think the paper can be accepted in the present form. 

 

Reviewer #3 (Remarks to the Author): 

The group is one of the pionner scientists to have developed the ATAS technique to reveal molecular 
dynamics. They have in the past studied several compounds using ATAS among which are some 
halogens : 

- Direct Observation of the Transition-State Region in the Photodissociation of CH3I by 

Femtosecond Extreme Ultraviolet Transient Absorption Spectroscopy ». JPCL 2015 . (ref36) 

-Transition State Region in the A-Band Photodissociation of Allyl Iodide—A Femtosecond Extreme 

Ultraviolet Transient Absorption Study ». JCP 2016 (ref40) 

-Tracking Dissociation Dynamics of Strong-Field Ionized 1,2-Dibromoethane with Femtosecond 

XUV Transient Absorption Spectroscopy . PCCP(2016) 

- Disentangling Conical Intersection and Coherent Molecular Dynamics in Methyl Bromide with 

Attosecond Transient Absorption Spectroscopy ». Nature Communications (2019) (ref15) 

The group now presents a study of the iso-propyl iodide and tert-butyl iodide molecules (i-C3H7I 

and t-C4H9I) dissociation which is caracterized by a larger amount of I/I* branching ratio relative 

to smaller alkyl iodides. The introduction, experimental results and their assignments are perfectly 

written and explained with the proper references. The reading is very easy and the experimental 

data beautiful to see (only 2mOD ! Thanks to the high-frequency post-process analysis). It was a 

real pleasure to read it. If the below mentioned concerns and minor items can be addressed 

satisfactorily, one could consider publication in Nat. Com. 

Main concerns : 

1) In Fig2c for i-C3H7I, why does the feature at 47.6 eV, that is fragment signature , seems 

dominant before 120fs and not after ? 

2) In fig2d for t-C4H9I , what is the assignment of transients at 46.4-46.8 eV/40-90 fs ? 

3) It is really disappointing that there is no attempt to fit the rising and decay times of the features 

by a decomvolution with the apparatus function. Indeed, when the precision of the cross-correlation 

is +/-7fs, it means that the resolution of the dynamics will appear on the same range. It is especially 

true to go beyond the contour plot that gives the misleading first impression that product features at 

45. 9 eV appears as soon as 0fs for i-C3H7I. Similar treatments were already made in the JCP 2016 

for allyl iodine with a longer cross-correlation. If it is due to the limited space of Nat.Com, it could 

be included in the SI. It is also relevant to compare the two molecular systems and to conclude if 

the time-scales can also rationalize the different I*/I ratio observed. 

4) I don’t understand what is clearly done in ‘enforcing the convergence of I:I* branching ratios to 

values of 2:1 and 13:1 at 50 fs time delay’. A few sentences should be written to tell the reader that 

the dynamics in the simulation is twice as fast as in the experiment. Could this simulation be 

temporally broadened by a 50 fs Gaussian (as in the experiment) instead of 15 fs which does not 

make sense to me. 

Minor ones 

5) I would have expected a slight energy variation of the 1 and 2 features due to the chemical shift 
variation with the C-I distance. This is not observed : is it due to the long 50fs duration of the pump 
pulse ? It would be great to discuss the requirement on the experimental set-up to observe on such 
compounds a chemical shift variation. 

6) I don’t like the figure 1A because it give the wrong impression that R products appear without 

internal energy and that all the relased energy is of kinetic nature relative to the center of mass.