Peer review of: Ultraviolet photolysis of H2S and its implications for SH radical production in the interstellar medi (2020)

Reviewers' comments: 

 

Reviewer #1 (Remarks to the Author): 

 

Please find below my referee’s report for the manuscript “Ultraviolet Photolysis of H2S: Implications 
for SH Radical Density Estimate in the Interstellar Medium” by J. Zhou et al.. I recommend the paper 
for publication in Nature Communications as it stands. 


The new results – concerning the yield of ground-state SH radicals following VUV photolysis of 
hydrogen sulfide – are highly relevant to photochemical models of sulfur-bearing molecules in the 
interstellar medium. The new experimental results are used to show that the ground state SH yield is 
strongly wavelength dependent and, in fact, effectively goes to zero at wavelengths shortward of 
about 125 nm. When coupled with a standard model of the radiation field in the ISM and H2S 
photoabsorption cross sections, the wavelength-dependent photolysis yield nicely accounts for the 
observed depletion of SH relative to H2S in astronomical sources. In addition to the astrophysical 
applications that rely on a quantitative understanding of the H2S dissociation paths following VUV 
excitation, there is intrinsic merit on the basic molecular physics side of the ledger in elucidating 
these dissociation mechanisms. The paper is extremely well written and comprehensive; the 
experimental methods are presented at an appropriate level of detail should other researchers 
consider trying to reproduce the results. The experimental techniques, which utilize a pulsed VUV 
free-electron laser, velocity map imaging, and H-atom tagging, are state of the art. In short, this is an 
excellent piece of experimental work that is well-presented and whose results are applied to at least 
partly resolve an existing discrepancy between astronomical observations and photochemical 
models. 

 

I have only one question for the authors: In figure 4, the total photoabsorption cross section of H2S 
is plotted as a function of wavelength. That cross section is used in the convolution calculation that 
results in the yield of ground state SH. I believe that it is assumed in this calculation that the H2S 
photodissociation cross section is identical to the H2S photoabsorption cross section in this 
wavelength region. Is this known to be the case? There appears to be sharp structure in the 
photoabsorption cross section (at least at the level of resolution in the figure); that might indicate a 
non-zero probability of radiative relaxation of H2S, which would also lower the yield of SH. It would 
be helpful if the authors could clarify this one point. 



Reviewer #2 (Remarks to the Author): 

I was asked to provide a report on the paper entitled "Ultraviolet Photolysis of H2S: Implications for 
SH radical density estimates in the interstellar medium". This paper presents experimental results on 
the photolysis of H2S by UV photons. The goal here is to constrain the products of the 
photodissociation of gas-phase H2S to see if and how much HS is produced in order to explain the 
observed HS/H2S abundance ratio in diffuse interstellar medium. In fact, current astrochemical 
models assume that H2S photodissociation produces only one channel leading to HS. Using this 
branching ratio in chemical models produces a larger HS/H2S than what is observed. 

The present paper shows that the photodissociation of H2S only leads to something like 26% of HS 
and so concludes that astrochemical models should be changed according to this result and this may 
explain the low observed HS/H2S. 

Most of the paper focuses on the description of the results of the experiment. Not being an expert 
on these experiments, I am not able to understand in details what is explained. There is one 
paragraph on the implication of their experimental result on the study of HS in the interstellar 
medium. This paragraph is more a discussion than a demonstration that their results will explain the 
observations as they do not go beyond the estimation of the branching ratio. These new data has 
not been included in a chemical model and so its impact is speculative. 

Overall, such experimental results are crucial for astrochemical studies. These specific results can 
indeed be important and have an impact. The paper is however difficult to read for a general 
astrochemist not coming from this field of research. The very promising title of the paper is not 
completely reflecting the content of the paper. 

 

Reviewer #3 (Remarks to the Author): 

The article suggests to revise the relevant astrochemical models to justify the depletion of SH(X) in 
the interstellar medium. Based on the photolysis of jet cooled H2S molecules in the 122 - 155 nm 
range the authors show that the quantum yield to form SH(X) decreases from 1 (at the longest 
excitation wavelengths) to zero at short wavelengths. Convoluting the interstellar radiation field 
taken from the literature with the total absorption cross-sections of H2S the authors report that only 
26% of all photoexcitation events should result in SH(X) fragments. The 26% is a very challenging 
value. What are the error estimates for this value? There are many theoretical spectroscopic studies 
that describe the VUV photo-dissociation dynamics of H2S. This paper shows again another UV 
photolysis of H2S study but the obtained results cannot be used to estimate the SH radical density in 
the ISM as claimed by the author. Before publication the authors should pay attention on a few 
typos such as those on page 32 line 66 and in figure 4. I suggest also that they should change the title 
of this article and review the part concerning the SH density in the ISM. They should also link their 
study to the Chakraborty's work which was not mentioned and related to the sulfur isotopic 
fractionation in VUV photodissociation of H2S at 121.6, 139.1, and 157.6 nm, the same wavelengths 
used in the present study (PNAS 2013 110 (44) 17650-17655).