Review Reports - A Sustainable Slit Jet FTIR Spectrometer for Hydrate Complexes and Beyond

Round 1

Reviewer 1 Report

Excellent work! The reported sustainable slit jet FTIR apparatus design is highly novel, the methodology is very well described and the presented spectroscopic results supporting the author’s main conclusions are all very convincing. The unprecedented direct infrared absorption spectrum obtained for vacuum-isolated clusters up to the pentamer of H₂¹⁸O generated from 2700 doped neon gas pulses and a total consumption of only 120 mg of H₂¹⁸O is remarkable. The compact design together with the significant reduction of both carrier gas and substance consumption due to a continuous recycling approach make this instrument broadly applicable due to the less required laboratory space for this kind of experiment and the substantial widening of feasible target systems.

The manuscript is highly recommended for publication in Instruments either in the present form or after minor revisions as suggested below:

-The novel recycling approach developed for the gratin jet is sustainable in terms of sample and carrier gas consumption as documented in Table 4. However, the reduction of carrier gas consumption by a factor of 3.5 going from filet to gratin jet is offset by a 9-fold increase of total scan time. In the presented applications of the gratin jet, the total scan times often reach 24 hours. In the context of sustainability, it could be relevant to learn more about the estimated consumption of power by the high-capacity pumping systems over these longer time intervals.

- In section 2.2.4 it would be interesting to learn more about the potential implementation of Si-bolometer detectors for the detection of the class of large-amplitude intermolecular vibrational modes in the far-infrared spectral region.

- The beautiful water cluster spectra presented in Fig. 11 seem to reveal significantly more narrow OD stretching bands for (D₂O)₄ and (D₂O)₅ relative to the corresponding OH stretching bands for (H₂O)₄ and (H₂O)₅ after the first batches of co-added scans, which may deserve a comment? In addition, the rotational fine structure contour is visible for the (D₂O)₂ band but cannot be observed for the corresponding (H₂O)₂ band?

- In the convincing cluster size assignment procedure outlined for the different water cluster isotopologues, the authors compare spectra obtained for three different water vapor pressures in 750 hPa expansions. The observed cluster size distributions would probably be slightly different employing these same vapor pressures at different total stagnation pressures. It would perhaps make sense to use mixing ratios instead of vapor pressures in the discussion of cluster size distributions.

- This reviewer had to spend some time to fully understand some of the graphical illustrations fx Fig. 8 providing a graphical comparison of the performances of the gratin and filet jets. However, after careful inspection of the captions and the descriptions in the text, the reviewer appreciates the wealth of information contained in these illustrations.

- The “Noise Test Challenge” introduced on page 9 is very useful for the comparison between the gratin and filet jet sensitivities under different conditions. The plots of the quantity –log(RMSE), however, at first sight challenge the reader's intuition as high values of –log(RMSE) indicate lower levels of noise and the raw values of RMSE could probably work.

Author Response

We thank the reviewer for her/his expert and very positive judgement of our new instrument and for pointing out some very valid points to be clarified and elaborated on. We respond to all of these suggestions for minor review as follows (-reviewer, >response, * verbatim text changes made *):

-The novel recycling approach developed for the gratin jet is sustainable in terms of sample and carrier gas consumption as documented in Table 4. However, the reduction of carrier gas consumption by a factor of 3.5 going from filet to gratin jet is offset by a 9-fold increase of total scan time. In the presented applications of the gratin jet, the total scan times often reach 24 hours. In the context of sustainability, it could be relevant to learn more about the estimated consumption of power by the high-capacity pumping systems over these longer time intervals.

>The reviewer is right that apart from minor details, the savings relative to the filet jet do not extend to the electrical power (nominally in the 10 kW range) needed, so any extended measurement (the straightforward way to increase the S/N ratio) translates into more power consumption, in both spectrometers alike. However, we now point out an improvement potential associated with the new economic possibility to use neon carrier gas, after the first sentence of the outlook:

* Neon, in particular, offers a downscaling of the electrical power demand up to a factor of 2, by skipping the first compression stage without any significant compromise in the spectral quality or in the avoidance of carrier gas co-condensation. *

>The reviewer raises an interesting point on a challenging application. We now write at the end of the second paragraph of section 2.2.4:

* This can also be equipped with a Si-bolometer to extend measurements into the far infrared, assisted by the broadband beamsplitter and mirror optics, together with CsI windows and a cryogenic helium recovery port. Mechanical vibrations are not enhanced compared to the filet jet setup despite the more compact design. Therefore, an improved performance in this challenging and attractive spectral range [30] can be anticipated, but has not yet been explored. *

- The beautiful water cluster spectra presented in Fig. 11 seem to reveal significantly more narrow OD stretching bands for (D2O)4 and (D2O)5 relative to the corresponding OH stretching bands for (H2O)4 and (H2O)5 after the first batches of co-added scans, which may deserve a comment? In addition, the rotational fine structure contour is visible for the (D2O)2 band but cannot be observed for the corresponding (H2O)2 band?

>It is already known that deuteration narrows the tetramer and pentamer bands (independent on the location of the scans in the co-addition, once there is sufficient water deposited) and we now mention and cite this in the caption. There is also a slightly bimodal rotational contour for (H2O)2 seen in the figure, but other than for (D2O)2 it does not have a minimum between the P- and R-like branches and is therefore less suitable for rule-of-thumb estimates of the rotational temperature. Again, we now cite the appropriate cavity ring down literature.

Caption Fig 11 ... pentamer contributions (4/5) * , the latter known to narrow down upon deuteration [75-78] *. The...

Third-last paragraph in section 3.4.1 ... the somewhat more pronounced * [75,76] * ...

>We agree that for future pressure variation studies (which are largely equivalent to concentration variation studies), the mixing ratio may be a more intuitive variable (easily obtained from the two quoted pressures), but as the primary quantity in the actual filling procedure is the partial pressure (no need to prepare a gas mixture off-line, another virtue of the new setup) and that quantity is proportional to the mixing ratio for constant total pressure (assuming ideal gas behaviour), we prefer to give these primary quantities in the present manuscript, instead of their ratio to the total pressure.

- This reviewer had to spend some time to fully understand some of the graphical illustrations fx Fig. 8 providing a graphical comparison of the performances of the gratin and filet jets. However, after careful inspection of the captions and the descriptions in the text, the reviewer appreciates the wealth of information contained in these illustrations.

>We agree that Figs 6-9 and in particular Fig 8 contain a lot of subtle and composite information which we did not want to hide from the interested reader in such a performance-describing article. They are not necessary for a reader who is happy with the simplified messages and can certainly be skipped for those just interested in the scientific results based on the improved performance.

- The “Noise Test Challenge” introduced on page 9 is very useful for the comparison between the gratin and filet jet sensitivities under different conditions. The plots of the quantity –log(RMSE), however, at first sight challenge the reader's intuition as high values of –log(RMSE) indicate lower levels of noise and the raw values of RMSE could probably work.

> The reason why we use a logarithmic scale in these plots is that we want to cover also opaque regions and non-cooled detectors with poor noise performance. But we fully agree that the choice could be perceived as counterintuitive, and therefore add to the caption of Fig 5:

* Note that a higher value means lower noise and thus better performance. *

Reviewer 2 Report

This manuscript describes in a very detailed and rigorous manner the potentialities and limitations of the new experimental device "gratin jet". This device revives in a revolutionary way the use of FTIR spectroscopy to probe supersonic flows. The implementation of closed-circuit pumping makes it possible to achieve enormous savings in carrier gas, to use very expensive carrier gases such as neon, and to drastically reduce the quantity of active molecule injected. This gives the possibility of increasing the number of accumulated scans for the same quantity of molecules consumed and thus to gain an order of magnitude in the signal-to-noise ratio, which is extraordinary.

Thus, "Gratin jet" is particularly well suited to the study of hydrated molecular complexes in the OH region. Several examples illustrate the performance of the device for the study of such complexes. The water vapor example seems particularly important to me as it convincingly responds to a controversy over the attribution of the signatures of water dimer and trimer.

This manuscript dedicated to the new "gratin jet" device whetted my appetite, and it will undoubtedly have the same effect on all readers who will read it. I strongly recommend its publication as is.

I would like to congratulate the authors for this magnificent innovation which will stimulate many researchers across the world of spectroscopy.

Author Response

We thank the reviewer for his/her enthusiastic feedback. Indeed, the new setup also whets our appetite for more and better vibrational cluster spectroscopy using the intrinsically less sensitive FTIR approach, to complement laser-based techniques in an optimized and meaningful way. Long term operation will certainly reveal further improvement potential and limitations of the technique.